Whether in the movies, on television or in person, just about everyone is familiar with the ghostly green images portrayed by night vision equipment that helps humans see in the dark. This is relatively new technology for mankind, but many animals solved the dilemma of how to see in the dark many eons ago.
For most animals, seeing in the dark is easy, a combination of physical and chemical adaptations that take a little optical background to understand.
It starts with the retina.
The retina, or back of the eye, is covered with light-sensing cells called cones and rods. The cones are in the center and are responsible for vision during bright conditions. They also detect color.
Rods circle outside the cones in the retina. During the daytime, these cells are essentially inactive and are not very sensitive to color. They slowly become activated when light dims and the pupil dilates, and a protein, Rhodopsin (cones use Photopsin), is activated.
If you go from bright to dark, your eyes may require up to 45 minutes to fully charge the rods with Rhodopsin although most of the process will occur within six minutes. However, if you catch a quick flash of bright light, the Rhodopsin blanches and the process starts again. Red light doesn’t deactivate Rhodopsin and thus red lights are often used when humans work in light sensitive areas.
Animals utilize many strategies to essentially boost either the number of rods or their function. For many nocturnal animals, the nuclei of the rods undergo a pattern shift shortly after birth that enhances night vision. Their rods are also stacked in such a way as to create a multiplier effect, increasing light sensitivity by up to eight times with the same number of rods.
Often, nocturnal animals have larger eyes, which mean more rods. More rods mean better night vision.
For example, the tarsier, a tiny arboreal mammal, has eyes that are larger than its brain. It is reported to be able to see well enough to hunt in total darkness. Owls also have huge eyes. In fact, their eyes are so large that they cannot rotate in their sockets, which is why owls move their entire head to look at something.
Some animals have developed pupils that can open extra wide. This ensures that every rod is available for light detection.
Many animals also have a tapedum lucidum. This is a reflective layer at the back of the eye that reflects light back through the retina increasing the light available to see. This is also the layer that is responsible for eyeshine, that eerie red or green glow that we see when an animal is caught in the headlights.
Like so often in life, excellent night vision comes with a cost. For many nocturnal animals, excellent night vision costs them color vision as more rods mean fewer cones. The tapedum lucidum also decreases sharpness of the image the eye sees.
The ability to see at night — to hunt, to forage or to escape predation, opened up one of the great habitat partitions in the natural world. With night and darkness ruling half our lives, it was one smart move.