When light catches the eyes of certain animals in the dark, they can appear to glow. This common nocturnal observation, known as eyeshine, is a fascinating natural phenomenon. While it might seem as though these animals are emitting light, the effect is actually a reflection.
Understanding Eye Shine
Possums’ eyes do indeed appear to “glow” or reflect light when illuminated in low-light conditions. This is not true bioluminescence, where an organism produces its own light, but rather a strong reflection of an external light source. Many other nocturnal animals, such as cats, raccoons, and deer, also exhibit eyeshine.
The color of the reflected light can vary among species and even within the same species, depending on the angle of observation and the specific biological makeup of the eye. For possums, the eyeshine typically appears red or pink, though some observations report green or gold. This reflective quality aids these animals in navigating their environment after sunset.
The Biology of Nocturnal Vision
The mechanism responsible for eyeshine is a specialized layer of tissue located within the eye called the tapetum lucidum. This Latin term translates to “bright tapestry.” Situated immediately behind the retina, the tapetum lucidum acts like a biological mirror. When light enters the eye and passes through the retina without being absorbed by photoreceptor cells, it then strikes the tapetum lucidum.
The tapetum lucidum reflects this unabsorbed light back through the retina, essentially giving the photoreceptors a “second chance” to capture the photons. This process significantly amplifies the amount of light available to the eye, enhancing visual sensitivity in dim conditions. The specific composition of the tapetum lucidum varies across different animal groups; for example, it can be made of crystals, collagen fibers, or, as in the case of possums, lipid spheres.
Possum Sight in the Dark
The tapetum lucidum is a significant adaptation for possums, which are primarily nocturnal creatures. This reflective layer enhances their ability to see in very low light, which is crucial for their survival activities such as foraging for food and detecting predators. Possums possess proportionally large eyes compared to their body size, which helps them gather more ambient light.
Their retinas contain a high concentration of rod photoreceptor cells, which are highly sensitive to low light levels. Approximately 97% of the photoreceptors in a possum’s retina are rods, enabling them to detect faint traces of light. This combination of a tapetum lucidum, large pupils, and a rod-dominant retina provides possums with exceptional night vision, estimated to be 7 to 8 times greater than human light sensitivity. While their night vision is highly effective, their vision in bright daylight is less sharp, and their color perception is limited, primarily seeing shades of gray.