Blane Klemek: Night vision is an amazing wildlife adaptation
I remember several years ago when I stepped outside for a breath of fresh air before turning in for the night. It was winter, about this time of year.
As I stepped outside the house I was surprised by the sight of a small mammal scurrying from the ground directly below the birdfeeder that hung from the eve of the house. The little animal bounded over the snow to the yard-light pole, quickly climbed halfway up, stopped, and then didn’t move a muscle. I knew immediately that I was looking at a flying squirrel.
I haven’t seen many of these remarkable little squirrels in my life, but I knew that the nocturnal species of squirrel are actually quite common in Minnesota’s woodlands. This particular flying squirrel had been taking advantage of sunflower seeds that the finches, nuthatches, and chickadees drop from the feeder throughout the day.
A few minutes later while watching the little fellow from inside the house through a bedroom window, it abruptly climbed several feet higher, turned its body in the direction of the birdfeeder, and leapt from the pole. Spreading its legs to reveal the full breadth and purpose of those folds of skin and fur, the squirrel glided through the cold night air like a kite — its dorsally flattened tail trailing behind like a rudder — in a delightful display of grace, speed, and precision. It landed a couple of feet from where it had earlier been and resumed feeding on seeds seconds later.
One may wonder how such an animal even evolved. Or why they are nocturnal. Indeed, both species of flying squirrels in Minnesota, the northern and the southern flying squirrel, are active only at night. Their bulging eyes are huge and, in proportion to the head, seem quite oversized. However, eyes like these are necessary for such night 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 (opening of the eye) are large too. The iris (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. However, they can move their heads nearly completely around (about 270 degrees). This is possible because of its long neck and number of vertebrae — fourteen 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.
The ability to see is certainly amazing in its own right. But without specially adapted eyes, being nocturnal would not be an option for many animals. And even more amazing is the amount of variation of eyes and eyesight in everything from ants to elephants. Our fortune comes with being able to appreciate and observe the wonders of Nature as we get out and enjoy the great outdoors.
(Klemek is the DNR Area Wildlife Supervisor in Detroit Lakes. You can contact him at firstname.lastname@example.org.)