Edward Tyson was first to dissect a rattlesnake and provide a detailed description of the viper for the 17th century scientific community. His description included the loreal pits. His interpretation of the pits was ―they were ears. The true function of the pits was not discovered until the 1950’s when Theodore Bullock and F. P. J. Diecke experimentally demonstrated the loreal pits of pit-vipers and the labial pits of boids and pythonids were in fact heat sensors.
[caption id="attachment_144" align="aligncenter" width="650" caption="Heat sensing pits in a Mojave Rattlesnake, an Emerald Treeboa, and a Ball Python. Photos copyright John C. Murphy."]
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Now, Elana O. Gracheva and colleagues at the University of California, San Francisco have revealed the mechanism used to transduce the infra-red heat into nerve impulses. Using unbiased transcriptional profiling, the research team identified TRPA1 channels as the infra-red receptors on sensory nerve fibers. TRPA1 channels are the most heat sensitive ion channels known in vertebrates.
In humans, the TRPA1 orthologue is activated by allyl isothiocyanate, the molecule that gives wasabi and mustard their kick. And, in most vertebrates the TRPA1 molecule is associated with the detection of noxious chemicals, but in some invertebrates (such as fruit flies) it is known to detect heat, and it is used by vampire bats to detect heat.
Gracheva and colleagues tested TRPA1 orthologues in Western Diamondback Rattlesnakes (Crotalus atrox), Texas Rat Snakes (Pantherophis obsoleta lindheimeri), Western Coachwhips (Masticophis flagellum testaceus), Ball Pythons (Python regius), and Amazonian Treeboas (Crorallus hortulanus). The molecules were most active in the pits of the rattlesnake. The heat sensing molecules were obtained from the trigeminal nerve ganglia of each species.
Rattlesnake TRPA1 was found to be inactive at room temperatures, but it became robustly active at temperatures of about 28°C. The Rat Snake’s TRPA1 was also sensitive to heat, but did not become active until it reached 36°C, and of course, the Texas Rat Snake and Coachwhip lack specific organs to detect heat.
The boa and python tested showed TRPA1 heat sensitivity also, both had a higher threshold to heat, with the molecules becoming active at temperatures of about 30° and 33°C respectively. However, the constrictors’ labial pits were 5 to 10 times less sensitive to heat than were the pit vipers.
To be sure, pit vipers and the large constrictors evolved the heat sensing mechanisms independently of each other. Furthermore, the boids and pythonids are no longer known to be each other’s closest relatives. Thus, the question becomes did the labial pits of boas and pythons also evolve independently from each other, or were they present in some ancient common ancestor?
Citation: Gracheva, E., Ingolia, N., Kelly, Y., Cordero-Morales, J., Hollopeter, G., Chesler, A., Sánchez, E., Perez, J., Weissman, J., & Julius, D. (2010). Molecular basis of infrared detection by snakes Nature DOI: 10.1038/nature08943