Comparative anatomy of the encephalon of new world primates with
emphasis for the 
Human evolutionary history is directly associated with brain development, considering
that the construction of society depends on the high cognition observed in the
humankind . Indeed, the
human brain is the most sophisticated instrument of cognition and, putatively, the
most complex material structure known . Therefore, the neural system has been
studied extensively in multidisciplinary domains, but many questions about brain
functions remain answered  One of the approaches to understand the cerebral physiology is to study the
historical/archaeological data from primitive humans and to observe and to use
extant models of evolution, such as other primates, to understand the anatomical
features that made humans a species with higher intelligence showing unique
characteristics such as language, self-ornamentation, and artistic and religious
manifestations , inter
alia. However, the use of ancestral fossils for brain studies is limited because
soft tissues are not well conserved. Therefore, only the bone impressions of the
encephalon remain, which are insufficient for a deep understanding of its anatomical
history. Therefore, chimpanzees and gorillas are the most studied models for human
evolution, social cognition, hand abilities, behavioral flexibility, and tool use,
certainly because they are the most cognitive non-human primates and closer to
humans in terms of phylogenesis, brain size  and DNA similarities ; in terms of phylogenetic proximity  chimpanzees, bonobos,
gorillas, and orangutans are considered, in this order, as the main models to study
the human evolutionary history. Various studies about evolution of the mind and intelligence of modern humans are
based mainly on the behavior of apes and other primates—social and solitary
species—using tests of cognition about memory and learning; however, for the
neurophysiological research, macaques are the main studied primates . Despite that, data of gross anatomy of the
cerebrum are scarce, especially when considering comparative studies. Behavioral (ethological) studies are sometimes performed before a structural analysis
 of the animals,
leading to some misinterpretations . However,
studies do not indicate a positive correlation of brain anatomy with behavior,
cognition, or other complex abilities, except, perhaps, with brain size [ , , ]; indeed, proportional analysis indicates no
significant difference between the cerebral mass of modern humans and baboons, for
instance . Thus, a study of the gross anatomy of the brain performed in many primate species
from different taxa may generate a negative answer for the following question: “Does
the brain anatomical structure correspond the level of cognitive development in
phylogenetic evolution?” Such a study is essential to better understand the
relationship between cognitive development and brain anatomy; however, to date, such
a study has not been performed in the species we describe here. It is well established that some gyri and sulci are common for all primates, whereas
others exist in non-human primates but not in modern humans . However, an accurate comparative study of
the brains of neotropical primates, Old World primates, apes, and modern humans,
analyzing aspects such as the main gyri and sulci, brain size, and gyrencephaly has
not yet been performed. Among all primates, the neotropical ones are scarcely studied, except for the genus Sapajus (bearded capuchins), which, remarkably and
unexpectedly, exhibit characteristics such as memory, cognition, social behavior,
and tool use similar to chimpanzees [ , – ] a big brain in relation to
their body mass ; and high motor development
. Additionally, there is
some research on the brain of Saimiri collinsi , Sapajus
apella (formerly Cebus apella )  and Callithrix sp. , but the studies did not
include a detailed comparative anatomy or investigate the correlations with behavior
and evolution. Furthermore, a study investigated the brain anatomy of Sapajus , but some information was missing . Therefore, the present study aims to perform a comparative analysis of experimental
data of the main aspects of the gross anatomy of the brain of neotropical primates
such as Callithrix penicillata , Saimiri ustus , Brachyteles arachnoides , and Sapajus
libidinosus and data from previous studies on Saimiri
collinsi , Galago sp., Papio sp., Macaca spp., Pongo sp., Gorilla spp., chimpanzees, and modern humans. Allometric
measurements of the sulci along a straight line and along the sinuosities were
performed to generate a dataset to facilitate comparison among the species in terms
of the degree of gyrencephaly for dissected specimens; moreover, the encephalization
quotient was calculated for neotropical primates to allow comparison with the data
for other primates from previous studies. When possible, we investigated the
association between the obtained data with the general behavioral aspects of the
respective primate species. 

Macroscopic description of the encephalon of Callithrix
penicillata The height, length, and width measurements of C . penicillata brain were 18.08, 29.10, and 21.43 mm,
respectively; its volume was 0.6 mL, and EQ was 1.7. The lissencephaly of the
frontal, parietal and occipital lobes made the delimitation of cerebral
hemispheres challenging; however, it was possible to identify the longitudinal
fissure and lateral and temporal sulci on the convex aspect of the brain ( ). The lateral sulcus is
a small sulcus in the oblique direction. The superior temporal sulcus in the
temporal lobe is visible only in the lateral view, in a direction approximately
parallel to lateral sulcus ( ). On the medial aspect, it was possible to visualize the calcarine, rhinal,
hippocampal sulci and sulcus of the corpus callosum. This region presents few
gyri and could be considered as lissencephalic ( ). The posterior portion of the sulcus
of the corpus callosum ends at the junction between the hippocampal and
calcarine sulci ( ).
The rhinal sulcus is a short, shallow and arcuated sulcus that is located
posteriorly to the temporal lobe; the hippocampal sulcus—found posteriorly to
the calcarine sulcus and sulcus of the corpus callosum—terminates in the uncus
( ). The telencephalic sulci shows negligible gyrencephaly or degree of sinuosity
( ), indicating
a straight trajectory, and therefore exhibits few gyri. For instance, neither
the central sulcus nor the precentral and postcentral sulci were evident, and
thus the precentral and postcentral gyri could not be observed ( ). The other encephalic structures of C . penicillata are similar to the corresponding structures in
the human brain but with different anatomical organization; for instance, the
corpus callosum is appears straighter than other primates, as indicated by the
sinuosity of the sulcus of the corpus callosum ( ), and the angle of the encephalic
trunk in relation to thalamus is greater than that in humans. No specific differences were observed in other inferior structures, such as the
pellucid septum; corpus callosum that presents genu, trunk, splenium, and an
almost imperceptible rostrum; diencephalon; mesencephalon; pons; cerebellum; and
medulla oblongata ( ). Tables and – show the absolute measures of the sulci,
gyri, encephalic trunk, and other structures. Macroscopic description of the encephalon of Saimiri
ustus The height, length, and width measurements of the Samiri ustus encephalon were 28.48, 44.38, and 34.10 mm, respectively. The encephalon volume
was 22 mL, and EQ was 2.25. There is a deep longitudinal fissure separating the hemispheres and a few sulci
and gyri on the convex aspect ( ). The central sulcus is shallow and short, and anteriorly
from it, the frontal lobe is lissencephalic. In contrast, the posterior parietal
area presents two big gyri or lobules generated by the continuation of the
lateral sulcus ( )—the
rostral and caudal lobules. The lateral sulcus is deep and separates the
frontal, parietal, and temporal lobes; it is present in an inclined/oblique
direction and continues in the medial aspect ( ). The superior temporal sulcus in the temporal lobe is parallel to the extension of
the lateral sulcus and generates two gyri: the superior and inferior temporal
gyri ( ). The occipital
lobe is lissencephalic on the convex aspect, but it is possible to observe a
small depression called lunatus sulcus ( ). On the medial aspect, the
sulcus of the corpus callosum and parieto-occipital, rhinal, hippocampal,
collateral, and calcarine sulci are visible ( ). The cingulate sulcus does not follow the arcuate curve of the corpus callosum; it
is a straight sulcus in the frontal-to-parietal direction that delimits
superiorly the cingulate gyrus, which is posteriorly separated from the
occipital lobe by the subparietal sulcus ( ). The cingulate sulcus is delimited
anterior-inferiorly by the sulcus of the corpus callosum and
posterior-inferiorly by the calcarine sulcus ( ) in the region of the isthmus of the
cingulate gyrus. Posteriorly, the sulcus of the corpus callosum meets the calcarine sulcus. The
corpus callosum presents a rostrum, genu, trunk, and splenium and the structure
is somewhat arcuated ( ). Continuous with the lateral sulcus but medially, the
parieto-occipital sulcus separates the parietal and occipital lobes; anterior to
the parieto-occipital sulcus is the subparietal sulcus, which is delimited
anteriorly by the precuneus and posteriorly by the isthmus of the cingulate
gyrus ( ). The calcarine sulcus begins in the temporal lobe on the medial aspect and
continues to the occipital lobe, where it bifurcates into superior and inferior
rami; it joins the sulcus of the corpus callosum anteriorly ( ). Situated between the
parieto-occipital sulcus and superior ramus of the calcarine sulcus is the
cuneus gyrus ( ). The
collateral sulcus begins from the calcarine sulcus in the occipital lobe and
ends in the anterior medial region of the temporal lobe. It delimits the lingual
gyrus in its posterior inferior portion and the parahippocampal gyrus in its
anterior inferior portion ( ). The lingual gyrus is found between the calcarine and collateral sulci; it is
delimited superiorly by the hippocampal sulcus. Located below the collateral
sulcus is the fusiform gyrus ( ). The hippocampal sulcus begins posteriorly in the bifurcation
between the sulcus of the corpus callosum and the anterior portion of the
calcarine sulcus and terminates cranially below the uncus ( ). The diencephalon presents structures and features similar to those observed in
humans and other primates; the encephalic trunk is present at an angle greater
than 90° in relation to a vertical axis passing through the telencephalon, and
it exhibits the general characteristics found in humans and other non-human
primates ( ).
Evidently, the absolute measurements of the brain structures differ among these
species. Tables – present the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. Macroscopic description of the encephalon of Sapajus
libidinosus In this study we focus our description on the macroscopic features of the lateral
part of the Sapajus libidinosus brain, diencephalon and
encephalic trunk, with limited comments on the medial part, already
comprehensively described be Pereira-de-Paula . On the medial aspect of Sapajus libidinosus encephalon, superiorly, the cingulate
sulcus and gyrus are visible ( ). The cingulate gyrus is delimited inferiorly by the sulcus of the
corpus callosum and superiorly by the cingulate sulcus. Located above the
cingulate sulcus is the medial frontal gyrus ( ). The cingulate sulcus is present anterior and superior to the corpus callosum; it
courses in the anterior-to-posterior direction at a small inclination angle.
Approximately in the posterior third section, it sharply ascends to the superior
border and terminates before reaching the superior border; the sharply ascending
section is called marginal ramus ( ). A small and solitary sulcus is observed in the medial and anterior part of the
frontal lobe, here designated as the rostral sulcus ( ). The sulcus of the corpus callosum
contours the corpus callosum; it starts above the rostrum and terminates below
the splenium, where it joins the calcarine and hippocampal sulci ( ). The calcarine sulcus,
another sulcus that starts close to the splenium, courses in the posterior
direction towards the occipital lobe, where it bifurcates into superior and
inferior rami ( ).
Lateral and inferior to the anterior part of the calcarine sulcus is the lingual
sulcus, which also delimits the isthmus of the cingulate gyrus. The middle and
superior parts of the calcarine sulcus delimit the cuneus in the superior part
of the occipital lobe ( ). The cuneus is anteriorly delimited by the occipital sulcus
( ). At the beginning of the calcarine sulcus running in the anterior direction in the
medial superior part of the temporal lobe is situated the hippocampal sulcus,
which terminates in the uncus ( ). The collateral sulcus begins inferior to the middle third of the
calcarine sulcus and delimits the anterior section of the lingual gyrus and the
posterior section of the fusiform gyrus. The occipital-temporal sulcus is
located anterior and inferior to the collateral sulcus and terminates
approximately inferior to the starting point of the rhinal sulcus. The rhinal
sulcus is shallow and situated in the middle of the temporal pole ( ). The fusiform gyrus is delimited inferiorly by the occipital inferior sulcus,
which contours the inferior portion of the occipital lobe and appears on the
convex aspect of this lobe, terminating in the posterior part of the temporal
lobe ( ). Located
inferiorly in the temporal lobe is the occipital-temporal sulcus, which delimits
the parahippocampal gyrus inferiorly and separates this gyrus from the inferior
temporal gyrus ( ). The
parieto-occipital sulcus separates the parietal and occipital lobes medially and
is continuous with the subparietal sulcus in most of the antimeres  ( ). The structures present in the diencephalon and encephalic trunk are very similar
to the corresponding structures found in other non-human primates and humans,
but the absolute dimensions and angulation in relation to a vertical axis
through the telencephalon are different ( ). Tables – present the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. Macroscopic description of the encephalon of Brachyteles
arachnoides Located centrally on the convex aspect of the B . arachnoides brain is the central sulcus, which separates
the frontal and parietal lobes. It begins close to superior border of the
hemisphere and terminates before encountering the lateral sulcus ( ). Three more sulci are
observed on the convex aspect of the frontal lobe: the frontal, superior
frontal, and precentral sulci ( ). The superior frontal sulcus has a longitudinal superior position
close to the superior border of the hemisphere; oblique and almost vertical. The
precentral sulcus is located superiorly to the inferior frontal sulcus ( ). Located in the posterior part of the hemisphere behind the central sulcus is the
postcentral sulcus in the parietal lobe, which is a small and deep sulcus
partially parallel to the central sulcus ( ). A long, deep, and oblique sulcus
marks the convex aspect of the hemispheres in the B . arachnoides brain—the lateral sulcus that courses across
from the inferior to the superior border of the brain hemispheres and, after two
curvatures, terminates in a bifurcation into the posterior region of the
parietal lobe ( ). Two sulci are observed in an inclined temporal lobe—the superior and inferior
temporal sulci. The superior temporal sulcus is deep and runs parallel to the
lateral sulcus across the temporal lobe, thus delimiting the superior and middle
temporal gyri. A small, straight, and shallow inferior temporal sulcus scarcely
separates the middle from the inferior temporal lobe ( ). On the convex aspect, the lunatus sulcus, which separates the parietal and
occipital lobes, presents differences between the right and left hemispheres;
for instance, the lunatus sulcus joins the lateral sulcus in the right
hemisphere not in the left hemisphere ( ). On the convex aspect, an
inferior occipital sulcus appears in the middle part of the inferior occipital
pole in both hemispheres; however, a superior occipital sulcus is prominent in
the left occipital lobe but shallow and insignificant in the right occipital
lobe ( ). A straight sulcus observed on the medial aspect of the frontal lobe is the
rostral sulcus ( ). The
cingulate sulcus contours the cingulate gyrus from its anterior portion;
differing in its posterior portion between the two hemispheres ( ). The parieto-occipital
sulcus clearly separates the occipital and parietal lobes on the medial aspect
( ). Contouring the corpus callosum is the arcuated sulcus of the corpus callosum; it
ends at the junction between the hippocampal and calcarine sulci ( ). The hippocampal
sulcus is arcuated in the cranial direction and ends in the uncus medially and
inferiorly in the temporal pole ( ). From the occipital lobe, the calcarine sulcus emerges in a
horizontal trajectory and terminates generating the superior and inferior rami
prominently visible in the left hemisphere ( ). Visible on the inferior surface of the occipital and temporal lobes is the
collateral sulcus, including the inferior part of the hippocampal gyrus. The
rhinal sulcus starts after the collateral sulcus in the anterior inferior part
of the temporal lobe ( ). shows structures such
as the corpus callosum, diencephalon, and encephalic trunk. All these structures
are anatomically similar to the corresponding structures in other primates;
however, the angle of the encephalic trunk to a vertical axis through the
telencephalon is greater than 90°. Tables – show the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. Allometric data of the studied encephalons Data regarding the presence or absence of the main sulci in the medial region of
the brain in the primate species investigated in the present study and
previously published data were concatenated ( ). The absolute measurements of the brain structures of the primate species studied
in the present study are shown in part of the and Tables – . These measurements indicate the difference
in the size of different structures, distance of sulci and gyri to the poles,
and dimensions of the encephalon. These measurements indicate provide an
approximatione of view about the size [(volume)] of some lobes. A table and graph with straight versus curve dimensions of main gyrus were made
to indicate the sinuosity degree among the primates studied this work ( ). For some primates
was not possible to obtain all data. presents the
relative values to facilitate a better comparison among the encephalon
structures. These data permit to verify approximately the real relation of
dimensions of the cerebral structures. The EQ values were calculated for the primate species included in the present
study and for data obtained from previous studies ( ). 

