Humans are extremely dependent on their eyes; aside from the blind, who adapt with a heightened awareness of other senses, we use our eyes constantly to gather an enormous variety of information. But many animals use other organs and senses to view the physical world around them, seeing without eyes.
Pit Vipers See Heat
Pit vipers, the family of snakes that includes rattlesnakes and moccasins, are so named for their heat-sensing pits. These pits allow them to see the heat given off by the body of another animal or find a cool place on a hot day, even if the difference in temperature is extremely slight. With one pit under each eye, the vipers get a three-dimensional image with depth perception, similar to the effect of having binocular (two-eyed) vision.
A rattlesnake’s eyes do not have a fovea, the cluster of color-sensing cone photoreceptors that give humans a sharp focal point in the center of the visual field. This makes it difficult for them to perceive images clearly with just their eyes. Rather, they overlay their infrared heat vision with their ordinary eyes, which they use primarily to sense movement, to create a unified and precise image.
Bats and Dolphins See With Sound
Several species use another sense to create mental images of their surroundings: sound. In echolocation, animals create noises in order to listen to the echoes created. By analyzing the time and volume of sound as it echoes back to each ear, and the difference from one ear to the other, these animals can create a three-dimensional mental representation of their surroundings.
Bats make the sounds with the larynx, like humans, and hear them with their ears. In toothed whales, the family that includes dolphins and certain whales, the noise is created by the blowhole and structures surrounding it, and focused by the “melon” — the fatty organ that forms a large bump on the forehead of these creatures. Toothed whales have developed another organ to receive sound, fatty structures in the lower jaw that transmit the sound acoustically to the ear.
Some blind humans are able to use echolocation; they can tell the size and proximity of nearby objects from the echoes of sounds they produce.
Sharks Sense Electricity
Perhaps the most incredible method of seeing without eyes is electrolocation, used by sharks, rays, bony fish, dolphins, platypuses, and even bees. Certain fish practice active electrolocation, which is like echolocation; the fish generates an electric current and detects distortions in the field as it returns. Other species, like sharks, can sense the weak electric fields generated in all living animals by active nerve and muscle tissues.
Bees naturally alter the electric charge of flowers as they harvest pollen; as a result, other bees can use electroreception to see which flowers have been visited recently and avoid them, maximizing pollen-collection efficiency.
Electric eels, known for subduing their prey (or would-be predators) with intense jolts of electric current, can also emit weaker currents for electrolocation and communication.
Remarkably, many separate species have independently developed these astonishing senses for seeing without eyes, through convergent evolution. For all of these animals, alternative sense allow them to see their environment when conditions are not optimal for optical vision, at night or in murky waters, conferring a distinct ecological advantage.
Image source: Flickr