Why Do Nocturnal Animals Have Large Pupils?

Large Pupils Have Greater Surface Area

• Nocturnal animals have larger pupils because there is a limited amount of light available to them to see by at night. The pupils of a nocturnal animal are large because it increases the amount of area available to collect light particles, which maximize the amount of visual information available for the eye of a nocturnal animal's brain to interpret. For this reason, some nocturnal animals also have larger eyes, because this further increases the surface area to maximize their ability to see in dim light.

How The Pupil Works

• A pupil is usually located in the center of an animal's eye, is black, and forms a hole through which light shines through. This light is the source the animal's brain uses for information regarding its immediate surrounding environment. Light particles -- photons -- get translated into visual information by an animal's brain. Nocturnal animals tend to avoid daylight because the amount of photons their large pupils collect can overwhelm them with visual information.

How A Nocturnal Animal Sees

• Human eyes evolved to be able to see in the daylight and to see things differently than an animal that spends its time awake during the night. Pupils in eyes contain tiny organs called rods and cones, which further refine the visual information the animal's brain is able to interpret. Nocturnal animals have significantly more rods in their eyes than cones. Cones help an animal see color, and since nocturnal animals lack many cones, they see mostly in black and white.

Other Adaptations In Nocturnal Vision

• The eyes of nocturnal animals appear to flash or to glow in the dark. This is due to the tapetum, which is another adaptation in the eyes of nocturnal animals. The tapetum is a highly thin and reflective membrane behind the pupil in the eye of nocturnal animals that acts like a mirror. This adaptation further maximizes how much light the eye of a nocturnal animal is able to collect for it vision.