Callithrix
penicillata The height, length, and width measurements of C . penicillata brain were 18.08, 29.10, and 21.43 mm,
respectively; its volume was 0.6 mL, and EQ was 1.7. The lissencephaly of the
frontal, parietal and occipital lobes made the delimitation of cerebral
hemispheres challenging; however, it was possible to identify the longitudinal
fissure and lateral and temporal sulci on the convex aspect of the brain ( ). The lateral sulcus is
a small sulcus in the oblique direction. The superior temporal sulcus in the
temporal lobe is visible only in the lateral view, in a direction approximately
parallel to lateral sulcus ( ). On the medial aspect, it was possible to visualize the calcarine, rhinal,
hippocampal sulci and sulcus of the corpus callosum. This region presents few
gyri and could be considered as lissencephalic ( ). The posterior portion of the sulcus
of the corpus callosum ends at the junction between the hippocampal and
calcarine sulci ( ).
The rhinal sulcus is a short, shallow and arcuated sulcus that is located
posteriorly to the temporal lobe; the hippocampal sulcus—found posteriorly to
the calcarine sulcus and sulcus of the corpus callosum—terminates in the uncus
( ). The telencephalic sulci shows negligible gyrencephaly or degree of sinuosity
( ), indicating
a straight trajectory, and therefore exhibits few gyri. For instance, neither
the central sulcus nor the precentral and postcentral sulci were evident, and
thus the precentral and postcentral gyri could not be observed ( ). The other encephalic structures of C . penicillata are similar to the corresponding structures in
the human brain but with different anatomical organization; for instance, the
corpus callosum is appears straighter than other primates, as indicated by the
sinuosity of the sulcus of the corpus callosum ( ), and the angle of the encephalic
trunk in relation to thalamus is greater than that in humans. No specific differences were observed in other inferior structures, such as the
pellucid septum; corpus callosum that presents genu, trunk, splenium, and an
almost imperceptible rostrum; diencephalon; mesencephalon; pons; cerebellum; and
medulla oblongata ( ). Tables and – show the absolute measures of the sulci,
gyri, encephalic trunk, and other structures. 

Saimiri
ustus The height, length, and width measurements of the Samiri ustus encephalon were 28.48, 44.38, and 34.10 mm, respectively. The encephalon volume
was 22 mL, and EQ was 2.25. There is a deep longitudinal fissure separating the hemispheres and a few sulci
and gyri on the convex aspect ( ). The central sulcus is shallow and short, and anteriorly
from it, the frontal lobe is lissencephalic. In contrast, the posterior parietal
area presents two big gyri or lobules generated by the continuation of the
lateral sulcus ( )—the
rostral and caudal lobules. The lateral sulcus is deep and separates the
frontal, parietal, and temporal lobes; it is present in an inclined/oblique
direction and continues in the medial aspect ( ). The superior temporal sulcus in the temporal lobe is parallel to the extension of
the lateral sulcus and generates two gyri: the superior and inferior temporal
gyri ( ). The occipital
lobe is lissencephalic on the convex aspect, but it is possible to observe a
small depression called lunatus sulcus ( ). On the medial aspect, the
sulcus of the corpus callosum and parieto-occipital, rhinal, hippocampal,
collateral, and calcarine sulci are visible ( ). The cingulate sulcus does not follow the arcuate curve of the corpus callosum; it
is a straight sulcus in the frontal-to-parietal direction that delimits
superiorly the cingulate gyrus, which is posteriorly separated from the
occipital lobe by the subparietal sulcus ( ). The cingulate sulcus is delimited
anterior-inferiorly by the sulcus of the corpus callosum and
posterior-inferiorly by the calcarine sulcus ( ) in the region of the isthmus of the
cingulate gyrus. Posteriorly, the sulcus of the corpus callosum meets the calcarine sulcus. The
corpus callosum presents a rostrum, genu, trunk, and splenium and the structure
is somewhat arcuated ( ). Continuous with the lateral sulcus but medially, the
parieto-occipital sulcus separates the parietal and occipital lobes; anterior to
the parieto-occipital sulcus is the subparietal sulcus, which is delimited
anteriorly by the precuneus and posteriorly by the isthmus of the cingulate
gyrus ( ). The calcarine sulcus begins in the temporal lobe on the medial aspect and
continues to the occipital lobe, where it bifurcates into superior and inferior
rami; it joins the sulcus of the corpus callosum anteriorly ( ). Situated between the
parieto-occipital sulcus and superior ramus of the calcarine sulcus is the
cuneus gyrus ( ). The
collateral sulcus begins from the calcarine sulcus in the occipital lobe and
ends in the anterior medial region of the temporal lobe. It delimits the lingual
gyrus in its posterior inferior portion and the parahippocampal gyrus in its
anterior inferior portion ( ). The lingual gyrus is found between the calcarine and collateral sulci; it is
delimited superiorly by the hippocampal sulcus. Located below the collateral
sulcus is the fusiform gyrus ( ). The hippocampal sulcus begins posteriorly in the bifurcation
between the sulcus of the corpus callosum and the anterior portion of the
calcarine sulcus and terminates cranially below the uncus ( ). The diencephalon presents structures and features similar to those observed in
humans and other primates; the encephalic trunk is present at an angle greater
than 90° in relation to a vertical axis passing through the telencephalon, and
it exhibits the general characteristics found in humans and other non-human
primates ( ).
Evidently, the absolute measurements of the brain structures differ among these
species. Tables – present the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. 

