By Paul Donovan
The detection mechanism of prey within the snake families is predominantly achieved through the use of the tongue (via pheromones), and eyes (visually); this is a pretty standard organization, irrespective of the species or family. But imagine supplementing your senses with the ability to see the world around you as a thermal heat source, where other animals can be detected according to their body heat. And the more heat you give off, the brighter the image. Well, that is just how the pythons and boas (Pythonidae and Boidae) and the New World vipers, Crotalidae – or more commonly referred to as Pit vipers, can see the world. These snakes supplement the use of pheromones and visual perception with that of heat detection.
Both groups of snakes employ the services of special sensory structures called heat pits which, in pythons and boas, resemble ‘holes’ located in the upper, and sometimes lower labial scales (essentially the upper and lower lips) of which there may be up to 13 pairs present. In pit vipers, a single pit between the nostrils and eyes is present. Why pythons and boas have more pit structures than pit vipers, is difficult to reason, but is possibly due to them being less sophisticated, and having more makes up for this discrepancy.
Discovery
First described by Desmoulins in 1824, he observed that these pits were rich in nerve endings and attributed them to some sort of sensory function. However, it was not until 1957 when Bullock and Fox showed them to be perceptive to infrared radiation, that we fully understood their purpose.
What is interesting about this sensory organ, is that it evolved in two very different classes of snakes; the pythons and boas which are referred to as being primitive snakes and the pit vipers which are considered more advanced, due to the presence of venom which is used to kill prey. From an evolutionary standpoint, these facial pits underwent a parallel evolution although, for various reasons, did evolve in slightly different positions on the face.
Another evolutionary trait is that while it only evolved a single time in the pit vipers, it appears to have evolved multiple times in the pythons and boas. It is therefore difficult to try and rationalize as to why only these two groups of snakes evolved these structures and not all of them. Considering many venomous and non-venomous snakes share the same type of habitat and, within these groups, feed on the same prey types as well as evolving around the same time, why is it so limited given that thermal detection is so successful?
Superficially different
Although the structures outwardly show remarkable similarities to one another, they do differ in structure internally, which I will cover in a moment. But before I do, a question, “why are they so different?” Essentially, the pits enable the snake to “see” infrared radiation at wavelengths in the 5 and 30μm range. There is some evidence suggesting that far from evolving simply as an organ to detect heat given off by prey, they may also serve as an aid in behavioural thermoregulation, giving them a multi-purpose function. In other words, the snake can find the optimum temperature to bask at employing the services of these heat pits. But this begs the question again; if that is the case, why did all snakes (and lizards for that matter) not evolve them? This is one of those pondering questions.
There is a common analogy given in some literature, that the heat pits work along similar lines to that of a pinhole camera where light is focused onto a photographic plate through a tiny hole. Likewise, in the heat pits case, the heat source detected from a warm blooded animal, passes through a small pit on the snake’s face onto an underlying membrane. However, it does not hit a single point, but spreads out to hit quite a large surface area. That being so, as with the pinhole camera, the image seen is probably not very clear, resembling a blurred image, rather than a pin sharp one.
Anatomy
Structurally, the heat pits seen in Pit vipers are made up from an external pit, in which a thin sensory membrane of 0.010mm is suspended above an air-filled pocket. This helps to insulate the detector from the surrounding tissue. The membrane has a narrow tube linking the inner and outer pockets, and can be opened or closed by surrounding muscles. By opening or closing the tube, the pressure on the inside and outside of the membrane remains equal; it functions a bit like the eustachian tube of the ear. The membrane is highly vascular and richly supplied with sensory nerve endings, approximately 1600 in number which arise from the Trigeminal nerve masses (TNM’s).
The labial pits of the pythons and boas in contrast, have a fixed membrane richly supplied with nerve endings, rather than a suspended one. In addition, the concentration of nerve endings is less than that seen in pit vipers.
Built-in air-conditioning
Both types of pits have built-in ‘air-conditioning’. They receive air from the surrounding atmosphere which helps to cool them down to what is called a thermo-neutral state. This cooling down is important, as if they were not cooled down after being heated by thermal radiation from an external source, the receptors would be constantly in a state of warmth, and their efficiency reduced. They would, in effect, give the snake false information, making it difficult for them to identify the heat source.
