Underwater environments are dominated by blue light. Ironically, whales and seals cannot see blue, because they have independently lost their short-wavelength opsins.

Monochromacy, where the retina contains only one type of cone, has evolved a number of times independently in the mammals, in the terrestrial as well as the marine realm. In bright light, monochromats are most likely colour-blind, but in dim light, when cones as well as rods are active, interactions between the two photoreceptor types might permit at least some colour discrimination. This has been indicated by results from behavioural tests in several monochromatic species. Interestingly, all monochromats studied so far have lost the visual pigment sensitive to short wavelengths (S-opsin) but retained the longwave-sensitive one (L-opsin). Losing L-opsin would probably be much more detrimental, as it is not only involved in colour vision but also important for other visual functions. It seems that the lack of a functional S-cone is generally due to pseudogenisation – the S-opsin gene contains deleterious mutations preventing its translation into a functional protein.

Monochromacy in terrestrial mammals

L-cone monochromacy is likely to be relatively rare among terrestrial mammals. It seems to be universal in Lorisiformes, such as the nocturnal brown greater galago (Otolemur crassicaudatus), implicating that the S-cone loss occurred early in the evolution of this group. Also a number of other primates are monochromatic, as well as some carnivores, bats (a few horseshoe bats and fruit bats in the genera Rousettus, Eidolon and Epomophorus) and several rodents (e.g. golden hamsters Mesocricetus auratus and flat-haired mice Mus platythrix). Many of these species have dichromats (which have colour vision similar to that of red-green colour-blind humans) as close relatives. For example, in the Procyonidae, a family of small, New World carnivores, the nocturnal kinkajou (Potos flavus), common raccoon (Procyon lotor) and crab-eating raccoon (P. cancrivorus) have only a single cone type, whereas the diurnal coati (Nasua nasua) has two.

Monochromacy was generally assumed to be linked to nocturnality, as the ability to see colour is probably of little adaptive value in nocturnal animals. However, many nocturnal species are still dichromatic and some animals without functional S-cones might still be active during the day (such as the common raccoon). The New World owl monkeys in the genus Aotus are the only nocturnal anthropoids, and it was generally assumed that the loss of S-opsin observed in these primates is linked with their nocturnal lifestyle. However, not only the nocturnal A. trivirgatus and A. nancymaae show an S-opsin gene defect, but also the cathemeral A. azarai, suggesting that Aotus has probably lost the capacity for colour vision early in its evolutionary history and that the absence of colour vision is not necessarily tied to nocturnality.

Monochromacy in marine mammals

While monochromacy is probably the exception among terrestrial species, it appears to be the rule in marine mammals. All of the 13 cetacean and ten pinniped species studied so far lack functional S-opsin. As the terrestrial relatives of both groups are dichromats, it is very likely that the loss of S-opsin in the distantly related cetaceans and pinnipeds was convergent (it might even have occurred independently in toothed and baleen whales) and adaptive. However, the advantage of losing the S-cone in a marine environment is not easy to see. It presents in fact a paradox, as clear open waters are dominated by blue light (other wavelengths are largely absorbed and scattered) and L-cones usually have their maximum sensitivity in the green part of the spectrum. One could argue that colour vision might not be particularly useful in a dimly lit underwater environment, but retaining the S-cones would still be beneficial, as they would be best suited for the perception of brightness and contrast information. It is considered likely that the S-cones were lost very early in cetacean and pinniped evolution when these mammals still inhabited coastal waters. Here, the underwater light spectrum is shifted towards red, because blue light is absorbed by organic and inorganic material. Losing the S-cones would thus have entailed no cost but probably a benefit in terms of simplifying the processing of visual information. Some descendants then stayed in coastal waters (and for them the loss is still useful or at least neutral), while others conquered the open ocean. Certainly they would have profited from functional S-cones, but could not reverse the pseudogenisation. However, the spectral tuning of their L-cones (and rods) is shifted towards shorter wavelengths, which compensates for the loss of S-opsin at least partly. Furthermore, marine mammals have evolved remarkable non-visual capacities for orientation and prey location, such as echolocation in toothed whales.

In contrast to the cetaceans and pinnipeds, the likewise aquatic manatees (genus Trichechus) as well as several semi-aquatic mammals (pygmy hippopotamus Choeropsis liberiensis, European otter Lutra lutra and polar bear Ursus maritimus) have retained two cone types. Manatees live and feed in shallow waters, where the light is probably bright and spectrally broad. The semi-aquatic species still show considerable terrestrial activity and this might have favoured keeping cone dichromacy.

Colour-blindness in humans

Compared with other Old World primates, the selective pressures acting on human colour vision are relaxed and colour vision defects significantly more common. A relatively large proportion of humans, particularly males, are red-green colour-blind. Interestingly, this type of colour vision is strongly convergent with the dichromatic elephants, whose photopigments have virtually identical light sensitivities. There is considerable regional variation in the rate of red-green colour-blindness between human populations, with, for example, 8% of Caucasian, but only 3% of Asian males being affected. The M-opsin gene (of which many humans have multiple copies, which probably reduces the risk of its elimination) and the L-opsin gene lie in close proximity on the X-chromosome and are highly homologous. This increases the probability of unequal recombination, resulting in gene deletion or the formation of hybrid genes that account for the majority of red-green colour-blindness.

There are also other, less common colour vision defects. Blue-yellow colour vision deficiency is analogous to the monochromacy described above, as functional S-cones are lacking, resulting in blue-yellow colour confusion. The extremely rare blue-cone monochromacy is an X-linked disorder, where colour discrimination capacity is severely reduced due to, interestingly, the loss of functional L- and M-cones. Achromatopsia or total colour blindness is a genetic dysfunction of all three cone types, i.e. only the rods function normally, which not only renders colour discrimination impossible, but largely impairs vision in general. Interestingly, this disorder is relatively common on the Pingelapese Islands of Micronesia, affecting 5% of the population, which is probably the result of a bottleneck effect in combination with genetic isolation.