Sapajus
libidinosus In this study we focus our description on the macroscopic features of the lateral
part of the Sapajus libidinosus brain, diencephalon and
encephalic trunk, with limited comments on the medial part, already
comprehensively described be Pereira-de-Paula . On the medial aspect of Sapajus libidinosus encephalon, superiorly, the cingulate
sulcus and gyrus are visible ( ). The cingulate gyrus is delimited inferiorly by the sulcus of the
corpus callosum and superiorly by the cingulate sulcus. Located above the
cingulate sulcus is the medial frontal gyrus ( ). The cingulate sulcus is present anterior and superior to the corpus callosum; it
courses in the anterior-to-posterior direction at a small inclination angle.
Approximately in the posterior third section, it sharply ascends to the superior
border and terminates before reaching the superior border; the sharply ascending
section is called marginal ramus ( ). A small and solitary sulcus is observed in the medial and anterior part of the
frontal lobe, here designated as the rostral sulcus ( ). The sulcus of the corpus callosum
contours the corpus callosum; it starts above the rostrum and terminates below
the splenium, where it joins the calcarine and hippocampal sulci ( ). The calcarine sulcus,
another sulcus that starts close to the splenium, courses in the posterior
direction towards the occipital lobe, where it bifurcates into superior and
inferior rami ( ).
Lateral and inferior to the anterior part of the calcarine sulcus is the lingual
sulcus, which also delimits the isthmus of the cingulate gyrus. The middle and
superior parts of the calcarine sulcus delimit the cuneus in the superior part
of the occipital lobe ( ). The cuneus is anteriorly delimited by the occipital sulcus
( ). At the beginning of the calcarine sulcus running in the anterior direction in the
medial superior part of the temporal lobe is situated the hippocampal sulcus,
which terminates in the uncus ( ). The collateral sulcus begins inferior to the middle third of the
calcarine sulcus and delimits the anterior section of the lingual gyrus and the
posterior section of the fusiform gyrus. The occipital-temporal sulcus is
located anterior and inferior to the collateral sulcus and terminates
approximately inferior to the starting point of the rhinal sulcus. The rhinal
sulcus is shallow and situated in the middle of the temporal pole ( ). The fusiform gyrus is delimited inferiorly by the occipital inferior sulcus,
which contours the inferior portion of the occipital lobe and appears on the
convex aspect of this lobe, terminating in the posterior part of the temporal
lobe ( ). Located
inferiorly in the temporal lobe is the occipital-temporal sulcus, which delimits
the parahippocampal gyrus inferiorly and separates this gyrus from the inferior
temporal gyrus ( ). The
parieto-occipital sulcus separates the parietal and occipital lobes medially and
is continuous with the subparietal sulcus in most of the antimeres  ( ). The structures present in the diencephalon and encephalic trunk are very similar
to the corresponding structures found in other non-human primates and humans,
but the absolute dimensions and angulation in relation to a vertical axis
through the telencephalon are different ( ). Tables – present the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. 

Brachyteles
arachnoides Located centrally on the convex aspect of the B . arachnoides brain is the central sulcus, which separates
the frontal and parietal lobes. It begins close to superior border of the
hemisphere and terminates before encountering the lateral sulcus ( ). Three more sulci are
observed on the convex aspect of the frontal lobe: the frontal, superior
frontal, and precentral sulci ( ). The superior frontal sulcus has a longitudinal superior position
close to the superior border of the hemisphere; oblique and almost vertical. The
precentral sulcus is located superiorly to the inferior frontal sulcus ( ). Located in the posterior part of the hemisphere behind the central sulcus is the
postcentral sulcus in the parietal lobe, which is a small and deep sulcus
partially parallel to the central sulcus ( ). A long, deep, and oblique sulcus
marks the convex aspect of the hemispheres in the B . arachnoides brain—the lateral sulcus that courses across
from the inferior to the superior border of the brain hemispheres and, after two
curvatures, terminates in a bifurcation into the posterior region of the
parietal lobe ( ). Two sulci are observed in an inclined temporal lobe—the superior and inferior
temporal sulci. The superior temporal sulcus is deep and runs parallel to the
lateral sulcus across the temporal lobe, thus delimiting the superior and middle
temporal gyri. A small, straight, and shallow inferior temporal sulcus scarcely
separates the middle from the inferior temporal lobe ( ). On the convex aspect, the lunatus sulcus, which separates the parietal and
occipital lobes, presents differences between the right and left hemispheres;
for instance, the lunatus sulcus joins the lateral sulcus in the right
hemisphere not in the left hemisphere ( ). On the convex aspect, an
inferior occipital sulcus appears in the middle part of the inferior occipital
pole in both hemispheres; however, a superior occipital sulcus is prominent in
the left occipital lobe but shallow and insignificant in the right occipital
lobe ( ). A straight sulcus observed on the medial aspect of the frontal lobe is the
rostral sulcus ( ). The
cingulate sulcus contours the cingulate gyrus from its anterior portion;
differing in its posterior portion between the two hemispheres ( ). The parieto-occipital
sulcus clearly separates the occipital and parietal lobes on the medial aspect
( ). Contouring the corpus callosum is the arcuated sulcus of the corpus callosum; it
ends at the junction between the hippocampal and calcarine sulci ( ). The hippocampal
sulcus is arcuated in the cranial direction and ends in the uncus medially and
inferiorly in the temporal pole ( ). From the occipital lobe, the calcarine sulcus emerges in a
horizontal trajectory and terminates generating the superior and inferior rami
prominently visible in the left hemisphere ( ). Visible on the inferior surface of the occipital and temporal lobes is the
collateral sulcus, including the inferior part of the hippocampal gyrus. The
rhinal sulcus starts after the collateral sulcus in the anterior inferior part
of the temporal lobe ( ). shows structures such
as the corpus callosum, diencephalon, and encephalic trunk. All these structures
are anatomically similar to the corresponding structures in other primates;
however, the angle of the encephalic trunk to a vertical axis through the
telencephalon is greater than 90°. Tables – show the absolute
measurements of the sulci, gyri, encephalic trunk, and other structures. 

Data regarding the presence or absence of the main sulci in the medial region of
the brain in the primate species investigated in the present study and
previously published data were concatenated ( ). The absolute measurements of the brain structures of the primate species studied
in the present study are shown in part of the and Tables – . These measurements indicate the difference
in the size of different structures, distance of sulci and gyri to the poles,
and dimensions of the encephalon. These measurements indicate provide an
approximatione of view about the size [(volume)] of some lobes. A table and graph with straight versus curve dimensions of main gyrus were made
to indicate the sinuosity degree among the primates studied this work ( ). For some primates
was not possible to obtain all data. presents the
relative values to facilitate a better comparison among the encephalon
structures. These data permit to verify approximately the real relation of
dimensions of the cerebral structures. The EQ values were calculated for the primate species included in the present
study and for data obtained from previous studies ( ). 

