Thermal ecology of Massasauga Rattlesnakes (Sistrurus catenatus) near their northern range limit Daniel S. Harvey and Patrick J. Weatherhead Abstract: Using populations at their northern range limits, we compared the thermoregulatory behaviour of Massasauga Rattlesnakes (Sistrurus catenatus (Rafinesque, 1818)) to published results for Ratsnakes (Elaphe obsoleta (Say in James, 1823)) and Northern Watersnakes (Nerodia sipedon (L., 1758)) to test the hypothesis that given similar benefits of thermo- regulation, costs associated with foraging ecology should shape thermoregulatory behaviour. More than 32 000 body tem- perature measurements from 34 individual snakes over 4 years were used to quantify thermoregulation by Massasauga Rattlesnakes using standard indices and a new index (%MaxE) that describes how much of the thermoregulatory potential available to a snake is realized. On average, Massasaugas were much cooler (db = 6.9 8C) than their preferred body tem- perature range (30–33.6 8C) but warmer (de – db = 3.3 8C) than were they not thermoregulating. Massasaugas realized more than half of their environmental potential for effective thermoregulation (%MaxE = 64%). Consistent with there being less conflict between foraging and thermoregulation for ambush predators than active foragers, Massasaugas were more effective thermoregulators than Ratsnakes or Watersnakes during the day. All three species were effective thermoreg- ulators at night, supporting the assumption that species in a cool environment will thermoregulate when there are few com- peting interests. Re ´sume ´: En utilisant des populations a ` la limite nord de leur aire de re ´partition, nous comparons le comportement thermore ´gulateur du crotale massasauga (Sistrurus catenatus (Rafinesque, 1818)) aux donne ´es publie ´es pour les couleuvres obscures (Elaphe obsoleta (Say in James, 1823)) et les couleuvres d’eau du Nord (Nerodia sipedon (L., 1758)) afin de tes- ter l’hypothe `se selon laquelle, lorsque les be ´ne ´fices de la thermore ´gulation sont semblables, les cou ˆts associe ´s a ` l’e ´cologie alimentaire devraient contro ˆler le comportement thermore ´gulateur. Plus de 32 000 mesures de la tempe ´rature corporelle chez 34 individus sur une pe ´riode de 4 ans nous ont servi a ` de ´terminer la thermore ´gulation chez les crotales massasaugas a ` l’aide d’indices standards et d’un nouvel indice (%MaxE) qui de ´crit la proportion du potentiel thermore ´gulateur dispo- nible au serpent qui est en fait re ´alise ´e. En moyenne, les massasaugas se retrouvent a ` une tempe ´rature beaucoup plus fraı ˆ- che (db = 6,9 8C) que leur gamme de tempe ´rature pre ´fe ´re ´e (30–33,6 8C), mais a ` une tempe ´rature supe ´rieure (de – db = 3,3 8C) a ` celle qu’ils auraient sans thermore ´gulation. Les massasaugas re ´alisent plus de la moitie ´ de leur potentiel envi- ronnemental de thermore ´gulation effective (%MaxE = 64 %). En accord avec le fait qu’il existe moins de conflit entre l’alimentation et la thermore ´gulation chez les pre ´dateurs qui chassent a ` l’affu ˆt que chez ceux qui recherchent activement leurs proies, les massasaugas font une thermore ´gulation plus efficace durant le jour que les couleuvres obscures et les cou- leuvres d’eau. Les trois espe `ces font une thermore ´gulation efficace la nuit, ce qui appuie la proposition que les espe `ces place ´es dans un environnement frais vont faire de la thermore ´gulation s’il y a peu d’autres inte ´re ˆts en compe ´tition. [Traduit par la Re ´daction] Introduction The thermal dependence of physiological processes makes body temperature (T b ) regulation a fundamental aspect of ectotherm ecology (Huey 1982; Heatwole and Taylor 1987; Peterson et al. 1993). In addition to contributing to our gen- eral knowledge of ectotherm ecology, understanding the thermoregulatory behaviour of ectotherms is also important to anticipating how ectotherm populations are likely to be affected by climate warming (Deutsch et al. 2008; Kearney et al. 2009). Although for some time we have had both a general theory (Huey and Slatkin 1976) and the analytical tools for quantifying thermoregulation (Hertz et al. 1993; Christian and Weavers 1996; Brown and Weatherhead 2000; Blouin-Demers and Weatherhead 2001a), comprehen- sive empirical studies of the thermal ecology of free-living reptiles remain scant. Two general research approaches for identifying the ecological factors that shape thermoregula- tory strategies are to document how the thermal ecology of the same species varies across environmental gradients, or to compare how species that differ ecologically deal with sim- ilar thermal challenges (Blouin-Demers and Weatherhead 2001a). Our general goal here was to follow the latter ap- proach and compare the thermal ecology of three species of snakes near their northern range limits. We do so by documenting daily, seasonal, and sex-specific patterns of Received 7 January 2010. Accepted 26 October 2010. Published on the NRC Research Press Web site at cjz.nrc.ca on 21 December 2010. D.S. Harvey 1,2 and P.J. Weatherhead. Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL 61801, USA. 1 Corresponding author (e-mail: harvey_da@yahoo.ca). 2 Present address: 2395 Conservation Road, Campbellville, ON L0P 1B0, Canada. 60 Can. J. Zool. 89: 60–68 (2011) doi:10.1139/Z10-094 Published by NRC Research Press