Most of the 5,000 or so living species of mammals have eyes and, in many, the keenness of their vision (visual acuity) is at least equivalent to that of humans. A few mammals have very limited vision, such as river dolphins (Platanistidae, Lipotidae, Pontoporiidae, and Iniidae) that live in extremely murky water or moles (Talpidae) that live in total darkness; indeed, in some moles, the optic nerve has actually degenerated. In mammals’ eyes, a lens focuses light on the retina, a layer of light-
Different chemicals (photopigments) in the cells of the retina convert optical information to electrical signals that are transmitted via the optic nerve to the brain. The retina has two main types of photosensitive cells: rods (that respond to black and white) and cones (that respond to color, which are different wavelengths of light). Color vision in mammals is uncommon, being present mainly in primates, some rodents, and some carnivores. In nocturnal mammals such as any microchiropteran bats, rodents (Muridae), and shrews (Soricidae), rods are often prevalent, while cones may be absent. To these mammals, the world is black, white, or shades of gray. The eyes of some diurnal mammals (for example, primates in the families Lorisidae and Leumuridae, or rodents in the Sciuridae) have both rods and cones, and these mammals can see color. Other mammals such as some cats (Felidae) have color vision, but only perceive a few colors.
Mammals show a range of overlap between the field of view of left and right eyes-
Arboreal animals such as many species of primates (lemurs, galagos, and lorises) tend to have higher degrees of binocularity than more terrestrial species (horses, cows, and pigs, in the orders Perrisodactyla and Artiodactyla, respectively). Finally, in some cases, the significance of binocularity in the animal’s life is not known (for example, in the case of the wrinkle-
It is common for nocturnal mammals to have a tapetum lucidum behind the retina. The tapetum lucidum is a layer of cells on the back of the eye that reflects light back through the retina, amplifying the stimulation of retinal cells by ensuring one round of stimulation as the light goes through, and another as it is reflected back. Tapeta lucida account for the “eyeshine” when catching a house cat or raccoon (Procyon lotor) in a car’s headlights or in the beam of a flashlight. Pinnipeds (Phocidae, Otariidae, and Odobenidae) and odontocetes (toothed whales and dolphins) also have tapeta lucida for helping gather available light at dark ocean depths, resulting in keen underwater vision. Visual displays from the tapetum lucidum are also common to the communication of diurnal mammals, but require that the individuals be close in proximity to each other.