This week, in honour of the superbly spook-tacular, spine-tingling run up to halloween, we thought we'd look at the humble feline in an altogether different light...
Have you ever looked outside, very late at night? Staring out from your window into the pitch-black landscape... Your chest tightens as a pair of eyes look back at you... Staring, unblinking, glowing... Your breathing quickens, your body breaks out in goosebumps... Then, your motion-activated outdoor lights kick into action and the intruder is revealed...
Oh, it's just your neighbour's cat...
Sound familiar? Yes? No?
Ever wondered why cats eyes, as such, glow in the dark? Because, we sure have! And, you probably know by now that our eyes most definitely don't have that same quality. Shame, it would've made one hell of a halloween costume...
The reason that cats eyes glow in the dark, is due to something called tapetum lucidum. We know it sounds scary, but it's really quite simple! Tapetum lucidum is a membranous* layer of the eye that is present in some, but not all, animals.
*Membrane: a thin sheet of tissue or layer of cells acting as a boundary, lining, or partition in an organism. (Anatomy, zoology.)
*Membrane: a microscopic double layer of lipids and proteins forming the boundary of cells or organelles. (Biology)
*Membranous: of, relating to, or resembling membrane.
Tapetum lucidum can be found in both vertebrate and invertebrate species but is more common in mammals. The purpose of the tapetum lucidum is to improve vision for animals that are nocturnal or live in spaces with poor or little light. Many species of nocturnal animals have this layer in their eyes. though humans do not. In order to produce eyeshine, a light source must be directed toward the animal’s eyes, causing it to be reflected off of the tapetum lucidum. However, there's a notable trade-off here for improved night vision. The animals that can produce the brightest eyeshine (increased tapetum lucidum) have fewer cones* in their eyes. As a result, they have limited colour vision or may be completely colour blind.
Cone cells, or cones; are photoreceptor cells in the retinas of vertebrate eyes, including the human eye.
They respond differently to light of different wavelengths, and are thus responsible for colour vision, and function best in relatively bright light, as opposed to rod cells, which work better in dim light.
Many of the animals with a tapetum lucidum are nocturnal predators. 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 actually have a tapetum lucidum layer in their eyes.
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.