Receptors Create Thermal Image

a green viper snack portait with a black background

Biological Strategy

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Sense Temperature Cues From the Environment

Some living systems use their ability to perceive temperature to find prey or avoid predators, or as a way to gain information about their environment, such as whether they are near a warm or cold place. Temperature gradients can be subtle and modulate as they travel through water, air, or solids. Living organisms must therefore have a variety of thermal sensors appropriate to a given medium (liquid, gas, solid). Some even have a way to “visualize” a signal’s source. For example, the rattlesnake has heat sensors with thousands of nerve endings located in pit-shaped holes on each side of its face. The sensitivity of the pits overlaps, giving the snake a bifocal image of the heat’s source.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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Thermoreceptors found in pits in a viper's face provide it with a bifocal thermal image of prey because the fields of thermal sensitivity overlap.

Image: Elroy Serrao /

Image: World of Warmth /

“The pit viper’s ability to register heat is so sensitive that they can feel the temperature variation produced by a mouse from 6 inches (15 cm) away. The heat sensors are located in the pit-shaped holes on their faces that give them their name. Positioned on each side of the snake’s head between the eye and nostril, these small, shallow pits point forward, and their tiny, pinhole openings are supplied with a grid of 7,000 nerve endings from a branch of the trigeminal nerve leading to the head and face. Toward the base of this pit is a membrane, similar to the retina of the eye, which has minuscule thermoreceptors, numbering 500-1,500 per square millimeter. Because the fields of sensitivity of the two pits overlap, a pit viper can see heat in stereo. This bifocal thermal vision provides the snake with a fiery infrared image of its prey and enables it to judge how far away it is. The pit viper’s sensory awareness is coupled with quick reactions, allowing it to respond to a heat signal in under 35 milliseconds.” (Shuker 2001:17-18)

Last Updated August 18, 2016

References

Book

The Hidden Powers of Animals: Uncovering the Secrets of Nature

26/02/2021 | Dr. Karl P. N. Shuker

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Reference

Journal article

Molecular basis of infrared detection by snakes

Snake infrared detection unravelled

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