NOTE / NOTE Minimal overwintering temperatures of red-sided garter snakes ( Thamnophis sirtalis parietalis): a possible cue for emergence? Deborah I. Lutterschmidt, Michael P. LeMaster, and Robert T. Mason Abstract: Red-sided garter snakes (Thamnophis sirtalis parietalis (Say in James, 1823)) in Manitoba, Canada, undergo 8 months of continuous winter dormancy prior to spring emergence. As in other ectothermic species, increases in ground temperature may be the cue for emergence from winter dormancy in these populations. To test this hypothesis, we meas- ured body temperatures during winter dormancy by surgically implanting small temperature loggers into 32 female red- sided garter snakes before they entered their native hibernaculum. The following spring, we recaptured seven of the snakes implanted with temperature loggers. Body temperature declined gradually from mid-September (14.7 ± 0.24 8C, mean ± SE) to early April (1.1 ± 0.16 8C, mean ± SE) during winter dormancy, reaching minimal values approximately 1 month prior to spring emergence. Body temperatures of emerging snakes ranged from 0.5 8C during early spring to 6.3 8C during late spring (3.4 ± 0.84 8C, mean ± SE). These results do not support the hypothesis that an increase in ground temperature (and hence body temperature) is necessary for emergence from winter dormancy. We suggest that critically low tempera- tures (i.e., 0.5–1 8C) are a Zeitgeber entraining an endogenous circannual cycle that regulates snake emergence. These re- sults offer new insight into the mechanisms regulating seasonal emergence from winter dormancy. Re ´sume ´: Les couleuvres raye ´es a ` flancs rouges (Thamnophis sirtalis parietalis (Say in James, 1823)) du Manitoba, Can- ada, comple `tent 8 mois continus de dormance d’hiver avant leur e ´mergence au printemps. Comme c’est le cas chez d’autres espe `ces ectothermes, l’accroissement de la tempe ´rature du sol est peut-e ˆtre le signal pour l’e ´mergence de la dorm- ance hivernale chez ces populations. Afin de ve ´rifier cette hypothe `se, nous avons mesure ´ la tempe ´rature corporelle de 32 couleuvres raye ´es a ` flancs rouges femelles durant la dormance d’hiver en leur inse ´rant par chirurgie de petits enregistreurs de tempe ´rature avant qu’elles n’entrent dans leurs hibernacles d’origine. Le printemps suivant, nous avons re ´cupe ´re ´ sept des couleuvres porteuses d’enregistreurs de tempe ´rature. La tempe ´rature corporelle de ´cline graduellement de la mi-septembre (14,7 ± 0,24 8C, moyenne ± erreur type) au de ´but d’avril (1,1 ± 0,16 8C, moyenne ± erreur type) durant la dormance d’hiver, atteignant un minimum environ un mois avant l’e ´mergence printanie `re. La tempe ´rature corporelle des couleu- vres a ` l’e ´mergence varie de 0,5 8C au de ´but du printemps a ` 6,3 8Ca ` la fin du printemps (3,4 ± 0,84 8C, moyenne ± erreur type). Ces re ´sultats n’appuient pas l’hypothe `se qui veut qu’un accroissement de la tempe ´rature du sol (et par conse ´quent de la tempe ´rature corporelle) soit ne ´cessaire pour l’e ´mergence de la dormance d’hiver. Nous croyons que les tempe ´ratures basses critiques (c.-a `-d., 0,5–1 8C) agissent comme Zeitgeber pour e ´tablir un cycle circannuel qui re `- gle l’e ´mergence des couleuvres. Ces re ´sultats ouvrent de nouvelles perspectives sur les me ´canismes re ´gulateurs de l’e ´mergence saisonnie `re de la dormance d’hiver. [Traduit par la Re ´daction] Introduction Most vertebrates exhibit some seasonality in many behav- ioral and physiological processes. One of the most reliable environmental cues thought to function in regulating season- ality in vertebrates is photoperiod. Unlike other environmen- tal signals (e.g., temperature and humidity) that can vary quite dramatically both within seasons and among years, changes in photoperiod length accurately and reliably reflect changing environmental seasons. Many ectotherms inhabit- ing north-temperate climates, however, undergo periods of prolonged winter dormancy prior to spring breeding. Animals that occupy underground hibernacula during winter dor- mancy are not exposed, or receive little exposure, to chang- ing photoperiodic conditions (e.g., Whittier et al. 1987; Grobman 1990). Thus, photoperiod is likely not a critical Received 10 November 2005. Accepted 2 March 2006. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 6 June 2006. D.I. Lutterschmidt 1 and R.T. Mason. Department of Zoology, Oregon State University, Corvallis, OR 97331, USA. M.P. LeMaster. Department of Biology, Western Oregon University, Monmouth, OR 97361, USA. 1 Corresponding author (e-mail: luttersd@science.oregonstate.edu). 771 Can. J. Zool. 84: 771–777 (2006) doi:10.1139/Z06-043 # 2006 NRC Canada