Comparative morphology The morphological data were compared with the anatomical descriptions of the
brains of Galago senegalensis senegalensis (Strepsirrhini) and Callithrix jacchus (Platyrrhini); both exhibit lower
complexity of brain structure than the primate species investigated in the
present study— Alouatta seniculus (howler monkey) and Ateles geoffroyi (spider monkey), which are New World
primates; Macaca fascicularis , Macaca mulatta (rhesus monkey), and Papio cynocephalus (baboons), which are
Old World primates; Pan troglodytes (chimpanzees), which use
tools; and humans ( Homo sapiens ). The central sulcus is absent in Galago senegalensis senegalensis  and in C . penicillata ( ). In C . jacchus  and Saimiri ustus ( ), this sulcus is little developed. In Sapajus libidinosus , the sulcus shows higher length and
sinuosity, little inclination, and location at the longitudinal midpoint of the
telencephalon . In
the genera Macaca , Papio , Pan , and Homo , the central sulcus does not
communicate with the lateral sulcus and has two curvatures, which are more
inclined in Pan and Homo [ , , ]. The frontal lobe of Galago , Callithrix , and Saimiri is characterized by the absence of sulci (Figs and ). In Sapajus , there are vertical and superior longitudinal sulci
, which are
respectively similar to the superior and inferior precentral sulci of Alouatta  and Brachyteles ( ). In Ateles , the
precentral sulcus is long, and arched and has a horizontal branch; it terminates
adjacent to the central sulcus . In Macaca , although there is divergence in the
nomenclature—superior and inferior precentral sulci  or superior and inferior arches [ – ]—there is consensus in the description
of the trajectory. The superior precentral sulcus is short and shallow, whereas
the inferior precentral sulcus is arched and C-shaped, with the concavity
directed cranially, and has a horizontal branch similar to that found in Ateles [ , – ]. In Papio and Pan , these sulci are similar to
those observed in Macaca  with a difference in nomenclature
 have called
these as the superior and inferior frontal sulci. In Homo
sapiens , the precentral sulcus is segmented and delimits the
precentral gyrus, which is related to motricity . A longitudinal sulcus on the inferior surface of the frontal lobe is similar in
the primate species studied. In Sapajus , it is named inferior
longitudinal sulcus 
in Ateles , Alouatta , Pan , and Papio , it is called sulcus rectus ; in Brachyteles , it is
known as inferior frontal sulcus; in Macaca , it is called
frontal sulcus . In Homo sapiens , superior and inferior frontal sulci are
present, which increase the cortical surface area [ – ]. The postcentral sulcus in Sapajus  is delimited on the posterior portion
of the postcentral gyrus and attached posteriorly to the lunatus sulcus, but in Brachyteles ( ) and Macaca , this sulcus is short and does
not join any other sulcus [ , , ]. In Alouatta , Ateles , Papio ,
and Pan , it divides into superior and inferior postcentral
sulci . In Homo sapiens , it is parallel to the central sulcus and
joins the intraparietal sulcus, similar to observations in Pan [ , , ]. The convex aspect of the Galago and Callithrix telencephalon ( ) is
almost lissencephalic, except for the lateral sulcus, which begins at the height
of the olfactory stria; delimited by the temporal lobe, it ascends in the
posterior direction . In contrast, in Saimiri , the lateral sulcus ( ) has a cranial curvature, and it
terminates at the height of the longitudinal fissure. In Sapajus and Brachyteles ( ), this sulcus terminates
in the parietal lobe, such that there are one and two curvatures, respectively
. In Alouatta and Ateles , the lateral sulcus is
continuous with the intraparietal sulcus; in the former, it ends in a
bifurcation, whereas in the latter, it terminates close to the lunatus sulcus
. In Macaca , Papio , and Pan ,
the lateral sulcus courses to the parietal lobe with variations in the degree of
sinuosity, which increases in these primate species, respectively [ , , , ]. Homo sapiens shows
the most highly developed lateral sulcus, which is very deep and has three
branches: ascending, anterior, and posterior  report that the simplest configuration
of the lateral sulcus in primates exposes the insular lobe, in contrast to the
observations in humans, in which this sulcus is deeper and more developed . The superior temporal sulcus is absent in Galago . In Callithrix ( ) and Saimiri ( ), only the superior temporal sulcus is
present on the temporal lobe. In Sapajus , Ateles , Alouatta , Brachyteles ( ), Papio , Pan , and Homo sapiens, there are superior and inferior temporal
sulci present. The superior temporal sulcus shows a more variable pattern across
species. In Sapajus , this sulcus joins the lateral sulcus
; in Alouatta and Brachyteles ( ), it does not join other
sulci. In Macaca and Papio , the superior
temporal sulcus terminates in a bifurcation posterior and superior to the
lateral sulcus [ , , ]. Furthermore, in Macaca , there is a middle temporal sulcus, which is
discontinuous and inferiorly delimits the superior temporal gyrus . In Pan , the posterior part of the superior temporal sulcus
joins the lunatus sulcus . In Homo sapiens , the superior temporal sulcus
runs parallel to the lateral sulcus and they rarely join . The inferior temporal sulcus in Pan and Homo sapiens is discontinuous
owing to several interruptions . The lunatus sulcus is absent in Galago and Callithrix ( ), whereas in Saimiri ( ), it is rudimentary, which makes it
difficult to identify the parietal and occipital lobes on the convex aspect. In Sapajus , this sulcus is continuous with the postcentral
sulcus ; in contrast,
in Alouatta , the lunatus sulcus is located inferiorly to the
lateral sulcus, does not characterize a specific lobe, and does not join any
other sulcus . In Ateles , it varies in shape (a straight path or arch) and
joins the intraparietal sulcus . In Brachyteles ( ), the lunatus sulcus joins the lateral
sulcus. In Macaca , it is continuous with the parieto-occipital
sulcus and delimited by the angular gyrus in combination with the superior
temporal sulcus . In Papio and Pan , the lunatus sulcus joins
the intraparietal sulcus  and is located posteriorly, thus increasing the extension of the
parietal lobe . In Homo sapiens , this sulcus is absent in most cases; when
present, it is segmented and displaced posteriorly and ventrally [ , , , ]. According to Armstrong et al. , the difference in the
position of the lunatus sulcus in primates may be associated with an increase in
the associative cortex in the caudal part of the parietal lobe, whereas
according to Holloway , this expansion contributed to the advancement of communication, use
and creation of tools, social complexity, and long-term memory. In addition, it
is observed that in humans, the primary visual cortex (Brodman’s area 17) and
the peri-striatum (Brodman’s area 18) were pushed posteriorly, which displaced
the lunatus sulcus to a more caudal position than that observed in Pan . The organization of the parietal cortex is important as an area that shows large
differences among primates, including humans. The parietal sulcus, as a
hand–vision coordination area, may have its development intrinsically related to
tool use behavior ,
as well as complex social behavior that may require a, possibly rudimentary,
theory of mind . The
cortical area in the lateral sulcus also varies greatly among primates , with functions ranging
from basic somatosensory to complex associative functions , which may explain mirror
self-recognition being observed in Sapajus and Brachyteles —an ability thought to be limited to humans and
apes until recently—but not in other New World primates . In Galago , Callithrix ( ), and Saimiri ( ), the occipital lobe is
lissencephalic; in contrast, Sapajus exhibits the inferior
occipital sulcus ( ). In Alouatta , Ateles , Brachyteles ( ), Macaca , and Papio , there
are superior and inferior occipital sulci. The inferior occipital sulcus is
larger and deeper than the superior occipital sulcus and the latter is variable
among species [ , , ]. Moreover, in Papio ,
the lateral calcarine sulcus is present. In Pan , the inferior
occipital sulcus is variable in its trajectory; it may join the middle temporal
sulcus or occipitotemporal sulcus or not join any other sulcus . In Homo
sapiens , the path of the sulci and gyri on the convex surface of
the occipital lobe varies more than that in the other lobes . In general, there are
two occipital sulci that subdivide the occipital lobe into three gyri: superior,
middle, and inferior [ – ]. The medial region shows the presence of the cingulate sulcus. In Galago , it is short, whereas in C . penicillata ( ) and C . jacchus , it is
absent . In Saimiri ( ), its distal portion ascends slightly; however, in Sapajus ( ), Alouatta , Ateles , Brachyteles ( ), Macaca , Papio , and Pan , this sulcus ascends and forms the marginal branch
[ , , , , , , ]. In Homo sapiens , this
sulcus is curved and has an approximate shape of a horizontal “s” . In the posterior
region, it divides into the paracentral and marginal branches, which are
ascending, and the subparietal sulcus, which continues caudally . Paus  and Tamraz and Comair  have described
anatomical variations in this sulcus in humans, such as duplication,
interruptions in its trajectory, and formation of new branches in the
telencephalon. The rostral sulcus is absent in Galago , Callithrix ( ) ; and Saimiri ( ) and has a similar
trajectory in Sapajus ( ), Brachyteles ( ), Alouatta , Ateles , Macaca , Papio , and Pan [ , , , ]. In Homo sapiens , this
sulcus separates the frontal gyrus medially into superior and inferior portions
. In some
telencephalons, this sulcus may be joined to the cingulate sulcus, according to
Paus  or appear
duplicated, with the presence of an accessory rostral sulcus . The sulcus of the corpus callosum is present in and has a similar trajectory in Callithrix , Saimiri , Sapajus , Brachyteles (Figs – respectively), and Macaca .
However, in Macaca , the caudal portion of this sulcus joins the
calcarine and hippocampus sulci . In Homo sapiens , the sulcus of the corpus
callosum and hippocampal sulcus are continuous, and they are separate from the
calcarine sulcus . This sulcus has
not been described for the other primate species considered in the present
study. In Galago , the calcarine sulcus consists of three small and
continuous sulci: precalcarine, retrocalcarine, and paracalcarine. The first is
near the hippocampal sulcus on the temporal lobe; the second is continuous with
the cingulate sulcus; and the third goes to the occipital pole . In Callithrix ( ), this sulcus goes to
the occipital pole without dividing into the superior and inferior branches
. In Saimiri , Sapajus , and Brachyteles (Figs – , respectively) Macaca , Papio , and Pan , the calcarine sulcus
divides into the superior and inferior sulci on the occipital lobe [ , , ]. Connolly  described the calcarine sulcus in Alouatta and Ateles as two sulci, the
first is similar to that in Sapajus , and the second is the
paracalcarine sulcus, which resembles a hook and originates in the occipital
lobe before the parieto-occipital sulcus. In Homo sapiens , the
calcarine sulcus bifurcates into the anterior and posterior calcarine sulci
. In Galago and Callithrix ( ), the parieto-occipital and subparietal
sulci are absent . In Saimiri and Sapajus (Figs and , respectively), the parieto-occipital
sulcus does not join the calcarine sulcus, and the subparietal sulcus appears
before the parieto-occipital sulcus or is continuous with it. In Alouatta , Ateles , and Brachyteles ( ), the parieto-occipital sulcus is present but the subparietal
sulcus is absent . Moreover, in Ateles , there is an accessory parieto-occipital sulcus
. In Saimiri and Sapajus , the subparietal
sulcus is continuous with the lunatus sulcus on the convex surface of the
cerebral hemisphere, and it is restricted on the parietal lobe; this criterion
was used to differentiate it from the paracalcarine sulcus that is present in
other primates . In
the genera Macaca , Papio , and Pan , the parieto-occipital and calcarine sulci do not join,
and the subparietal sulcus is variable in its trajectory; in most cases, it is
posteriorly connected to the parieto-occipital sulcus. Other possibilities are
the absence of this sulcus or connections with other sulci [ , , ]. In Homo sapiens , the
parieto-occipital sulcus is located only on the medial aspect and terminates
forming a right angle with the calcarine sulcus . Although the parieto-occipital sulcus
and calcarine sulcus appear continuous, they are separated by one or more small
gyri . The
subparietal sulcus is derived from the cingulate sulcus and is not continuous
with the parieto-occipital sulcus . In Galago , the hippocampal sulcus courses in the
posterior-to-anterior direction and contours the hippocampal tubercle on the
temporal lobe . In Callithrix , Saimiri , Sapajus (Figs – respectively), Alouatta , Ateles , Brachyteles ( ), Macaca , Papio , and Pan , the hippocampal sulcus begins near the splenium of the
corpus callosum and joins caudally to the sulcus of the corpus callosum and
calcarine sulcus. Then, it continues to the temporal pole, where it terminates
separating the para-hippocampal gyrus from the uncus [ , , , , ]. In Homo sapiens , the
trajectory of the hippocampal sulcus is similar to that in other primates, with
the difference that the hippocampal sulcus is continuous only with the sulcus of
the corpus callosum . The collateral sulcus is absent in Galago and Callithrix ( ) .
Connolly  reports
that this sulcus has a very primitive organization in Nycticebus (family Lorisidae). In Saimiri ( ), Ateles , and Brachyteles ( ), the collateral and
occipitotemporal sulci are continuous with each other, whereas in Saimiri , the collateral sulcus is connected to the
calcarine sulcus and ends its trajectory close to the temporal pole. In Sapajus ( ), Alouatta , Macaca , Papio , and Pan , this sulcus is well
developed and exhibits two parts: the posterior part is joined to the calcarine
sulcus is the collateral sulcus, and the anterior portion is joined to the
occipitotemporal sulcus [ , , , , ]. In Homo sapiens , the
collateral and occipitotemporal sulci are well developed [ , , ]. In Galago , Callithrix , and Saimiri (Figs and ), the
rhinal sulcus is similar to shallow vascular depressions and separates the
piriform lobe from the rest of the temporal lobe [ , , ]. In Sapajus ( ), Brachyteles ( ), Alouatta , Ateles , Macaca , Papio , Pan , and Homo sapiens , this sulcus is evident and marks the boundary
between the paleocortex and the neocortex in the temporal lobe . In Homo
sapiens , this sulcus is often continuous with the collateral
sulcus, and this condition was also observed in four Sapajus brain specimens. It is suggested that this sulcus appeared in mammals after the
ventral displacement of the pririform cortex due to the development of the
neocortex . Allometric approach The initial studies about the sulci and gyri of the cortex did not indicate any
shared patterns among primates and humans . After Gratiolet Pearce  , the main
sulci and gyri were defined for mammalian studies from the simplest to more
complex (i.e., in humans). The concepts of lissencephaly and gyrencephaly were refined and could be
explained by the requirement of larger cortical areas (i.e., the circumvolutions
of the brain) in relation to the volume of the cranial box (this approach is not
universally accepted; for more detailed discussion, see . Indeed, this relationship could be
explained by a decrease of the encephalon by a power of 2/3 . That means, an object, when increased
shows a non-linear relationship between area and volume; the volume increases by
the third power, whereas the area increases by the second power . Accordingly, the energy expended for a
biological structure to become larger and change its shape seems to be higher
than the energy to increase its area without affecting the volume, according to
previous evolutionary studies . In this case, to save energy, an increase in the area of the
brain, a soft tissue, is favored rather than an increase in bone mass or total
volume of the cranium. Indeed, it seems easier to fold a soft brain tissue than
to increase a cranial box with repercussions on the weight and animal movement
and equilibrium, to say the least. However, this pattern is not observed in all
mammals; in some mammalian species, the brain does not exhibit most gyri found
in apes and humans. Another aspect considered in the present study in relation to comparative brain
analysis is EQ, an allometric measure associated with an increase in brain size,
a subject of interest in many fields of neuroscience . Indeed, brain size varies among
animals, with larger and smaller animals having larger and smaller brains,
respectively. The EQ is not a unanimously accepted concept . There are
discrepancies on the evolutionary scale in EQ values calculated according to the
following formula that takes into consideration certain metabolic parameters
: E Q = brain weight 0.12 × ( body weight ) 0.66 For instance, the EQ of dolphins (5.3) is larger than that of
chimpanzee (2.2–2.5) 
( ).
Accordingly, some authors do not consider EQ to be an ideal measure to estimate
brain complexity , but EQ seems to
be precise for analysis of a close group, such as primates . In general, apes and
other primates show a higher EQ than other animals . In fact, the differences in the brains are smaller for the phylogenetically
closer primates and larger for the more phylogenetically distant ones  however, EQ values calculated for
prosimians using metabolic parameters are not different than those for monkeys
 still presenting
a statistically significant relationship between body weight and brain weight
 In the present study, EQ values from some non-human primates to humans were
plotted against cerebral mass; a complete linear regression of EQ was not
observed, but a regression of degree 3 was observed. However, between Callithrix and Gorilla , a linear
regression was observed ( ). For neotropical primates, there are discrepancies within the group, both for EQ
and behavioral aspects, mainly for Sapajus . Indeed, many
studies on Sapajus behavior suggest that they present high
cognition comparable to that in chimpanzees [ , – ] and unexpectedly complex behavior for a
New World primate, but a possible explanation may be provided on the basis of EQ
(2.54 and 4.79 for Sapajus and Cebus . These values of EQ for Sapajus have been discussed in detail by many authors on
the basis of metabolism, suggesting that Saimiri and Sapajus present high EQ because of their high metabolic
needs, whereas Callithrix and Brachyteles show
lower metabolism . In
fact, the EQ values of the latter two are similar, and the EQ values of Saimiri and Sapajus are also similar
( ). Excluding
the cited discrepancies owing to the metabolism rate, the EQ seems to increase
proportionally with the phylogenetic scale but with discrepancies in the group
of New World primates because the calculated EQ for Saimiri and Sapajus is larger than that for Macaca . However, while the EQ of Sapajus may be justified by their high
cognitive behavior, the same cannot be said for Saimiri .
Considering the evolutionary scale of cognition, among the tropical primates,
the EQ of Saimiri is inconsistent. In general, EQ seems to be
an indicator for comparisons of cognition among primates, but the discrepancies
must be considered. Calculating the EQ for other primates, mainly the
less-studied New World primates  may indicate whether more or less differences would be found.
The EQ for tropical primates is expected to be approximately 1.7, on the basis
of the results obtained for Callithrix and Brachyteles ( ). Another factor to evaluate are cerebral gyri and sulci. For humans and apes, the
bilateral differences in some gyri and sulci indicate asymmetry of the cerebral
hemispheres [ – ], an important
evolutionary aspect associated with cognitive aspects of primates. In general,
for some primates and other mammals, the asymmetry seems inconclusive . Phylogenetically, the first sulcus to appear in the brain was the hippocampal
sulcus, which delimits the archicortex, and the second was the rhinal sulcus,
which separates the paleocortex from the neocortex . Remarkably, these two sulci are also
present in the almost lysencephalic primates such as Galago and Callithrix ( )  and persist in
more complex brains such as those of apes  and humans. The highly conserved sulci
in primate brains are the lateral, cingulate, calcarine, hippocampal, and rhinal
sulci and longitudinal fissure  in Callithrix , the cingulate sulcus was absent
( ), but it is
not appropriate to consider this primate as completely lissencephalic. In general, differences in primate brains in comparison with other mammals are
the increase in new parts of the encephalon, neocortex, and neencephalon and: 1)
enhanced visual capabilities with concomitant complexity of the occipital cortex
and optical pathways, generating a stereoscopic vision, 2) enhanced tactile
sensibility in the limb extremities (hand/feet or pads), and 3) comparative
diminution of olfactory capabilities . Nevertheless, considering the difficulty in evaluating the cerebral differences
with respect to the gross neuroanatomy, some measures could generate data to
elucidate the differences among the primates’ brains, such as allometry. In general, the permanent sulci and other variations present on both medial and
lateral aspects in human brain may be observed because of the increased
complexity due to the large cortical area; they are not so prominent in primates
presenting a certain degree of gyrencephaly, such as the tropical primates
investigated in the present study. Among the primates analyzed in this study,
only the genera Galago  and Callithrix have no
central sulcus. Indeed, their brains could be considered as lissencephalic to a
certain extent, mainly on the convex aspect; however, on the medial aspect, at
least the calcarine, rhinal, and hippocampal sulci are evident for all
primates. In gross anatomy, the degree of gyrencephaly is cited as a visual criterion of
the convex aspect of the brain; however, it was purposed in this study.
Moreover, a proportional measure between the straight and sinuous measurements
for the main sulci for Sapajus  and for humans (Féré cited by  was used. The ratio of
the straight by sinuous measurement, the degree of sinuosity, indicates the
relative value for gyrencephaly for each sulcus and facilitates interspecies
comparison; for sulci of the convex aspect, the ratio indicates more folding of
the cortex. The absolute size of the sulci cannot be considered for interspecies
analysis; however, the relative values permit a comparative analysis. The
relative data [straight measurement/sinuous measurement] offers a direct and
simple approach to estimate the relative cortical area to verify the hypothesis
that the more gyrencephalic the brain is, the more gyrencephalic are the sulci,
and to perform a comparative analysis with other genera. According to our analysis, there was a high correlation between these data and
visual observations for the level of gyrencephaly (Tables , , , and and ). The degree of sinuosity, which
represents the relative size of the sulci, shows lower values for more sinuous
gyri. Accordingly, among the New World primates studied in this work ( ), the genus Saimiri exhibits a lower degree of gyrencephaly than Sapajus and Brachyteles on the basis of
the sulci on the convex aspect, as can be observed in Figs – for Saimiri , Sapajus and Brachyteles , respectively. For
humans, the highest degree of gyrencephaly was observed for the central sulcus
( ) as reported
by Féré . In Callithrix , the absence of most sulci indicates a high
level of lissencephaly ( and Tables , , and ); in fact, on the convex
aspect, only the lateral and temporal sulci were observed ( ). For the gyrencephaly analysis, we measured the brain structures and calculated
the degree of sinuosity ( ) and also performed a mathematical analysis ( ), which is more objective and permits
a more accurate evaluation of interspecies differences. For posterior studies,
the degree of sinuosity of the sulci could be calculated for a better
interspecies comparison of allometric aspects. The distance from the superior and inferior edges of the central sulcus to both
the occipital and frontal poles, taken separately, is an indirect indicator for
the size of the anterior and posterior brain parts. These measurements were
first performed by Féré, Passet  and Giacomini  for humans but have been scarcely used in neuroanatomical
studies since. In the present study, the distance from the edges of the central
sulcus to the cerebral poles was performed for dissected specimens of
neotropical primates that presented a complete central sulcus. In this respect
and other aspects evaluated in relation to gyrencephaly, Sapajus brain is closer to other New World primates. 

