As I sat on the beach of the East Bay sand bar on a blisteringly hot day last July, I was thinking that perhaps that month should carry that label. Although the sun’s incoming intensity is less in February, about 85 per cent of it is reflected back into our eyes by the snow. For cold-hardy biosphere predators, the reflecting blanket of snow makes it easier to spot prey in moonlight.
For moon-less nights when the reflected light is not so abundant, many animals rely on the tapetum lucidum to turbo-boost their night vision. The tapetum is the reflective surface on the back of the eye that acts like a mirror, sending some of the light that hits the back of the eye to the photoreceptors. It often gives animals that deer in the headlights look if a light is shone in their direction. The Latin name roughly translates to “bright carpet.” It is also called “eye-shine” and it is not a characteristic shared by all animals.
What animals have a bright carpet in their eyeballs other than that deer in the headlights? The evolution of a tapetum in some fish is proposed to have occurred about 360 million years ago to facilitate prey detection in the deep, dark ocean. Amphibians such as frogs and toads are not blessed with a tapetum and the only reptiles which possess one are alligators and crocodiles. Birds with a tapetum are rare. Lobsters, shrimp, scallops and many insects benefit from a tapetum.
Mammals are generally well-endowed for night vision. The only group of mammals which doesn’t have a tapetum are the higher primates, including humans. Wouldn’t it be nice to easily make your way from your car to your house on a dark night with turbo-boosted night vision? Alas, we are not as well-equipped with bright carpets in our eyeballs as are many other biosphere mammals. On the positive side, we do have illuminated cell phones to assist in our night-time maneuverings.
As you might expect, those animals which concentrate activities in daylight are generally not blessed with a tapetum. This shining carpet in the eyeball enhances visual sensitivity in low light, but there is a trade-off. When there is lots of available light, the presence of the bright carpet lessens the amount of detail that the eye can discriminate.
There is just too much light bouncing around inside the eye. This happens to varying degrees in different animals and it is a function of the construction of their tapetum. Researchers have found that the composition of the tapetum is often adapted to reflect the wavelength of light that is most useful to the particular animal. For example, the tapetum of the biosphere’s white-tailed deer reflects light in the blue to yellow range and is concentrated on the upper portion of the retina, amplifying the light from the darker ground.
Researchers feel that the evolution of the deer’s tapetum allows the animals to more effectively forage for the choicest plants on a dark forest floor. When the animal steps out in the open fields during the bright sun time in February, the retina is protected by a constriction of the pupil and a decrease in sensitivity of the photoreceptors. It is quite a complex system.
For whatever reason, as you are reading this, you may wonder whether the complex visual system of a white-tailed deer can detect your orange reflective vest as you saunter through the forest. The answer is yes but that may be more a function of how bright the vest is rather than the colour itself.
If you enjoy evening strolls or quiet drives along country roads, watch for those reflective eyeballs. In the Bras d’Or Lake Biosphere you can be sure that you are not looking at an alligator. Animals that you may be seeing include your neighbour’s cat or dog. Those shining eyeballs may belong to a white-tailed deer, moose, raccoon, coyote or black bear.
You probably won’t see brightly glowing eyes flying through the air or perched on a branch. The birds that will be out and about in the winter nights in the biosphere are probably owls and they have not evolved tapeta. They rely on large eyes with an abundance of cones for their night vision.
Enjoy your nighttime excursion to see and be seen and watch for little bright carpets moving around the biosphere!
Dr. Annamarie Hatcher is a consulting ecologist and a board member of the Bras d’Or Lake Biosphere Reserve Association. For more information about the Bras d’Or Lake Biosphere Reserve Association, please visit http://blbra.ca/ or check out our Facebook page.