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The Tapetum
The tapetum lucidum tends to lie behind or within the retina. This specialized structure essentially reflects light to allow for another round of reabsorption into the eye. There are different types of tapeta in various groups of animals. For example, carnivores and some other creatures have a tapetum rich in crystals of a compound called guanine; this is called the tapetum lucidum cellulosum. A sea lion or walrus, by contrast, has a tapetum lucidum fibrosum, defined by fibers of collagen. The striking shine of an animal's eye in the glare of a flashlight or headlight reveals the reflective tapetum, with the color of the eyeshine -- yellow, green, blue and such -- varying between species.
Mammals
A whole range of mammals possess tapeta, many of them either fully nocturnal -- active mainly at night -- or crepuscular, meaning they are primarily foraging around dusk and dawn. Many carnivores hunt at such times, their tapeta improving their vision in darkness. Prominent among these are the cats, nearly all of which favor twilight or nighttime prowling. Tapetum-possessing pinnipeds -- seals, sea lions and walruses -- are classifed as carnivores. Some ungulates also have tapeta, as do cetaceans like whales, dolphins and porpoises, fruit bats, rodents and a handful of lower primates. Humans, apes and most monkeys, all mainly diurnal by nature, lack tapeta.
Other Animals
Among reptiles, crocodilians sport tapeta. These mostly large, predatory creatures include crocodiles, alligators, gharials and caimans. They are often nocturnal, spending the day basking lethargically on shorelines and sandbars or in shallows, coming alive at night to snag prey. A flashlight leveled upon a slough or backwater in the southeastern U.S., where American alligators are abundant, often reveals constellations of yellow eyeshines, the reflected glow of the big reptiles' tapeta. Bony fish and sharks both sport tapeta. Some birds, such as certain species of nightjar, kiwis and the boat-billed heron, also have the feature.
Red Eye
Exposed to the flash of a camera, many animals -- including humans -- may show a red shine to their eyes in a photograph. This is not due to the tapetum or lack thereof. Rather, this glow comes from light reflected off capillaries in the eyeball.