The morphological data were compared with the anatomical descriptions of the
brains of Galago senegalensis senegalensis (Strepsirrhini) and Callithrix jacchus (Platyrrhini); both exhibit lower
complexity of brain structure than the primate species investigated in the
present study— Alouatta seniculus (howler monkey) and Ateles geoffroyi (spider monkey), which are New World
primates; Macaca fascicularis , Macaca mulatta (rhesus monkey), and Papio cynocephalus (baboons), which are
Old World primates; Pan troglodytes (chimpanzees), which use
tools; and humans ( Homo sapiens ). The central sulcus is absent in Galago senegalensis senegalensis  and in C . penicillata ( ). In C . jacchus  and Saimiri ustus ( ), this sulcus is little developed. In Sapajus libidinosus , the sulcus shows higher length and
sinuosity, little inclination, and location at the longitudinal midpoint of the
telencephalon . In
the genera Macaca , Papio , Pan , and Homo , the central sulcus does not
communicate with the lateral sulcus and has two curvatures, which are more
inclined in Pan and Homo [ , , ]. The frontal lobe of Galago , Callithrix , and Saimiri is characterized by the absence of sulci (Figs and ). In Sapajus , there are vertical and superior longitudinal sulci
, which are
respectively similar to the superior and inferior precentral sulci of Alouatta  and Brachyteles ( ). In Ateles , the
precentral sulcus is long, and arched and has a horizontal branch; it terminates
adjacent to the central sulcus . In Macaca , although there is divergence in the
nomenclature—superior and inferior precentral sulci  or superior and inferior arches [ – ]—there is consensus in the description
of the trajectory. The superior precentral sulcus is short and shallow, whereas
the inferior precentral sulcus is arched and C-shaped, with the concavity
directed cranially, and has a horizontal branch similar to that found in Ateles [ , – ]. In Papio and Pan , these sulci are similar to
those observed in Macaca  with a difference in nomenclature
 have called
these as the superior and inferior frontal sulci. In Homo
sapiens , the precentral sulcus is segmented and delimits the
precentral gyrus, which is related to motricity . A longitudinal sulcus on the inferior surface of the frontal lobe is similar in
the primate species studied. In Sapajus , it is named inferior
longitudinal sulcus 
in Ateles , Alouatta , Pan , and Papio , it is called sulcus rectus ; in Brachyteles , it is
known as inferior frontal sulcus; in Macaca , it is called
frontal sulcus . In Homo sapiens , superior and inferior frontal sulci are
present, which increase the cortical surface area [ – ]. The postcentral sulcus in Sapajus  is delimited on the posterior portion
of the postcentral gyrus and attached posteriorly to the lunatus sulcus, but in Brachyteles ( ) and Macaca , this sulcus is short and does
not join any other sulcus [ , , ]. In Alouatta , Ateles , Papio ,
and Pan , it divides into superior and inferior postcentral
sulci . In Homo sapiens , it is parallel to the central sulcus and
joins the intraparietal sulcus, similar to observations in Pan [ , , ]. The convex aspect of the Galago and Callithrix telencephalon ( ) is
almost lissencephalic, except for the lateral sulcus, which begins at the height
of the olfactory stria; delimited by the temporal lobe, it ascends in the
posterior direction . In contrast, in Saimiri , the lateral sulcus ( ) has a cranial curvature, and it
terminates at the height of the longitudinal fissure. In Sapajus and Brachyteles ( ), this sulcus terminates
in the parietal lobe, such that there are one and two curvatures, respectively
. In Alouatta and Ateles , the lateral sulcus is
continuous with the intraparietal sulcus; in the former, it ends in a
bifurcation, whereas in the latter, it terminates close to the lunatus sulcus
. In Macaca , Papio , and Pan ,
the lateral sulcus courses to the parietal lobe with variations in the degree of
sinuosity, which increases in these primate species, respectively [ , , , ]. Homo sapiens shows
the most highly developed lateral sulcus, which is very deep and has three
branches: ascending, anterior, and posterior  report that the simplest configuration
of the lateral sulcus in primates exposes the insular lobe, in contrast to the
observations in humans, in which this sulcus is deeper and more developed . The superior temporal sulcus is absent in Galago . In Callithrix ( ) and Saimiri ( ), only the superior temporal sulcus is
present on the temporal lobe. In Sapajus , Ateles , Alouatta , Brachyteles ( ), Papio , Pan , and Homo sapiens, there are superior and inferior temporal
sulci present. The superior temporal sulcus shows a more variable pattern across
species. In Sapajus , this sulcus joins the lateral sulcus
; in Alouatta and Brachyteles ( ), it does not join other
sulci. In Macaca and Papio , the superior
temporal sulcus terminates in a bifurcation posterior and superior to the
lateral sulcus [ , , ]. Furthermore, in Macaca , there is a middle temporal sulcus, which is
discontinuous and inferiorly delimits the superior temporal gyrus . In Pan , the posterior part of the superior temporal sulcus
joins the lunatus sulcus . In Homo sapiens , the superior temporal sulcus
runs parallel to the lateral sulcus and they rarely join . The inferior temporal sulcus in Pan and Homo sapiens is discontinuous
owing to several interruptions . The lunatus sulcus is absent in Galago and Callithrix ( ), whereas in Saimiri ( ), it is rudimentary, which makes it
difficult to identify the parietal and occipital lobes on the convex aspect. In Sapajus , this sulcus is continuous with the postcentral
sulcus ; in contrast,
in Alouatta , the lunatus sulcus is located inferiorly to the
lateral sulcus, does not characterize a specific lobe, and does not join any
other sulcus . In Ateles , it varies in shape (a straight path or arch) and
joins the intraparietal sulcus . In Brachyteles ( ), the lunatus sulcus joins the lateral
sulcus. In Macaca , it is continuous with the parieto-occipital
sulcus and delimited by the angular gyrus in combination with the superior
temporal sulcus . In Papio and Pan , the lunatus sulcus joins
the intraparietal sulcus  and is located posteriorly, thus increasing the extension of the
parietal lobe . In Homo sapiens , this sulcus is absent in most cases; when
present, it is segmented and displaced posteriorly and ventrally [ , , , ]. According to Armstrong et al. , the difference in the
position of the lunatus sulcus in primates may be associated with an increase in
the associative cortex in the caudal part of the parietal lobe, whereas
according to Holloway , this expansion contributed to the advancement of communication, use
and creation of tools, social complexity, and long-term memory. In addition, it
is observed that in humans, the primary visual cortex (Brodman’s area 17) and
the peri-striatum (Brodman’s area 18) were pushed posteriorly, which displaced
the lunatus sulcus to a more caudal position than that observed in Pan . The organization of the parietal cortex is important as an area that shows large
differences among primates, including humans. The parietal sulcus, as a
hand–vision coordination area, may have its development intrinsically related to
tool use behavior ,
as well as complex social behavior that may require a, possibly rudimentary,
theory of mind . The
cortical area in the lateral sulcus also varies greatly among primates , with functions ranging
from basic somatosensory to complex associative functions , which may explain mirror
self-recognition being observed in Sapajus and Brachyteles —an ability thought to be limited to humans and
apes until recently—but not in other New World primates . In Galago , Callithrix ( ), and Saimiri ( ), the occipital lobe is
lissencephalic; in contrast, Sapajus exhibits the inferior
occipital sulcus ( ). In Alouatta , Ateles , Brachyteles ( ), Macaca , and Papio , there
are superior and inferior occipital sulci. The inferior occipital sulcus is
larger and deeper than the superior occipital sulcus and the latter is variable
among species [ , , ]. Moreover, in Papio ,
the lateral calcarine sulcus is present. In Pan , the inferior
occipital sulcus is variable in its trajectory; it may join the middle temporal
sulcus or occipitotemporal sulcus or not join any other sulcus . In Homo
sapiens , the path of the sulci and gyri on the convex surface of
the occipital lobe varies more than that in the other lobes . In general, there are
two occipital sulci that subdivide the occipital lobe into three gyri: superior,
middle, and inferior [ – ]. The medial region shows the presence of the cingulate sulcus. In Galago , it is short, whereas in C . penicillata ( ) and C . jacchus , it is
absent . In Saimiri ( ), its distal portion ascends slightly; however, in Sapajus ( ), Alouatta , Ateles , Brachyteles ( ), Macaca , Papio , and Pan , this sulcus ascends and forms the marginal branch
[ , , , , , , ]. In Homo sapiens , this
sulcus is curved and has an approximate shape of a horizontal “s” . In the posterior
region, it divides into the paracentral and marginal branches, which are
ascending, and the subparietal sulcus, which continues caudally . Paus  and Tamraz and Comair  have described
anatomical variations in this sulcus in humans, such as duplication,
interruptions in its trajectory, and formation of new branches in the
telencephalon. The rostral sulcus is absent in Galago , Callithrix ( ) ; and Saimiri ( ) and has a similar
trajectory in Sapajus ( ), Brachyteles ( ), Alouatta , Ateles , Macaca , Papio , and Pan [ , , , ]. In Homo sapiens , this
sulcus separates the frontal gyrus medially into superior and inferior portions
. In some
telencephalons, this sulcus may be joined to the cingulate sulcus, according to
Paus  or appear
duplicated, with the presence of an accessory rostral sulcus . The sulcus of the corpus callosum is present in and has a similar trajectory in Callithrix , Saimiri , Sapajus , Brachyteles (Figs – respectively), and Macaca .
However, in Macaca , the caudal portion of this sulcus joins the
calcarine and hippocampus sulci . In Homo sapiens , the sulcus of the corpus
callosum and hippocampal sulcus are continuous, and they are separate from the
calcarine sulcus . This sulcus has
not been described for the other primate species considered in the present
study. In Galago , the calcarine sulcus consists of three small and
continuous sulci: precalcarine, retrocalcarine, and paracalcarine. The first is
near the hippocampal sulcus on the temporal lobe; the second is continuous with
the cingulate sulcus; and the third goes to the occipital pole . In Callithrix ( ), this sulcus goes to
the occipital pole without dividing into the superior and inferior branches
. In Saimiri , Sapajus , and Brachyteles (Figs – , respectively) Macaca , Papio , and Pan , the calcarine sulcus
divides into the superior and inferior sulci on the occipital lobe [ , , ]. Connolly  described the calcarine sulcus in Alouatta and Ateles as two sulci, the
first is similar to that in Sapajus , and the second is the
paracalcarine sulcus, which resembles a hook and originates in the occipital
lobe before the parieto-occipital sulcus. In Homo sapiens , the
calcarine sulcus bifurcates into the anterior and posterior calcarine sulci
. In Galago and Callithrix ( ), the parieto-occipital and subparietal
sulci are absent . In Saimiri and Sapajus (Figs and , respectively), the parieto-occipital
sulcus does not join the calcarine sulcus, and the subparietal sulcus appears
before the parieto-occipital sulcus or is continuous with it. In Alouatta , Ateles , and Brachyteles ( ), the parieto-occipital sulcus is present but the subparietal
sulcus is absent . Moreover, in Ateles , there is an accessory parieto-occipital sulcus
. In Saimiri and Sapajus , the subparietal
sulcus is continuous with the lunatus sulcus on the convex surface of the
cerebral hemisphere, and it is restricted on the parietal lobe; this criterion
was used to differentiate it from the paracalcarine sulcus that is present in
other primates . In
the genera Macaca , Papio , and Pan , the parieto-occipital and calcarine sulci do not join,
and the subparietal sulcus is variable in its trajectory; in most cases, it is
posteriorly connected to the parieto-occipital sulcus. Other possibilities are
the absence of this sulcus or connections with other sulci [ , , ]. In Homo sapiens , the
parieto-occipital sulcus is located only on the medial aspect and terminates
forming a right angle with the calcarine sulcus . Although the parieto-occipital sulcus
and calcarine sulcus appear continuous, they are separated by one or more small
gyri . The
subparietal sulcus is derived from the cingulate sulcus and is not continuous
with the parieto-occipital sulcus . In Galago , the hippocampal sulcus courses in the
posterior-to-anterior direction and contours the hippocampal tubercle on the
temporal lobe . In Callithrix , Saimiri , Sapajus (Figs – respectively), Alouatta , Ateles , Brachyteles ( ), Macaca , Papio , and Pan , the hippocampal sulcus begins near the splenium of the
corpus callosum and joins caudally to the sulcus of the corpus callosum and
calcarine sulcus. Then, it continues to the temporal pole, where it terminates
separating the para-hippocampal gyrus from the uncus [ , , , , ]. In Homo sapiens , the
trajectory of the hippocampal sulcus is similar to that in other primates, with
the difference that the hippocampal sulcus is continuous only with the sulcus of
the corpus callosum . The collateral sulcus is absent in Galago and Callithrix ( ) .
Connolly  reports
that this sulcus has a very primitive organization in Nycticebus (family Lorisidae). In Saimiri ( ), Ateles , and Brachyteles ( ), the collateral and
occipitotemporal sulci are continuous with each other, whereas in Saimiri , the collateral sulcus is connected to the
calcarine sulcus and ends its trajectory close to the temporal pole. In Sapajus ( ), Alouatta , Macaca , Papio , and Pan , this sulcus is well
developed and exhibits two parts: the posterior part is joined to the calcarine
sulcus is the collateral sulcus, and the anterior portion is joined to the
occipitotemporal sulcus [ , , , , ]. In Homo sapiens , the
collateral and occipitotemporal sulci are well developed [ , , ]. In Galago , Callithrix , and Saimiri (Figs and ), the
rhinal sulcus is similar to shallow vascular depressions and separates the
piriform lobe from the rest of the temporal lobe [ , , ]. In Sapajus ( ), Brachyteles ( ), Alouatta , Ateles , Macaca , Papio , Pan , and Homo sapiens , this sulcus is evident and marks the boundary
between the paleocortex and the neocortex in the temporal lobe . In Homo
sapiens , this sulcus is often continuous with the collateral
sulcus, and this condition was also observed in four Sapajus brain specimens. It is suggested that this sulcus appeared in mammals after the
ventral displacement of the pririform cortex due to the development of the
neocortex . 

