The tapetum lucidum is a membranous layer of the eye that is present in some, but not all, animals. It can be found in both vertebrate and invertebrate species but is more common in mammals. The tapetum lucidum is a reflective surface that causes the eyes of animals to look like they are glowing in the dark. Many species of nocturnal animals have this layer in their eyes. Humans eyes do not have a tapetum lucidum.
Eye Anatomy and Physiology
The eye contains photoreceptor cells called rods and cones. Photoreceptors are sensory cells that detect and process light energy. Rods differentiate light and dark and cones differentiate color. Rods and cones cover the retina, a layer of the eyeball that forms images and transmits signals to the brain via the optic nerve. Some animals, like humans, have a layer of pigmented cells called the choroid that is located behind the retina and absorbs light.
The Tapetum Lucidum Membrane
Some animals have an additional layer located in the back of the eye called the tapetum lucidum. This reflective membrane is located directly behind the retina. When light enters the eye, it bounces off the membrane. The tapetum lucidum gives these animals a quality known as eyeshine, which makes their eyes reflect light in dark settings. In order to produce eyeshine, a light source must be directed toward the animal’s eyes, causing it to be reflected off the tapetum lucidum. The reflected light makes the animals’ eyes appear to glow. The purpose of the tapetum lucidum is to improve vision for animals that are nocturnal or live in spaces where there is little light.
Eyeshine and Night Vision
The presence of a tapetum lucidum in the eye allows animals to see more accurately at night and in low-light settings. Most animals that have a tapetum lucidum in their eyes are mammals, although some reptiles, amphibians and invertebrates also have this reflective membrane. Eyeshine color can appear, orange, yellow, green or blue, depending on the animal. The eyes of different species “glow” in different colors due to coloration and shape of the eye as well as the angle of the light shining on the eye. Eyeshine color can also change as animals age. Some animals have eyes that glow more brightly at night than others. However, there is a trade-off for improved night vision. The animals that can produce the brightest eyeshine have fewer cones in their eyes. As a result, they have limited color vision or may be completely color blind.
Predators With Glowing Eyes
Many of the animals with a tapetum lucidum are nocturnal predators. A common sight is the glow of cat eyes in the dark. Members of the cat family, including big cats and house cats alike, have eyes that reflect light in darkness. Dogs and other canines, ferrets and alligators are other predators that exhibit eyeshine. Improved night vision makes it easier for these carnivores to find prey and track movement in low-light conditions. Many types of fish also have reflective membranes, which helps them seek prey in deep water where there is little light. While some species of birds – such as owls – have eyes that appear to glow, birds do not have a tapetum lucidum layer in their eyes.
Eyeshine in Non-Predators
Several types of ungulates, or hooved animals, have eyes with a tapetum lucidum layer. Deer are active during twilight and pre-dawn hours and benefit from improved vision after sunset and before sunrise. Cattle also have eyeshine, as well as horses, which are active during daytime and nighttime hours. While the tapetum lucidum may have evolved in predators to help them catch their prey in darkness, this membrane may have evolved in herbivores as defense mechanism to detect predators at night. Non-predators that don’t have a tapetum lucidum in their eyes include squirrels, pigs, kangaroos and camels.