Seeing In The Dark
Animals that see well in the dark usually have big eyes with large pupils. The bigger the eyes, the more light they can collect. Owls are known for their good night vision. Although an owl’s head is smaller than yours, its eyes are bigger and shaped different, too. A small Asian monkey called a tarsier has the biggest eyes of any mammal compared to its head size.
Certain eye structures help an animal see at night. A raccoon has big eyes with large pupils that help them gather as much light as it can in the dark. Their retinas have many more photoreceptors called cones and rods that can convert light waves into information for its brain. In addition, raccoons have a reflector in the backs of their eyes that double the amount of light their eyes can use. This reflector is called a tapetum. When you see a raccoon at night it looks like their eyes glow, but what you’re really seeing is the tapetum reflecting light. Other animals with tapetums include cats, cows, sharks, crocodiles, deer, zebra, lions and moths.
In photographs it sometimes looks like human eyes have a tapetum, we call it “red eye”, but humans do not have reflectors. Instead, what is being photographed is the light from the camera’s flash bouncing off the back of our eyes or retinas. Human retinas are red, hence the “red eye.” At night your pupil’s get bigger to let in more light. Special cells in your retina called rods turn on to gather that light. You have about 95 million rods scattered across your retina. Rods are good at gathering light, but they don’t see color or detail. That’s why it’s hard to match your socks in the dark.
Animals that see well in the dark don’t necessarily see well in the daytime. Their big eyes and large pupils can draw too much sunlight. A gecko, for example, has excellent night vision but is often awake during the day. To compensate, the gecko’s pupil closes to four tiny holes. These holes let in just enough light so the gecko can see without hurting its eyes. Fish that live in the deep dark sea are used to almost no light at all. Dr. Tammy Frank studies the eyes of deep sea animals. She sends a net down 2,000 feet, catching shrimp, squid and fish in a special light-proof container on the end of the net. Then she brings the animals to her lab to study their eyes under dim red light. She has to keep them in the dark because the bright light we’re used to would make these animals go blind.