Pitvipers are probably some of the scariest creatures on earth. Equipped with a deadly set of fangs and thermally sensitive facial pits, they can always track down a warm meal. Aaron Krochmal explains that until recently it was thought that prey detection drove the evolution of the facial pit receptors, but important new research is now challenging this assumption(p. 4231).
During his previous work at Indiana State University with George Bakken,Krochmal discovered that one pitviper species, the western diamondback rattlesnake, uses its facial pits to detect cool refuges to shelter from excessive heat, suggesting that these thermal radiation receptors may actually be a more general-purpose organ than previously thought. So Krochmal, Bakken,and their colleague Travis LaDuc, decided to go one step further. They tested whether other pitvipers' facial pits also allowed them to seek out cooler corners.
Krochmal and his colleagues chose 12 diverse species of pitviper to work with, and one closely related subfamily of true viper, the puff adder. Puff adders do not have facial pits. Instead, they are equipped with supranasal sacs, which may be able to detect thermal radiation from prey, but it was unclear whether they were involved in behavioural thermoregulation.
To test the snakes' ability to differentiate between hot and cold environments, the team designed a Y-shaped maze made of plumbing pipes for the vipers to move through, and placed it inside an environmental chamber at a scorching 40°C. By cooling the end of one branch of the structure to 30°C, they made a heat `refuge'. After releasing a snake into the bottom of the Y, the team waited to see which environment the reptile chose.
All species of pitviper tested moved toward the cool refuge and avoided the hot branch of the maze, suggesting that they could determine heat differences from a distance. The team also made absolutely sure that the snakes were using their receptors to choose between the hot and cooler branches by temporarily disabling their facial pits. They insulated each of the viper's facial pits with a small polystyrene ball, topped by a tiny piece of aluminium foil. Krochmal admits that this part of the study was probably the most nerve wracking, and, being acutely aware of the dangers of working with such venomous vipers, he'd spent the year prior to their arrival, `dreaming up ways to keep from dying'.
Fortunately, the planning paid off. Deprived of their facial pits, the pitvipers failed to home in on their cooler refuge, only ending up in the 30°C refuge by chance; the facial pits were involved in behavioural thermoregulation. However, when the team put the puff adders to the Y-test,these true vipers only found their way to the cooler refuge by chance. They weren't capable of behavioural thermoregulation using thermal radiation cues.
Krochmal and his colleagues were very excited. `When we started testing we saw that all pitvipers were able to make a thermoregulatory decision based on radiation and we really knew we were on to something; that what we had been seeing was unique to pitvipers, and an ancestral trait' he says. This uniqueness also suggests that this behaviour may have evolved at the same time as the facial pits. Maybe those pits are more than just prey detectors after all.