Blane Klemek Outdoors: The sense of sight is truly amazing

I haven't observed many flying squirrels in my life, but this nocturnal species of squirrel — the only squirrel active at night — are quite common in Minnesota's woodlands and forests. Indeed, they often take advantage of the leftover sunflower seeds and other birdseeds that wild birds drop from our backyard birdfeeders throughout the daytime.

One may wonder how such an animal evolved. Or why they are nocturnal in the first place. Their bulging eyes are huge and, in proportion to the head, seem quite oversized. However, eyes like these are necessary for such nighttime creatures. Large eyes gather all available light and provide them the ability to find food, detect predators and navigate safely throughout their environment. There are many species of animals so equipped.

The eyes of owls serve as a prime example of the ultimate in eyesight and night vision. Like us, owls have binocular vision. All this means is that an object can be seen with both eyes at the same time. But that’s where the similarity essentially stops.

First of all, owls’ eyes are very large. Both the outer portion of the eye, or cornea, and the pupil, or opening of the eye, are large, too. The iris, which is a membrane between the cornea and lens, controls the pupil’s size. On especially dark nights, an owl’s pupils are at their largest, which in turn allows the maximum amount of light to enter into the eye.

As light passes through the eye, special light-sensitive rod-shaped cells called “rods” located at the back of the eye (retina) receive the image. Cone-shaped cells called “cones” decipher color, but owls have many times more rods than they do cones. It is much more important for a nocturnal bird of prey, or any nocturnal animal for that matter, to have more of the light-sensitive rods than the color-sensitive cones.

Unlike any other nocturnal animal, owls’ eyes are fixed, or in other words, immobile, within their eye sockets. An owl cannot move their eyes, but they can move their heads nearly completely around (about 270 degrees!). This is possible because of its long neck and number of vertebrae — 14 in all. For comparison, we have just seven. Owls also have a wide field of view, but somewhat less than our own.

Of course, it is more than just eyes that work to make the owl such an efficient nighttime predator. Acute hearing that can pinpoint the location of a small rodent under the snow from dozens of feet away, soft feathers that are absolutely silent in flight for capturing prey undetected, and four long and sharp talons on each foot for holding onto its prey once caught, all work in unison, with their eyes, in outfitting this incredible bird of prey.

There are so many variations in vision throughout the animal kingdom. For example, the American woodcock has huge eyes located on the sides and near the top of its head. Eyes of this sort provide the bird with a 360-degree field of view.

Additionally, their position in the skull also allows the bird to see well when it probes the soil for earthworms, its favorite food, along with providing them with excellent night vision for migration, courtship activities, and foraging.

As almost everyone knows, a deer's vision is excellent. Their field of view encompasses 310 degrees, and they can detect the slightest of movement. Deer have no problem navigating throughout their environment in broad daylight, darkness and all light conditions in between.

The ability to see in low light and in darkness is the result of a special membrane in the rear of the eye called the tapetum lucidum. This reflective tissue "bounces" light coming into the eye back across the receptor cells of the retina. This bouncing back of incoming light across the receptor cells for a second time effectively brightens images that are dimly lit. This is also the reason why a deer’s eyes shine when lights are cast upon them.

The sense of sight is truly amazing. Yet without specially adapted eyes, being nocturnal or being able to see well at night, would not be an option for most creatures great or small, as we get out and enjoy the great outdoors.

Blane Klemek is a Minnesota DNR wildlife manager. He can be reached at bklemek@yahoo.com.