5 Animals That See Infrared

These animals have the natural ability to take thermal cues from infrared light

An American Bullfrog waiting for prey in a New York pond
Michael Lee / Getty Images

The discovery of infrared light can be traced back to Sir Frederick William Herschel, who conducted an experiment in the 1800s measuring the temperature changes between the colors of the electromagnetic spectrum. He noticed a new, even warmer temperature measurement beyond the visible red in a farther region of the spectrum — infrared light.

While there are plenty of animals that can feel heat, relatively few of them have the ability to sense it or see it with their eyes. The human eye is only equipped to see visible light, which represents just a small section of the electromagnetic spectrum where light travels in waves. While infrared isn’t detectable to the human eye, we can often sense it as heat on our skins; there are some objects, like fire, that are so hot that they emit visible light.

While humans have expanded our range of vision through technology like infrared cameras, there are a few animals who have evolved to detect infrared light naturally.

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Salmon

Spawning Sockeye Salmon on the Fraser River Run in North America
Stuart Westmorland / Getty Images

Salmon go through a lot of changes to prepare for their annual migrations. Some species may change their body shape to develop a hooked snout, humps, and large teeth, while others replace their silver scales with bright colors of red or orange; all in the name of attracting a mate.

As the salmon journey from the clear open oceans to murky freshwater environments, their retinas go through a natural biochemical reaction that activates their ability to see red and infrared light. The switch allows the salmon to see more clearly, making it easier to navigate through the water in order to feed and spawn. While conducting a study on zebrafish, scientists at Washington University School of Medicine in St. Louis discovered that this adaptation is connected to an enzyme that converts vitamin A1 into vitamin A2.

Other freshwater fish, such as cichlid and piranha, are believed to see far red light, a range of light that comes just before infrared on the visible spectrum. Others, like common goldfish, may have the ability to see far red light and ultraviolet light interchangeably.

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Bullfrogs

Bullfrog (Lithobates catesbeinus) Close up
Herman Bresser / Getty Images

Known for their patient hunting style, which basically consists of waiting for their prey to come to them, bullfrogs have adapted to thrive in multiple surroundings. These frogs use the same enzyme linked to vitamin A as salmon, adapting their sight to see infrared as their environment changes.

However, bullfrogs switch to predominantly A1 based pigments during their change from the tadpole phase into adult frogs. While this is common in amphibians, bullfrogs actually retain their retina’s ability to see infrared light (which is well-suited for their murky aquatic environment) rather than lose it. This may have to do with the fact that bullfrog eyes are designed for light environments of both open air and water, unlike salmon, which aren’t meant for dry land.

These frogs spend most of their time with their eyes just above the water’s surface, looking for flies to catch from above while watching for potential predators below the surface. Because of this, the enzyme responsible for infrared sight is only present in the part of the eye that looks into the water.

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Pit Vipers

A rattlesnake pit viper its pit organs to sense infrared light
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Infrared light is comprised of short wavelengths, around 760 nanometers, to longer wavelengths, around 1 million nanometers. Objects with a temperature above absolute zero (-459.67 degrees Fahrenheit) emit infrared radiation.

Snakes in the subfamily Crotalinae, which includes rattlesnakes, cottonmouths, and copperheads, are characterized by pit receptors that allow them to sense infrared radiation. These receptors, or “pit organs,” are lined with heat sensors and located along their jaws, giving them a built-in thermal infrared sensing system. The pits contain nerve cells that detect infrared radiation as heat on a molecular level, warming up the pit membrane tissue when a certain temperature is reached. Ions then flow into the nerve cells and trigger an electrical signal to the brain. Boas and pythons, both types of constrictor snakes, have similar sensors.

Scientists believe that the pit viper’s heat sensing organs are meant to complement their regular vision and provide a replacement imaging system in dark environments. Experiments conducted on the short-tailed pit viper, a venomous subspecies found in China and Korea, found that both visual and infrared information are effective tools for prey targeting. Interestingly, when researchers restricted the snake’s visual sight and infrared sensors on opposite sides of its head (making only a single eye and pit available), snakes completed successful prey strikes in less than half of trials.

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Mosquitoes

Mosquito Aedes Aegypti on a leaf in Brazil
Renato Augusto Bezerra / Getty Images

While hunting for food, many blood sucking insects rely on the odor of carbon dioxide (CO2) gas that humans and other animals emit. Mosquitos, however, have the ability to pick up on thermal cues by using infrared vision to detect body heat.

A 2015 study in Current Biology found that while CO2 triggers initial visual features in a mosquito, the thermal cues are what eventually guides the insects close enough (usually within 3 feet) to pinpoint the exact location of their prospective hosts. Since humans are visible to mosquitos from a distance of 16 to 50 feet, those preliminary visual cues are an important step for the insects to get within range of their warm-blooded prey. Attraction to visual features, CO2 odor, and infrared attraction to warm objects are independent of each other, and don’t necessarily have to go in any particular order for a successful hunt.

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Vampire Bats

Vampire bats in Manu National Park, Peru
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Similar to pit vipers, boas, and pythons, vampire bats use specialized pit organs around their noses to detect infrared radiation, with a slightly different system. These bats have evolved to naturally produce two separate forms of the same heat sensitive membrane protein. One form of the protein, which is what most vertebrates use to detect heat that would be painful or damaging, normally activates at 109 Fahrenheit and above.

Vampire bats produce an extra, shorter variant that responds to temperatures of 86 Fahrenheit. Essentially, the animals have split the function of the sensor to tap into an ability to detect body heat by naturally lowering its thermal activation threshold. The unique feature helps the bat find its warm-blooded prey more easily.