With the autumnal equinox behind us, daylight hours will be surpassed by darkness from now until mid-March. For many of us, this means reduced time for outdoor activities, but for nocturnal animals it expands the time when they have an advantage over daytime competitors.

Being active at night requires specific adaptations, and the most dramatic are related to sight. Some animals get along by hardly using their eyes. Bats and skunks both have poor eyesight. They depend more heavily on hearing and smell, respectively. Other species exaggerate their vision using various methods.

One approach is to have exceptionally large eyes. Owls, flying squirrels and even mice exhibit this feature. Owls’ eyes fill more than half the volume of their skull. Larger eyes allow for greater light-gathering capacity and enhanced functions within the eye. But other features add to superior night vision.

To understand how night vision works, it helps to know a bit about the structure of the eye and function of its parts.

In basic terms, light enters the eye through a hole called the pupil, the dark center in the middle of the eye. The human pupil is round (more on pupil shape later). The pupil dilates or constricts to control the amount of light entering the eye.

Next, light rays pass through a lens which helps focus the image, then hit the retina in the back of the eye.

From the retina, light is converted to electrical signals which travel down the optic nerve to the brain where the signals register the image.

The pupil and retina are particularly interesting in nocturnal animals. Some dark-adapted creatures have round pupils allowing for maximum light gathering when it is fully open. A disadvantage to this shape is that too much light can get in during the daytime and harm sensitive light receptor cells. To avoid this, owls have a third eyelid called the nictitating membrane, which acts like sunglasses.

Another strategy is to have a vertical slit-shaped pupil which lets in enough light at night but less during the day, particularly when an animal squints and dramatically reduces the exposed opening. Cats and foxes have vertical pupils.

The features of the retina are less noticeable, but even more effective in enhancing night vision. The retina is made up of photoreceptor cells called rods and cones. Cones work best with good light, help us see color and are responsible for high-resolution images. Rods are more sensitive to low light but lack acuity. Cone cells are concentrated in the center of the retina while rods are spread around the edges.

Nocturnal animals have a higher percentage of rod cells, which work well in dim light and are sensitive to movement. The tradeoff is a reduced number of cone cells, rendering them color blind and with somewhat fuzzy vision.

One additional feature in the eyes of nocturnal animals is a mirror-like structure called the tapetum lucidum located behind the retina. When light passes through the retina it is reflected off the tapetum and back to the retina, giving the rod cells a second chance to capture the light. The tapetum is the structure that creates “eye-shine.”

If you have ever caught the face-on view of an animal in the beams of your headlights or a flashlight you’ve probably noticed the bright reflective light from their eyes. The color of this light can be distinctive due to different pigments in the inner layer of the retina. Rabbits, foxes and owls have a reddish eye-shine. Eyes of cats and frogs reflect green light and raccoon eyes give a yellow glow.

Humans don’t have large eyes, extra rod cells or a tapetum lucidum but testing your own night vision can be fun. Go outside without artificial light for 30 to 45 minutes to allow your eyes to adjust to low light. Look at something straight ahead and then look at the object out of the corner of your eye (where the rod cells are concentrated). Which image is clearer? (It should be the second.) Also take some colorful objects and see how the darkness affects your ability to see color.

Those two activities can help you appreciate the visual adaptations of nocturnal animals and their abilities to succeed in the growing hours of darkness.

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