Pit Vipers Can 'See' You, Even in the Dark

pit viper
The highly venomous western diamondback rattlesnake (Crotalus atrox) is a type of pit viper. Aaron Horowitz/Getty Images

Rattlesnake Jake, the bad guy in the 2011 film "Rango," could've just as easily been named "Pit Viper Jake." Yeah, it's less catchy. But hear us out.

The rattlers depicted on yellow Gadsden flags and Craig Ferguson's coffee mug are all-American vipers. Noted for their venom, vipers make up a widespread snake family that's represented on most of the world's continents, Australia and Antarctica being the two exceptions.


All vipers carry venom in twin glands behind their eyes. The toxic brew is delivered through movable fangs that can be folded up against the roof of the mouth.

Rattlesnakes bring something extra to the table. Jake and his fellow rattlers belong to a viper subfamily called Crotalinae, or the "pit vipers." Such reptiles come with an amazing, built-in tool they use to locate prey and predator alike.


In the Heat of the Moment

"There are currently 260 species of pit vipers recognized by science," says herpetologist Emily Taylor via email. Many reside in the Americas, though the subfamily also extends into parts of Southeast Asia.

Besides rattlers, the list of pit vipers includes cottonmouths, copperheads and the bushmaster snakes.


We don't call them "pit" vipers because they hang out in ditches, or attended the University of Pittsburgh. Instead that name derives from the heat-sensitive holes (i.e., pits) located between their eyeballs and nasal openings.

"Pit vipers are distinguished by the two infrared-sensing pits on their faces. Other vipers lack these pits and also lack the ability to sense infrared radiation," says Taylor.

Wildlife biologist Andrew Durso explains in an email that while "boas and pythons also have pit organs," the "structure is different, although the function is the same."


Infrared Information Processing

Discovered in the year 1800, infrared radiation, also called "IR" or "infrared light," is a type of radiant energy on the electromagnetic spectrum that's invisible to the human eye.

You're giving off IR right now. Every object in the universe with a temperature above absolute zero (i.e., -459.8° Fahrenheit or -273.15° Celsius) emits some amount of infrared radiation. Things that are physically warmer give off more IR; our bodies feel that energy as heat.


Like night-vision goggles, pit vipers' facial pits are used to detect IR.

According to Durso, these structures are "essentially very simple eyes that see in the infrared spectrum. They have a narrow opening leading to a wider cavity, in the middle of which a membrane filled with infrared receptors is suspended away from the body, acting as a retina."

This lets the snakes produce "thermal images" of nearby animals. Since many pit vipers go hunting after dark for rodents, birds and other creatures warmer than themselves, it's a helpful superpower.

	Pit viper
The southern copperhead (Agkistrodon contortrix contortrix) is a species of venomous pit viper endemic to eastern North America.

"Their [real] eyes do not sense this radiation," Taylor explains. "However, the sensory information from their eyes (photoreception) and their pits (thermoreception) is likely 'merged' in their brain in some way."

"It's impossible to know for sure what it looks like to the snake, but it's likely that they combine the input from their eyes and from their pits into some sort of dual image," she adds.


Fangs-giving Dinner

When dinner's in sight, vipers lash out with sophisticated fangs. Hinged and capable of moving independently, these are long, tubular teeth. And they're hollow, too: Venom is released through a slit near the tip.

"Snake venoms are cocktails of thousands of chemicals, mostly proteins that disrupt physiological pathways in the prey," says Taylor.


Biological toxins fall into a number of different categories. Two of the major groups are known as hemotoxins and neurotoxins.

"Hemotoxins," Taylor informs us, "disrupt the coagulation of blood and cause internal bleeding." Meanwhile, neurotoxins are built to "interfere with normal nervous system functions at multiple levels, for example by blocking neurotransmitter release or reuptake or by preventing neurotransmitters from binding."

"The result is that muscles cannot contract, and envenomated prey often die when their diaphragm, which is the main muscle used for breathing, can no longer contract," says Taylor.

Depending on the species, a given snake might be able to inject you with both kinds of toxin. "Many venomous snakes, including pit vipers, have multiple hemotoxins and neurotoxins [in their venoms]," Taylor notes.


The Hunters and the Hunted

Ambush hunting is a pit viper specialty. Most of the time, the snakes wait for their prey to come to them instead of chasing it down. Why waste energy?

When a pit viper lashes out, the reptile can strike with up to half of its total body length. Some have been clocked moving their heads at 8 feet (over 2 meters) per second in the process.


That's not to say they always hit the target. Experiments involving Gloydius shedaoensis, a Chinese pit viper, show that adults strike more accurately than juveniles. Halfway around the world, kangaroo rats can use evasive maneuvers to dodge Mohave rattlesnakes in mid-strike.

Ironically, roadrunners actively goad rattlesnakes into striking. The famous birds eat small rattlers, despite having no natural resistance to their venom.

A roadrunner's first move is to pester one of these snakes until the reptile tries to strike back. Then — while the snake's body is fully extended — it'll grab the head. Before our rattler can react, its attacker bashes its skull against the ground. Red-tailed hawks employ a similar strategy.

Pit vipers are also killed by nonvenomous kingsnakes who swallow rattlers, cottonmouths and copperheads whole.

People would do well to keep their distance, though. While snake bite fatalities are rare in the United States, with only five to six human deaths occurring per year, pit viper envenomation may lead to shock, swelling, bruising, blistering, paralysis and other delightful symptoms.

We'll leave you on a rather more pleasant topic: babies. Although some pit vipers lay eggs, the majority give birth to live young. Certain rattlers can deliver as many as 25 infants per litter. Yep, old Rattlesnake Jake probably had lots of siblings ...