The light that is visible to humans is just one of the many types of light in the universe. First discovered in the 19th century, infrared is a light that we cannot see with our eyes, but that we can sometimes feel on our skin as heat.
It’s not possible for warm-blooded animals such as mammals and birds to see infrared light because their own bodies release heat. However, several cold-blooded animals evolved to see infrared light.
Some people refer to the infrared vision of snakes as a sixth sense. Snakes have receptors that allow them to see infrared in the dark thanks to protein channels that are activated by heat from the bodies of their prey.
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The snake family known as pit vipers, which includes pythons, boas, and rattlesnakes, have a particularly well-developed ability to sense heat in the dark thanks to their infrared vision. They have pits lined with heat sensors along their upper and lower jaws.
Blood-sucking insects, such as bedbugs and mosquitoes, rely on their infrared vision to feed themselves. They can "see" body heat and use the heat signature of the carbon dioxide (CO2) gas that humans and other animals naturally exhale to locate their prey.
For example, when an adult female mosquito looks for blood, she uses her infrared seeing skills to find a warm-blooded host to bite. She uses the protein and iron in the blood to make her eggs.
Some forms of fish such as goldfish, salmon, piranha and cichlid can see infrared light. Salmon and some other freshwater fish have an enzyme that switches their visual systems to activate infrared seeing, which helps them to navigate and hunt in murky waters.
In goldfish, eyesight is a highly developed sense, and it’s thought to be superior to humans. In fact, goldfish are the only members of the animal kingdom that can see both infrared and ultraviolet light.
As one of the most diverse species in the world, frogs are versatile animals. They come in all shapes and sizes, and they can survive anywhere except Antarctica. They are able to live on both land and water. Some frog types have infrared vision.
Bullfrogs, who can see infrared light, have eyes that can see both above and below the water surface. Bullfrogs use Cyp27c1, an enzyme linked to vitamin A, to supercharge their infrared vision. Bullfrogs’ infrared seeing ability adapts depending on the environment.