KROCHMAL, A.R.*; BAKKEN, G.S.; LADUC, T.J.; Indiana State University; Indiana State University; University of Texas, Austin: The role of facial pits in pitviper thermoregulation: behavioral and phylogenetic perspectives

Pitvipers (Viperidae: Crotalinae) possess unique organs, the facial pits, which allow them to detect subtle temperature fluctuations within their environments. It was previously thought that these organs were used solely to aid in prey acquisition. However, recent work has demonstrated that Western diamondback rattlesnakes (Crotalus atrox) use their facial pits to direct behavioral thermoregulation, suggesting that facial pits might be part of a generalized thermal sense used to drive a suite of behaviors. Alternatively, as C. atrox represents a highly derived pitviper which inhabits thermally-stressful areas, its ability to cue behavioral thermoregulation via the facial pits might represent a recent exaptation or a local adaptation to extreme thermal conditions. To investigate these issues, we conducted a phylogenetic survey of thermoregulatory behavior cued by thermal radiation. We assessed the thermoregulatory behavior of 12 pitviper species representing both key nodes in the evolution of pitvipers and a broad range of thermal environments and a single species of true viper (Viperidae: Viperinae), a closely related subfamily of snakes which lack facial pits but possess a putative thermal radiation receptor. All pitviper species were able to rely on their facial pits to direct thermoregulatory movements, while the one true viper tested was unable to do so. Our results suggest that facial pit-mediated thermoregulatory behavior is a universal role of facial pits and it likely represents an ancestral trait among pitvipers. These findings shed light onto the evolutionary origins of the crotaline facial pit.