Although structurally different, the pits function in a similar way in both species. The heat detecting membrane is sensitive to minute changes in temperature, and the nerve impulses in it are firing at a constantly slow (neutral) rate. Therefore, a heat source which is within a neutral range does not bring about any changes in the rate of firing. When an object is detected that exceeds a certain temperature limit (above the neutral range), the firing of the nerve impulses increases. Conversely, a cooler temperature will result in a decrease in the firing rate.
The neutral range is determined by the average (mean) temperature of all the objects within the organ’s receptive range. So sensitive are the nerve endings to changes in temperature that they can detect changes of <0.002C. They can also differentiate between the heat given off by an animal, and a rock warmed by the sun. Which is useful, otherwise the snake would be striking at any warm object indiscriminately, and the system would be continually overloaded.
Its function
Thermal imaging snakes use a combination of visual, pheromonal and infra-red information to detect and track an animal. This can be done with great speed and accuracy. Of course, there must be a point at which infra-red radiation influences detection over smell, or even vision, and this is when the prey animal comes within about a metre of the snake. The heat pits do not have limitless range perception; if they did, they would probably be more of a hindrance to the snake than a help. When within range and the pits come into play, the image seen by the snake is like a picture of its prey ‘painted’ as a heat source; the hotter the body part, the more intense the image is in that area. The snake can then strike with pinpoint accuracy. In fact, experiments with rattlesnakes showed that when the eyes were covered over, the snake could strike the chest area (the part giving off the hottest glow) of a rat 99% of the time.
The existence of heat pits in those species that possess them, is obviously of huge benefit. For one thing, it allows the snake to hunt in the dark, without placing reliance upon the need to actually visually see the animal it is hunting, or for it to even be moving for that matter. Many rodents adopt a motionless stance when they think a threat is nearby, but they are still ‘visible’ to these heat detectors. Also, should the eyesight, for whatever reason become impaired, the individual still has the capacity to hunt, which is a great advantage to it.
Although the snake can see its target both visually and through infrared means, it is still uncertain as to how these two images merge together to enable the snake to see a single image. It is doubtful that once within range, one sensory system takes total superiority over the other. They must work together.
Once the prey has been detected, interpretation of the ’image’ takes place in the region called the optic tectum. This region of the brain receives not only the infrared signals, but also signals from the eyes as well. Some neurons are singularly sensitive to infrared signals, others visual, while still further others a combination of both. This information is then relayed to the forebrain via the tectum where it is interpreted.
The pit organs, as I have said, become accustomed to continual stimuli, and if the stimuli is removed, will revert to another status. For example, if a warm object is placed in front of the pit, its nerve firing rate will increase until it reaches a peak level where the firing rate will level off; in effect it becomes accustomed to the heat source. When the source is then removed, the firing rate will reduce, eventually returning back to its ‘normal‘ state. This process is called the latency period, and takes little more than 50 milliseconds to adjust itself, which is an astonishingly quick time.
Horseradish?
What does wasabi (Japanese horseradish) and thermal heat pits have in common? Well, it would appear quite a lot as it turns out. The nerve receptors responsible for thermal imaging in snakes, produce the same protein as which detects the chemical pungency of this herb (and other chemical irritants) in humans. However, rather than detecting an odour, they have evolved to detect heat. This leads us on to the role genes play in establishing the infrared system, as they are responsible for this rather strange anomaly.
One gene in particular plays a crucial role in heat detection, and that is one called TRPA1 (Transient Receptor Potential Ankyrin). This gene is highly active in the trigeminal neurons of infrared detecting snakes, but less so in species lacking infrared detection. In humans and mammals, the receptors of this gene play a role in chemical detection. With changes in the structure of its receptors, it has evolved in snakes to allow them to detect infrared heat sources, which creates a thermal image in the snake’s brain.
Conclusion
The infrared system is a measure of external thermal fluctuations in temperature, allowing the snake to interpret an accurate measure of its distance away from the source. Thermal detection usually comes into play at a distance of about a metre between the snake and its prey. Although it has evolved in two different groups of snakes, structurally the pits differ quite appreciably. They are far more sensitive in the pit vipers than they are in the pythons and boas by approximately 10 times.
Although both pythons, boas, and pit vipers can detect infrared, the massive evolutionary distance between these two groups shows that they evolved infrared detection independently from one another. It also shows that a function does not always require a specific gene, but simply the repurposing of an existing one. If the mass media were to have us believe, snakes are primitive, primordial creatures. Nothing could be further from the truth. They are, as heat detection shows, highly evolved.
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