The initial studies about the sulci and gyri of the cortex did not indicate any
shared patterns among primates and humans . After Gratiolet Pearce  , the main
sulci and gyri were defined for mammalian studies from the simplest to more
complex (i.e., in humans). The concepts of lissencephaly and gyrencephaly were refined and could be
explained by the requirement of larger cortical areas (i.e., the circumvolutions
of the brain) in relation to the volume of the cranial box (this approach is not
universally accepted; for more detailed discussion, see . Indeed, this relationship could be
explained by a decrease of the encephalon by a power of 2/3 . That means, an object, when increased
shows a non-linear relationship between area and volume; the volume increases by
the third power, whereas the area increases by the second power . Accordingly, the energy expended for a
biological structure to become larger and change its shape seems to be higher
than the energy to increase its area without affecting the volume, according to
previous evolutionary studies . In this case, to save energy, an increase in the area of the
brain, a soft tissue, is favored rather than an increase in bone mass or total
volume of the cranium. Indeed, it seems easier to fold a soft brain tissue than
to increase a cranial box with repercussions on the weight and animal movement
and equilibrium, to say the least. However, this pattern is not observed in all
mammals; in some mammalian species, the brain does not exhibit most gyri found
in apes and humans. Another aspect considered in the present study in relation to comparative brain
analysis is EQ, an allometric measure associated with an increase in brain size,
a subject of interest in many fields of neuroscience . Indeed, brain size varies among
animals, with larger and smaller animals having larger and smaller brains,
respectively. The EQ is not a unanimously accepted concept . There are
discrepancies on the evolutionary scale in EQ values calculated according to the
following formula that takes into consideration certain metabolic parameters
: E Q = brain weight 0.12 × ( body weight ) 0.66 For instance, the EQ of dolphins (5.3) is larger than that of
chimpanzee (2.2–2.5) 
( ).
Accordingly, some authors do not consider EQ to be an ideal measure to estimate
brain complexity , but EQ seems to
be precise for analysis of a close group, such as primates . In general, apes and
other primates show a higher EQ than other animals . In fact, the differences in the brains are smaller for the phylogenetically
closer primates and larger for the more phylogenetically distant ones  however, EQ values calculated for
prosimians using metabolic parameters are not different than those for monkeys
 still presenting
a statistically significant relationship between body weight and brain weight
 In the present study, EQ values from some non-human primates to humans were
plotted against cerebral mass; a complete linear regression of EQ was not
observed, but a regression of degree 3 was observed. However, between Callithrix and Gorilla , a linear
regression was observed ( ). For neotropical primates, there are discrepancies within the group, both for EQ
and behavioral aspects, mainly for Sapajus . Indeed, many
studies on Sapajus behavior suggest that they present high
cognition comparable to that in chimpanzees [ , – ] and unexpectedly complex behavior for a
New World primate, but a possible explanation may be provided on the basis of EQ
(2.54 and 4.79 for Sapajus and Cebus . These values of EQ for Sapajus have been discussed in detail by many authors on
the basis of metabolism, suggesting that Saimiri and Sapajus present high EQ because of their high metabolic
needs, whereas Callithrix and Brachyteles show
lower metabolism . In
fact, the EQ values of the latter two are similar, and the EQ values of Saimiri and Sapajus are also similar
( ). Excluding
the cited discrepancies owing to the metabolism rate, the EQ seems to increase
proportionally with the phylogenetic scale but with discrepancies in the group
of New World primates because the calculated EQ for Saimiri and Sapajus is larger than that for Macaca . However, while the EQ of Sapajus may be justified by their high
cognitive behavior, the same cannot be said for Saimiri .
Considering the evolutionary scale of cognition, among the tropical primates,
the EQ of Saimiri is inconsistent. In general, EQ seems to be
an indicator for comparisons of cognition among primates, but the discrepancies
must be considered. Calculating the EQ for other primates, mainly the
less-studied New World primates  may indicate whether more or less differences would be found.
The EQ for tropical primates is expected to be approximately 1.7, on the basis
of the results obtained for Callithrix and Brachyteles ( ). Another factor to evaluate are cerebral gyri and sulci. For humans and apes, the
bilateral differences in some gyri and sulci indicate asymmetry of the cerebral
hemispheres [ – ], an important
evolutionary aspect associated with cognitive aspects of primates. In general,
for some primates and other mammals, the asymmetry seems inconclusive . Phylogenetically, the first sulcus to appear in the brain was the hippocampal
sulcus, which delimits the archicortex, and the second was the rhinal sulcus,
which separates the paleocortex from the neocortex . Remarkably, these two sulci are also
present in the almost lysencephalic primates such as Galago and Callithrix ( )  and persist in
more complex brains such as those of apes  and humans. The highly conserved sulci
in primate brains are the lateral, cingulate, calcarine, hippocampal, and rhinal
sulci and longitudinal fissure  in Callithrix , the cingulate sulcus was absent
( ), but it is
not appropriate to consider this primate as completely lissencephalic. In general, differences in primate brains in comparison with other mammals are
the increase in new parts of the encephalon, neocortex, and neencephalon and: 1)
enhanced visual capabilities with concomitant complexity of the occipital cortex
and optical pathways, generating a stereoscopic vision, 2) enhanced tactile
sensibility in the limb extremities (hand/feet or pads), and 3) comparative
diminution of olfactory capabilities . Nevertheless, considering the difficulty in evaluating the cerebral differences
with respect to the gross neuroanatomy, some measures could generate data to
elucidate the differences among the primates’ brains, such as allometry. In general, the permanent sulci and other variations present on both medial and
lateral aspects in human brain may be observed because of the increased
complexity due to the large cortical area; they are not so prominent in primates
presenting a certain degree of gyrencephaly, such as the tropical primates
investigated in the present study. Among the primates analyzed in this study,
only the genera Galago  and Callithrix have no
central sulcus. Indeed, their brains could be considered as lissencephalic to a
certain extent, mainly on the convex aspect; however, on the medial aspect, at
least the calcarine, rhinal, and hippocampal sulci are evident for all
primates. In gross anatomy, the degree of gyrencephaly is cited as a visual criterion of
the convex aspect of the brain; however, it was purposed in this study.
Moreover, a proportional measure between the straight and sinuous measurements
for the main sulci for Sapajus  and for humans (Féré cited by  was used. The ratio of
the straight by sinuous measurement, the degree of sinuosity, indicates the
relative value for gyrencephaly for each sulcus and facilitates interspecies
comparison; for sulci of the convex aspect, the ratio indicates more folding of
the cortex. The absolute size of the sulci cannot be considered for interspecies
analysis; however, the relative values permit a comparative analysis. The
relative data [straight measurement/sinuous measurement] offers a direct and
simple approach to estimate the relative cortical area to verify the hypothesis
that the more gyrencephalic the brain is, the more gyrencephalic are the sulci,
and to perform a comparative analysis with other genera. According to our analysis, there was a high correlation between these data and
visual observations for the level of gyrencephaly (Tables , , , and and ). The degree of sinuosity, which
represents the relative size of the sulci, shows lower values for more sinuous
gyri. Accordingly, among the New World primates studied in this work ( ), the genus Saimiri exhibits a lower degree of gyrencephaly than Sapajus and Brachyteles on the basis of
the sulci on the convex aspect, as can be observed in Figs – for Saimiri , Sapajus and Brachyteles , respectively. For
humans, the highest degree of gyrencephaly was observed for the central sulcus
( ) as reported
by Féré . In Callithrix , the absence of most sulci indicates a high
level of lissencephaly ( and Tables , , and ); in fact, on the convex
aspect, only the lateral and temporal sulci were observed ( ). For the gyrencephaly analysis, we measured the brain structures and calculated
the degree of sinuosity ( ) and also performed a mathematical analysis ( ), which is more objective and permits
a more accurate evaluation of interspecies differences. For posterior studies,
the degree of sinuosity of the sulci could be calculated for a better
interspecies comparison of allometric aspects. The distance from the superior and inferior edges of the central sulcus to both
the occipital and frontal poles, taken separately, is an indirect indicator for
the size of the anterior and posterior brain parts. These measurements were
first performed by Féré, Passet  and Giacomini  for humans but have been scarcely used in neuroanatomical
studies since. In the present study, the distance from the edges of the central
sulcus to the cerebral poles was performed for dissected specimens of
neotropical primates that presented a complete central sulcus. In this respect
and other aspects evaluated in relation to gyrencephaly, Sapajus brain is closer to other New World primates. 

