As the longest night of the year approaches, many of us spend more time outside in the dark. Walking from my car to the house after work and closing the chicken coop before dinner now occur long after sunset. On several recent evenings, I’ve encountered wildlife who seem more disturbed by my presence than by the lack of light.

Nocturnal animals have various adaptations that enable them to navigate in the dark. Some, such as bats and skunks, have poor eyesight and depend more heavily on hearing and smell to traverse their territory. Other species possess exceptional vision.

Owls, flying squirrels and even mice have unusually large eyes. Owl’s eyes fill over half the volume of their skull. Larger eyes allow for greater light gathering capacity and enhanced functions within the eye. 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 the function of its parts.

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. Light rays pass through a lens which helps focus the image, then they 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.

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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 impacts 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.