Monochromacy (from Greek mono, meaning "one" and chromo, meaning "color") is the ability of organisms or machines to perceive only light intensity, without respect to spectral composition (color). Organisms with monochromacy are called monochromats. Many mammals, such as cetaceans, the owl monkey and the Australian sea lion (pictured at right) are monochromats. In humans, absence of color vision is one among several other symptoms of severe inherited or acquired diseases, including achromatopsia or blue cone monochromacy, together affecting about 1 in 30,000 people. The affected can distinguish light, dark, and shades of gray but not color. Humans
Human vision relies on a duplex retina, comprising two types of photoreceptor cells. Rods are primarily responsible for dim-light scotopic vision and cones are primarily responsible for day-light photopic vision. For all known vertebrates, scotopic vision is monochromatic, since there is typically only one class of rod cell. However, the presence of multiple cone classes contributing to photopic vision enables color vision during daytime conditions. Most humans have three classes of cones, each with a different class of opsin. These three opsins have different spectral sensitivities, which is a prerequisite for trichromacy. An alteration of any of these three cone opsins can lead to colorblindness. Anomalous Trichromacy, when all three cones are functional, but one or more is altered in its spectral sensitivity. Dichromacy, when one of the cones is non-functional and one of the red-green or blue-yellow opponent channels are fully disabled. Cone Monochromacy, when two of the cones are non-functional and both chromatic opponent channels are disabled.
In the last half-century, however, a focus on behavioral and genetic testing of mammals has accumulated extensive evidence of at least dichromatic color vision in a number of mammalian orders. Mammals are now usually assumed to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions. Two mammalian orders containing marine mammals exhibit monochromatic vision:

Pinnipeds (including seals, sea lions and walruses)
Cetaceans (including dolphins and whales)Unlike the trichromacy exhibited in most primates, Owl monkeys (genus Aotus) are also monochromats. Several members of the family Procyonidae (raccoon, crab-eating raccoon and kinkajou) and a few rodents have been demonstrated as cone monochromats, having lost functionality of the S-cone (retaining the L-cone).The light available in an animals habitat is a significant determiner of a mammals color vision. Marine, Nocturnal or Burrowing mammals, which experience less light, have less evolutionary pressure to preserve dichromacy, so often evolve monochromacy.A recent study using through PCR analysis of genes OPN1SW, OPN1LW, and PDE6C determined that all mammals in the cohort Xenarthra (representing sloths, anteaters and armadillos) developed rod monochromacy through a stem ancestor. See also
Achromatopsia
Blue cone monochromacy
Cone dystrophy
Dichromacy
Trichromacy
Tetrachromacy

References
External links
Rossi, Ethan (February 2013). "Visual Function and Cortical Organization in Carriers of Blue Cone Monochromacy". PLOS ONE. 8 (2): e57956. Bibcode:2013PLoSO...857956R. doi:10.1371/journal.pone.0057956. PMC 3585243. PMID 23469117. Weleber, Richard (June 2002). "Infantile and childhood retinal blindness: A molecular perspective (TheFranceschetti Lecture)". Ophthalmic Genetics. 23 (2): 71–98. doi:10.1076/opge.23.2.71.2214. PMID 12187427. S2CID 30741530.