In “The Origin of Species,” Charles Darwin wrote about eyes as being nearly useless to moles and a few other burrowing rodents. He noticed that these animals’ eyes were small and often covered by skin or fur, and he considered this evidence for natural selection. While some moles have evolved to thrive underground despite being totally blind, most animals need some eyesight to survive. They use eyes to monitor prey and watch for predators. Like humans, animals have evolved to have eyesight that will suit their environment. This blog examines eyesight in various animal species and how it compares to human eyesight.
Bats Although people with poor vision may be rudely told they are “blind as a bat,” that’s not a fair comparison. All bat species have eyes that can see, just not the same way humans can. For example, many bats primarily use echolocation to identify their food. But although these bats depend more on their sense of hearing, they still need eyes to perceive changes in light. When it becomes dimmer outside, they know it’s time to hunt for dinner. These bats also use their eyes, not echolocation, to navigate over long distances. Bat eyesight is poor compared to human eyesight because bats have different photoreceptors than human eyes. In particular, bats that hunt at night have eyes suited for night vision, something human eyes are not built to do well. But humans can see more colours than night-hunting bats. Birds of Prey If someone tells you that you have “eagle eyes,” that’s a compliment, and unlike “blind as a bat,” this saying represents the animal involved with accuracy. Eagles and other birds of prey have eyesight that’s much sharper than human eyesight. For example, an eagle’s eyes are 4 to 8 times sharper than a human’s. Birds of prey need sharp eyesight to identify their prey from afar and surprise it from above. They can see small creatures wandering on the ground from heights close to 4,500 m (15,000 ft.). As they swoop in to capture their next meal, they maintain perfect focus on their target. Ancient Sea Scorpions You’ve likely never heard of ancient sea scorpions since they became extinct more than 400 million years ago. These large arthropods could grow to be 2 m (6.5 ft.) long with claws 0.6 m (2 ft.) in length. Their large bodies also featured large compound eyes, but researchers believe these eyes didn’t give the sea scorpions excellent eyesight. Despite having both huge and microscopic lenses, the eyes only assisted the sea scorpions a little as they hunted in dark waters. Colossal Squid This sea creature has the largest eyes of any animal. Based on the few specimens scientists have studied, colossal squid eyes measure upwards of 30 cm (11.8 in.) across. The lens alone is the size of an orange. Unlike an ancient sea scorpion, the colossal squid uses its big eyes to see well, even in dark underwater conditions. Each colossal squid has two eyes that face forward, like human eyes, to aid in distance perception. Colossal squid also have huge optic lobes, a sign that they have highly developed vision. They also have an organ near each eye that emits light so the squid can see its prey. Dogs Domestic dogs cannot perceive as many colours or the same intensity of colours as human eyes. They only have two types of cone receptors on their retinas. Humans have three, so we perceive a wider range of colours. Similarly, dogs used to have a difficult time perceiving TV and film images as movies. Their eyes need to see around 70 images per second to distinguish that the images show movement. By contrast, humans can perceive movement after viewing only 16 to 20 images per second. Older TVs used a slow image rate, so they probably looked like a slide show to dogs. Modern TVs show more images at a faster rate, meaning dogs can usually see moving images on TV the same way we do now. Mantis Shrimp Humans may have the advantage over dogs when it comes to colour perception, but we definitely lose compared to mantis shrimp. Human eyes have three types of colour receptors, but mantis shrimp eyes have 12. They can see ultraviolet and infrared light waves that humans can’t. This crustacean (which isn’t actually a shrimp) lives in tropical ocean waters and uses its eyes to hunt worms, fish, clams, crabs, and snails. Each eye rests on a long stalk which can rotate up to 70 degrees. As each eye rotates, it can perceive all three dimensions on its own, a trait called “trinocular vision.” Each eye also has compound lenses like a dragonfly. But, unlike a dragonfly, each row of lenses on a mantis shrimp performs a specialized function. Some sense light, others colour, and the eye itself—not the brain—interprets all these messages. After learning about eyes from the animal kingdom, you should recognize that your eyes are adapted to your needs. Human vision is rarely perfect, but we don’t need eyes that rotate or lenses the size of oranges. Now that you know how perfect your eyesight is for you, make the most of it. Wear glasses or contacts if your optometrist prescribes them so you can see the world around you.