Infant care not performed by the mother 
Allomothering, allomaternal infant care/handling, or non-maternal infant care/handling is performed by any group member other than the mother. Alloparental care is provided by group members other than the genetic father or the mother and thus is distinguished from parental care. Both are widespread phenomena among social insects, birds and mammals.
Allomothering comprises a wide variety of behaviours including: carrying, provisioning, grooming, touching, nursing (allonursing), and protecting infants from predators or conspecifics. Depending on age-sex composition of groups alloparents, helpers or "handlers" can be non-reproductive males in polyandrous systems, reproductive or non-reproductive adult females, young or older juveniles, or older brothers or sisters helping to raise their younger siblings.
Non-human primates.
The term allomother first appeared in a sociobiological analysis of reproductive strategies among langur monkeys and referred to group members other than the mother who share care of infants. Allomothering turns out to be common across the primate order and occurs in vervets, cebus monkeys, squirrel monkeys, various macaques, New World monkeys and prosimians as female or male group members assist the mother by carrying or guarding infants from predators, and in some New World monkeys such as tamarins and marmosets, helping to provision them. 
Allomaternal care varies greatly across and within different species, families, subfamilies, and groups of primates. Mothers within the same group often vary significantly in the amount of access they allow allomothers. Differing levels of allomaternal care are present in almost 75% of primate species for which there is data and among 100% of callitrichids. 
Allomaternal care by adult males is most often provided in species in which there is a relatively high degree of paternity certainty, such as within pair living species. However, unrelated adult males have been observed to provide allomaternal care as in fat-tailed lemurs and Barbary macaques.
The majority of allomaternal care in group living primate species is provided by females and juveniles. Juveniles are often older siblings, but do not necessarily provide allomaternal care exclusively to their siblings. Allomothering is most common in species with close female relationships and relaxed female dominance hierarchies. 
Number of allomothers.
The number of allomothers involved in the allomaternal care of a single infant varies by species. In hanuman langurs, infants receive allomaternal care from most of the females within the group, while in capped langurs, one adult female typically acts as the primary allomother for an infant. 
Infant's age.
The age at which infants receive care from allomothers also varies greatly by species. Research on wedge-capped capuchins has found that infants receive no allomaternal care during the first three months of their lives, and they receive the greatest amounts of allomaternal care between the ages of four and six months. However, potential allomothers show interest and investigate infants who are under three months old. Alternatively, research on wild capped langurs found that infants spent about ⅓ of their time with a single allomother during their first month of life, and after this point, time engaged in allomaternal care declined. Wild Formosan macaque infants receive the highest rates of allomaternal handling between the ages of four and seven weeks, and allomaternal care rates decrease greatly between 20 and 24 weeks of age. 
Infant's sex.
There is evidence that some primate species differentially provide allomaternal care based on the infant's sex. This sex-bias in allomaternal care is noted in wild Formosan macaques. In a study of this species, adult females participated in higher rates of allomaternal care with female infants than with male infants, while juvenile females engaged in higher rates of allomaternal care with male infants than with female infants.
Allonursing.
Cebus monkey females have been known to regularly nurse (allonurse) infants who are not their own (cf. wet nurse). In these species allonursing is performed by related and unrelated females. Moreover, about 10% of nursing bouts are attributed to allonursing. Allonursing is a widespread, though infrequent, behavior among female wedge-capped capuchins. Allomothering can also be performed by non-reproductive helpers like in the callitrichids (marmosets and tamarins). 
Cooperative breeding.
In the callitrichids, allomothering care goes beyond many other species and infants are spontaneously provisioned by all group members without a prior begging call on the part of the infants. These species practice facultative cooperative breeding, where a single dominant female reproduces and other group members (fathers, other males and non-reproductive juveniles) provide the majority of care to the infants.
Cognitive and socialization implications.
Many studies have shown that cooperative breeding and allomothering provide significant benefits for the development, learning, and socialization of offspring. Primates that are cooperative breeders are known for some behavioral elements including territory and group defense, caution, and transferring the offspring between different individuals. The cooperative group protects offspring and members physically and provision/sharing food through transferring offspring between different individuals. The researchers believe that there are cognitive abilities behind these cooperative behavioral elements that enable individuals to help others. These behavioral elements also lead to the enhancement of other cognitive abilities especially those that are related to social interactions.
In this relation, some studies have suggested that the potential evolutionary pressures may have formed the behavioral elements and even some morphological traits of infants in cooperative breeding animals. One of the possible pressures is the need for infants to develop some social skills that enable them to form strong bonds with multiple caregivers, as this can increase their chances to manipulate caregivers to help them survive. Another selective pressure that can lead to the evolution of distinctive vocalizations or other communicative cues in the offspring of cooperative breeders is the requirement for newborns to be able to effectively communicate with their caregivers.
As it is assumed in comparative studies, cooperative breeding may have led to the evolution of greater cognitive flexibility and problem-solving abilities in infants. This is because of this notion that navigating complex social situations with multiple caregivers requires a high level of cognitive ability, including understanding and responding to social cues, communicating effectively, and solving problems. It is important to know that these skills may be useful in other social situations rather than raising offspring and the current studies addressing this relationship emphasize further research. For example, in current comparative research on different species of primates, it is shown that cooperative-breeder species like callitrichids due to their greater social tolerance, and sensitivity to others' signals, may perform better on tasks requiring social learning, communication, and alliance-making than their independently reproducing sister taxa (squirrel monkeys).
