Snake Senses- Thermal Imaging

 

Snake senses- Thermal Imaging

 

Snakes are primarily nocturnal hunters. To assist them during low light conditions; the pythons the boas and the pit vipers have developed highly sophisticated thermo-sensory facial pits (heat receptors), which allows them to strike at their prey with incredible accuracy even in complete darkness.

Anatomy

Although they serve the same purpose, the anatomy of the heat pits found on the pythons and boas is different to that of the vipers.

On a viper there is a single external pit is located between the eye and nostril. Each has a thin sensory membrane suspended above an air filled pocket which acts as an insulator for the detector. This highly vascular membrane contains some 1600 sensory nerve endings, which sprout from around the trigeminal nerve. The membrane has a narrow tube linking the inner and outer pockets which can be opened and closed by the surrounding muscle. Being able to open and close the tube helps to maintain an equal amount of pressure either side of the membrane. Too much or uneven amounts of pressure would impair the heat peats.

This Copperhead viper has a single pit either side of its head, situated between the eye and nostril

In contrast the pythons and boas have a series of pits in and around the labial (lip) scales on either side of the head. The pits vary in size and number depending on the species; some are very shallow and almost unrecognisable, whereas others are deep and highly obvious. Although both types of snake posses heat pits, they both differ in how they are positioned. The heat pits from a Python are set into the labial scale itself; the Boas are set in between the scales. Each pit is lined with a membrane rather than a single suspended one, and although packed with nerve endings, their density is quite a bit lower than those found on the vipers. The lower density is probably attributed to the fact that there are multiple heat sensors as apposed to a single organ.


Here are the visible heat pits found on the Brazilian Rainbow Boa



 



The heat pits are not visible on a Boa Constrictor



It is fascinating how these two groups of snakes have evolved such a similar sensory ability, especially considering that pythons and boas are considered to be ‘primitive’ snakes in comparison to the more advanced pit vipers.

How do they work?

 The sensory membrane fires nerve impulses at a constant slow (neutral) rate. The neutral range is determined by the average temperature of all heat radiating objects within the heat pits range. When a particular heat source is within this neutral range it does not bring about any changes in the rate of firing. When a heat source exceeds a certain temperature, i.e. being above the neutral range, the nerve impulses begin to fire at an increased rate. The nerve endings are incredibly sensitive and can detect a change in temperature of less than 0.002°c. They can even differentiate the heat radiated from an animal and that from a rock warmed by the sun, a very useful function, which prevents the snake from striking out needlessly at anything giving off a heat signature.

The nerve impulses are sent to a region of the brain called the optic tectrum. The optic tectrum can process both thermal and visual information. Some of the neutrons (nerve cells) within the optic tectrum respond only to infrared signals, others to visual input and some to both inputs simultaneously. Here the two sets of information are sorted and combined to create a 3D thermal image.  Because of their position on either side of the head, the pits can work in stereo enabling the snake to better judge the range and bearing of its prey. A snake armed with heat receptors can strike at its prey with pinpoint accuracy. The neck and chest areas generate the most heat and are targeted most commonly.

Sensory overload

When in use the pits absorb a lot of thermal information. To cope with this thermal overload they have developed ways to remain functional. The pits inevitably become hot and need to be cooled. They do this receiving oxygen which in turn cools the membrane and returns it to a thermo-neutral state. Constantly absorbing new information means that the pits need to reset regularly. When a warm object is placed in front of the pit, its nerve firing rate will increase until it reaches its peak and levels off. At this point the pit has effectively become accustomed to the heat source. When the heat source is removed the firing rate decreases and reverts back to its thermo neutral state; a process which takes just over 50 milliseconds.