Most creatures' faces have the same standard arrangement of features; eyes at the top, nose beneath and mouth at the bottom. But pitvipers carry an extra set of facial indentations: facial pits. Staring into the dark hollows of these depressions, an age-old question glares back: what are they for? With thermally sensitive nerve endings concentrated in the pits, these organs respond to radiant heat, and allow snakes to detect and capture prey. But Aaron Krochmal thinks that this simple explanation could have distracted people from exploring alternative functions for the pits. Could they be involved in thermoregulation? Cold-blooded animals exploit their environment to regulate their body temperatures; they seek cool spots to escape the heat or warm places when it's cold. But little was known about the sensory cues that guide this behavior. Krochmal wondered whether the snakes might use their facial pits to direct them towards the sanctuary of a cool burrow(p. 2539).
Western diamondback rattlesnakes live in the sweltering deserts of the southwestern United States. If there were ever a place where snakes needed facial pits to help them find relief from the heat, this would be it. But before Krochmal and his advisor, George Bakken, could begin to test the snakes' ability to locate cool refuges, they had to win their cooperation. This was not an easy task; the snakes coiled into a defensive posture whenever they were placed in one of the open test arenas. But once they discovered that the snakes could be successfully moved into the arena in a cloth-covered carrying cage, Krochmal was ready to test the rattlesnakes'thermosensitivity.
In a series of tests, Bakken and Krochmal offered the snakes choices between hot or cool resting places, recording which direction the snakes moved. In the critical experiment, which simulated natural conditions, they included four artificial burrows for the snakes to choose between, one of which was cooled to 30°C to offer a cool refuge. Seeking relief, the snakes moved towards the cool burrow, which they detected from a distance of 1 m. The team then ingeniously blocked the animal's facial pits with an insulating polystyrene ball and a piece of aluminum to reflect thermal radiation. Krochmal and Bakken then gave the snakes the same choice of burrows but, this time, the snakes couldn't locate the cooler burrow to seek refuge from the heat. Fortunately, when the foil and polystyrene balls were removed,the hot snakes quickly found the cooler burrow again. The snakes were using their thermosensitive facial pits to seek refuge from the heat of the day.
With a newly documented function for crotaline facial pits, researchers will have to reconsider how they may have evolved. Krochmal suggests that facial pits may have first evolved for behavioral thermoregulation and were later recruited for prey detection. Alternatively, facial pit evolution may have been driven by some yet unidentified, alternative behavioral role. Phylogenetic studies, comparing facial pit use and thermoregulatory behavior across species, may well pave the way to understanding the evolutionary origin of this unique sensory organ.