In addition, the other important cognitive ability related to sociality and cooperative breeding is prosociality which has been addressed by some research. Comparative studies argue that cooperative breeding in nonhuman primates leads to cognitive changes that increase prosociality and enhance social cognition. These prosocial behaviors are not seen in nonhuman primates that are independent breeders.
The research on prosociality speaks about “shared intentionality” as an individual's capacity to share mental representations of goals and intentions with others based on the understanding of basic mental states. Consequently, this capability allows individuals to cooperate with each other and coordinate their actions. Particularly in humans, the characteristics related to shared childcare are thought to contribute to advanced cognitive skills including language, planning, cumulative culture, and intentional teaching. These characteristics are rare in other animals and need complicated interaction between cognitive development and social behavior.
Proposed explanations.
Multiple explanations have been proposed for the adaptive value of allomaternal care and who benefits from it: the mother, the infant, or the allomother.
Kin selection hypothesis.
Older siblings promoting their own genetic material via helping their younger siblings are explained by the inclusive fitness theory: offspring of the same parents are, on average, genetically equally close to their siblings as they would be to their own progeny. Under kin selection theory, related allomothers may improve their inclusive fitness if the allomothering behaviour contributes to the infant's survival or a faster reproductive rate for the mother since this will increase the related allomother's genetic success. Fairbanks, 1990 found that in captive Vervet Monkeys, juveniles were most likely to provide allocare to an infant sibling or the infant of a high ranking mother. In Wedge-capped Capuchins, the degree of relatedness best predicts allomaternal interactions, and female siblings are the most likely to act as allomothers to infants. However, kin selection does not account for all allomaternal behavior since non-kin subadults and females often are the ones providing allocare. Young females with siblings may just have more opportunities to care for related infants.
Learning to mother hypothesis.
A number of adaptive functions have been proposed to account for the widespread incidents of allomaternal care in mammalian and avian species. Jane Lancaster noted the reproductive benefits for primates as k-strategists in learning to be better mothers, or acquiring mothering skills. Her learning-to-mother hypothesis postulates that primate females with no children of their own participate in allomothering more frequently than expected, and evidence from studies by Sarah Hrdy and Lynn Fairbanks supports this hypothesis. However, experienced and pregnant mothers may also benefit from mothering practice. The hypothesis is supported by evidence of the success of allomothering as a learning technique. First time mothers have high rates of infant mortality, but the rate is reduced for females who engaged in allomaternal behavior as juveniles prior to the birth of their first offspring. So, more alloparenting as a juvenile corresponds with greater reproductive success for the female. Allomothers may face energetic, social, and reproductive costs, but are potentially benefited by learning how to parent and practicing parenting skills which results in higher survival rates for their first born offspring. This benefit to the allomother may be potentially costly to the infant and its mother. However, this hypothesis is disputed by evidence such as the observation that in wild Formosan Macaques, nulliparous and multiparous adult females engage in similar rates of allomaternal care.
Alliance formation hypothesis.
Other hypotheses include "alliance-formation", where subordinate allomothers endeavour to form social alliances with dominant mothers by interacting with their infants. Infants may also gain valuable social skills by interacting with allomothers. Infants may form social-alliances of their own, and improve their chances of having future dispersal partners. This is especially apparent in some species of Colobine old world monkeys where relationships are generally built less around kinship (as compared to Cercopithecine old world monkeys). In Colobines, allomaternal care may allow infants to form social networks and relationships that are separate from their mother's relationships. Allomaternal care may also be a form of reciprocal altruism between females in a group. In some cases, allomothering may also improve the chances for an infant to be adopted by another resident female should the mother die.
By-product hypothesis.
Another explanation is that selection for allomaternal behavior may just be a by-product of selection for maternal behavior, and that there is no specific adaptive value to allomaternal care. This theory is supported by observations that females who provide allomaternal care more often also end up being better mothers, so these females may be predisposed to care for infants. However, this hypothesis would not explain the high levels of allocare seen by juvenile, subadult or unrelated adult males in many [primate] species.
Reproductive fitness hypothesis.
An infant's birthmother, in a climate of allomothering, may gain time relieved from parental duties which can provide her with energetic advantages by allowing her to reduce levels of maternal care and expend less energy carrying her infant, and by allowing her to forage more efficiently. These energetic benefits may allow the mother to gain direct fitness benefits as she may be able to reproduce more quickly (i.e. reduce her inter-birth interval) due to allomaternal care providing her with the ability to more quickly invest in physical preparation for her next offspring. A reduction in inter-birth interval and a subsequent increase in the mother's reproductive rate may ultimately increase her lifetime reproductive success. Infants may also benefit from their mother's more effective feeding and allomaternal care through a faster maturation and growth rate or earlier weaning time (at a younger age but not at a lower weight).
Malicious behavior.
Allomothering care may not always be beneficial. In some cases, "aunting-to-death" has been reported, where females withhold an infant from their mother until the infant dies, which can be explained either as incompetence or as competition in favor of the aunts' own offspring. In other cases, infants may be kidnapped and receive life-threatening bites or hits from a supposed-alloparent. 
Little allomaternal care has been observed in cercopithecine old world monkeys and great apes. However, some cercopithecine