Ethics statement This work was approved by the Institutional Ethical Committee of the Federal
University of Goiás (CoEP-UFG 81/2008, authorization from the IBAMA number 15275
for the proceedings detailed here) and of the Federal University of Tocantins
(CEUA-UFT 23101-003220/2013-85). The details of animal use and welfare were in
accordance with the recommendations of the Weatherall report, “The use of
non-human primates in research.” Samples Seven, two, one, and one adult cadaveric specimens, respectively, of Sapajus sp., C . penicillata , Saimiri ustus , and B . arachnoides were used in this
investigation ( ).
No animal was killed or euthanized for the purposes of this study: five Sapajus sp. individuals suffered accidental deaths in their
natural habitat, and these specimens were donated by the Brazilian Institute of
Environment and Renewable Natural Resources (IBAMA) to the collection of the
Laboratory of Anatomy of the Federal University of Tocantins-Palmas-Tocantins
(Brazil). The remaining Sapajus libidinosus specimens and the C . penicillata and Saimiri
ustus specimens belonged to the Primates Center of University of
Brasília and were donated to the University of Tocantins for research. The B . arachnoides specimen was studied at the
Primatology Center of Rio de Janeiro (Guapimirim, RJ, Brazil). All specimens,
except B . arachnoides , presently belong to the
Laboratory of Anatomy of the Federal University of Tocantins, and access to them
requires permission from its chairperson. Preparation of the animals for dissection All procedures involving the animals were performed in accordance with the
guidelines of the Brazilian Society of Animal Experimentation (COBEA). After
trichotomy with a scalpel blade, the specimens were incubated in water at room
temperature for 10–12 h. All specimens were perfused and fixed, by injecting 10%
formaldehyde and 5% glycerin through the femoral vein. The animals were
conserved in 10% formaldehyde in covered opaque containers to avoid penetration
of light and evaporation of the preservative. Dissection and documentation Brains were removed from the skull, weighed, and measured with a caliper and
documented with a digital camera (Cannon, model EOS Kiss X3 lens 18–55). The
nomenclature of gyri, sulci, and other structures was based on descriptions from
previous studies on humans and other primates, mainly capuchins, which are New
World primates . Only
the visible and reliably delimited structures were studied for each genus. The
measurement of the major gyri in a straight line using a caliper and along the
sinuosity using an inextensible string was performed to assess the degree of
curvature, and the results were compared with the data from humans. The degree of sinuosity was calculated using the following equation: sinuosity degree = straight measure sinuous measure The major encephalon measure was performed longitudinally and transversely. The
degree of gyrencephaly was assessed for the lobes and major sulci. The
encephalization quotient (EQ) was calculated using the following equation: E Q = brain weight 0.12 × ( body weight ) 0.66 The distance from the superior and inferior extremities of the central sulcus to
the frontal and occipital poles was measured, when possible, to indicate the
degree of inclination (ID) of this sulcus, and as an indirect measure of the
frontal lobe ( ),
according to the following relation . For brains of C . penicillata ( ), Saimiri
ustus ( ), Sapajus libidinosus ( ), and B . arachnoides ( ), straight (Tables and ) and sinuos measurements of the sulci were
performed (Tables – ). The ratio of the
straight and sinuous measurements indicates a value that facilitates comparison
of the sulcus size among different species and the degree of sinuosity of those
sulci. Statistical analysis of the frequency of the presence of assessed structures and
central tendency measures (mean, standard deviation, and mean comparison) for
the brain measurements was performed, and linear regression was performed for
comparing the data of the studied species. 

