“The baby bat
Screamed out in fright,
‘Turn on the dark,
I’m afraid of the light.”
— Shel Silverstein
For bats and other nocturnal animals, nighttime is the nicest time. These animals prefer not to see the light of day, and have adaptations that make their night life shine. Bats are just one group of animals that can do it all in the dark. While “blind as a bat” is not illustrative of all bats (many fruit-eating bats have fine eyesight), some visually challenged bat species use their ears to see. By emitting high-pitched sounds and triangulating the audio that bounces back, bats can find their way and their prey in the dark.
For other creatures of the night, the eyes have it. What they have are specialized structures and techniques that differ from those of diurnal dwellers.
Bigger is better if you want to thrive after dark. Nocturnal animals generally have larger eyes, with wide pupils, large lenses, a wide cornea to increase their field of vision, and an increased retinal surface. In some cases the pupil will cover the entire front of the eye. These features help them collect the most ambient light even though less light is available at night.
Owls are a great example of an animal with large eyes; theirs fill half of their skull. To put that size in perspective, if human eyes shared the same eye-to-head ratio, our eyes would be the size of grapefruits! Though their eyes are large, owls cannot rotate their eyes in their sockets, instead they rotate their neck and head up to 270 degrees, or three-quarters of the way around!
The retina of an owl eye has both cones and rods like human eyes. However one difference is that owls have fewer cones and more rods. The function of cones is to see color in bright light and provide for a sharp picture. Rods are for low light and thus aid in night vision. So although owls can see better than us at night, their vision is less clear.
Tubular eyes rather than spherical ones provide telescoping vision for owls. Finally, don’t rule out the help of asymmetrical ears for depth perception, facial discs for funneling sound to the ears, and slits to adjust the size of the ear’s opening — all make an owl’s hearing more acute and their survival more assured in the dark.
Another eye-opening (and tongue tackling) feature of those nocturnal eyes is the tapetum lucidum. The tapetum lucidum, or bright carpet, is a layer of tissue behind the retina that acts as a reflector or mirror. It collects and remits light back to the retina a second time, giving animals a second chance to use the limited light in the night. It is this feature that causes the eye-shine that we see when we catch an animal in our headlights.
Many vertebrates have a tapetum lucidum. Dogs, cats, deer, cattle, horses, ferrets and raccoons have one, while humans, squirrels, kangaroos and pigs do not. It is a case of the have and the have-nots in the field of night vision.
So while we humans may rue our lack of adaptations to the dark as we fumble and strain to see what is right in front of us, we should respect and admire those creatures that desire the dark. Their talents for navigating the challenges of the night shouldn’t deter us as long as we remember to always look on the bright side.
Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.