Daily and annual patterns of thermoregulation in painted turtles (Chrysemys picta marginata) living in a thermally variable marsh in Northern Michigan

Abstract

The capacity for an ectothermic reptile to thermoregulate has implications for many components of its life history. Over two years, we studied thermoregulation in a population of Midland painted turtles (Chrysemys picta marginata) in a shallow, thermally variable wetland during summer in Northern Michigan. Mean body temperature (Tb) of free-ranging turtles was greater in 2008 (25.8°C) than in 2010 (19.7°C). Laboratory determined thermoregulatory set point (Tset) ranged from 25°C (Tset-min) to 31°C (Tset-max) and was lower during the fall (17–26°C). Deviations of Tb distributions from field measured operative temperatures (Te) and indices of thermoregulation indicated that C. picta marginata were capable of a limited degree of thermoregulation. Operative temperatures and thermal quality (de=|Tset-min−Te| and |Te−Tset-max|) cycled daily with maximal thermal quality occurring during late morning and late afternoon. The accuracy of thermoregulation (db=|Tset-min−Tb| and |Tb−Tset-max|) was maximal (db values were minimal) as Tb declined and traversed Tset during the late afternoon–early evening hours and was higher on cloudy days than on sunny days because relatively low Te values decreased the number of Tb values that were above Tset. Our index of thermal exploitation (Ex=frequency of Tb observations within Tset) was 36%, slightly lower than that reported for an Ontario population of C. picta marginata. Regression of db (thermal accuracy) on de (thermal quality) indicated that turtles invested more in thermoregulation when thermal quality was low and when water levels were high than when they were low. There were no intersexual differences in mean Tb throughout the year but females had relatively high laboratory determined Tb values in the fall, perhaps reflecting the importance of maintaining ovarian development prior to winter.