This work was approved by the Institutional Ethical Committee of the Federal
University of Goiás (CoEP-UFG 81/2008, authorization from the IBAMA number 15275
for the proceedings detailed here) and of the Federal University of Tocantins
(CEUA-UFT 23101-003220/2013-85). The details of animal use and welfare were in
accordance with the recommendations of the Weatherall report, “The use of
non-human primates in research.” 

Seven, two, one, and one adult cadaveric specimens, respectively, of Sapajus sp., C . penicillata , Saimiri ustus , and B . arachnoides were used in this
investigation ( ).
No animal was killed or euthanized for the purposes of this study: five Sapajus sp. individuals suffered accidental deaths in their
natural habitat, and these specimens were donated by the Brazilian Institute of
Environment and Renewable Natural Resources (IBAMA) to the collection of the
Laboratory of Anatomy of the Federal University of Tocantins-Palmas-Tocantins
(Brazil). The remaining Sapajus libidinosus specimens and the C . penicillata and Saimiri
ustus specimens belonged to the Primates Center of University of
Brasília and were donated to the University of Tocantins for research. The B . arachnoides specimen was studied at the
Primatology Center of Rio de Janeiro (Guapimirim, RJ, Brazil). All specimens,
except B . arachnoides , presently belong to the
Laboratory of Anatomy of the Federal University of Tocantins, and access to them
requires permission from its chairperson. 

All procedures involving the animals were performed in accordance with the
guidelines of the Brazilian Society of Animal Experimentation (COBEA). After
trichotomy with a scalpel blade, the specimens were incubated in water at room
temperature for 10–12 h. All specimens were perfused and fixed, by injecting 10%
formaldehyde and 5% glycerin through the femoral vein. The animals were
conserved in 10% formaldehyde in covered opaque containers to avoid penetration
of light and evaporation of the preservative. 

Brains were removed from the skull, weighed, and measured with a caliper and
documented with a digital camera (Cannon, model EOS Kiss X3 lens 18–55). The
nomenclature of gyri, sulci, and other structures was based on descriptions from
previous studies on humans and other primates, mainly capuchins, which are New
World primates . Only
the visible and reliably delimited structures were studied for each genus. The
measurement of the major gyri in a straight line using a caliper and along the
sinuosity using an inextensible string was performed to assess the degree of
curvature, and the results were compared with the data from humans. The degree of sinuosity was calculated using the following equation: sinuosity degree = straight measure sinuous measure The major encephalon measure was performed longitudinally and transversely. The
degree of gyrencephaly was assessed for the lobes and major sulci. The
encephalization quotient (EQ) was calculated using the following equation: E Q = brain weight 0.12 × ( body weight ) 0.66 The distance from the superior and inferior extremities of the central sulcus to
the frontal and occipital poles was measured, when possible, to indicate the
degree of inclination (ID) of this sulcus, and as an indirect measure of the
frontal lobe ( ),
according to the following relation . For brains of C . penicillata ( ), Saimiri
ustus ( ), Sapajus libidinosus ( ), and B . arachnoides ( ), straight (Tables and ) and sinuos measurements of the sulci were
performed (Tables – ). The ratio of the
straight and sinuous measurements indicates a value that facilitates comparison
of the sulcus size among different species and the degree of sinuosity of those
sulci. Statistical analysis of the frequency of the presence of assessed structures and
central tendency measures (mean, standard deviation, and mean comparison) for
the brain measurements was performed, and linear regression was performed for
comparing the data of the studied species. 

In this study, we investigated the association between the gross anatomy with some
behavior and physiology. The gross anatomy of the brain of Sapajus improved the data available for this primate; moreover, some concepts of allometry
were used to analyze its complex behavior in an evolutionary context and the
polynomial regression between brain size and body mass. The genus Sapajus includes primates with a degree of gyrencephaly
lower than that of Old-World primates but similar to that of New World primates, and
this feature does not seem to account for the complex behavior in this genus, which
often comparable to that of chimpanzees. Remarkably, despite this, the EQ of Sapajus is close to that of chimpanzees. Research taking a histological approach may explain the unexpectedly high cognitive
abilities observed in Sapajus . As the present study offers a
limited analysis only on the basis of gross anatomy, certain measures, except for
EQ, appear insufficient to explain the complexity of behavior in Sapajus . However, the evolutionary relationships deduced from
gyrencephaly and EQ seem consistent, follow the already generally accepted
notions. 

