Thermoregulation in a North Temperate Population of Midland Painted Turtles (Chrysemys picta marginata): Temporal Patterns and Intersexual Differences
The regulation of body temperature (Tb) is important for energy acquisition in ectotherms but may be challenging and costly. We studied the thermal biology of a north temperate population of Midland Painted Turtles (
Chrysemys picta marginata
) in a small pond in central Michigan. Cycling of mean daily body temperature (Tb) began as early as March and continued through October. The thermoregulatory setpoint range was 25–31°C, and turtles largely maintained Tb near or within the Tset range throughout the day between May and September. Mean hourly Tb was slightly (+0.5°C) higher on sunny days when compared to cloudy days. Turtles had relatively low investment in thermoregulation during the fall months potentially as an energy conservation measure prior to hibernation. Thermal exploitation values (Ex) indicated that our turtles spent as much, or more, time within the Tset range than individuals in other north temperate Painted Turtle populations and other reptile species studied to date, which was most likely due to the relatively high thermal quality of our aquatic environment. We did not find differences between males and females in terms of thermal exploitation, which suggests little or no differences in thermal energy needs that might be associated with potential energetic allocations to reproductive activities (e.g., ovarian development or mate location in males).
Mean (± SE) daily atmospheric biophysical basking model (Tmodel) and water column temperatures (Tw). Data are a composite of readings recorded between 2010 and 2013 and averaged by day-of-year.
Diel variations in (A) adjusted mean hourly operative temperatures (Te) under different weather conditions during summer (June–August; 2010–2013), (B) hourly water and biophysical model temperatures measured during summer, and (C) least square mean hourly Te values during spring and fall. The shaded regions represent spring and summer Tset and fall Tset is bounded by the dashed lines.
Thermal quality (de = |Te–Tset|) at Clark Pond expressed as mean hourly values during (A) summer (June–August) and under variable weather conditions and (B) spring and fall.
Mean hourly thermal quality values (de = |Te–Tset|) for the (A) water column (average of shallow and deep temperatures), (B) atmospheric basking model, and (C) partially submerged model measured between April and October at Clark Pond.
Adjusted mean hourly Tb in C. picta marginata during (A) summer (June–August, n = 12,274 hourly observations in 17 turtles, 2010–2013) and (B) spring (April, n = 775 hourly observations in 3 turtles 2011 and 3 in 2012; May, n = 857 hourly observations in 3 turtles in 2011 and 4 in 2012) and fall (September, n = 2,297 hourly observations in 15 turtles, 2010–2012; October, n = 3,378 hourly observations in 12 turtles, 2010–2012). The shaded regions represent spring and summer Tset and presumed fall Tset is bounded by the dashed lines.
Adjusted mean (± SE) hourly Tb in C. picta marginata derived from GLMs (by month) that included sex, hour-of-day, and daily weather conditions as main effects and all interaction terms.
Adjusted mean (±SE) derived from GLMs of (A) daily mean Tb of C. picta marginata derived from hourly observations averaged per individual per month and (B) daily mean, minimum, and maximum Tb values recorded at 15 min intervals for 17 turtles, 2010–2013.
Mean (± SE) percentage of the total number of Tb observations tallied below, within, and above Tset for each individual during each month (A) overall and (B) when Te values were within Tset.
Distributions of Tb and Te values in 5°C increments for (A) spring months, (B) summer months, and (C) fall months.
Mean (± SE) hourly (A) number of C. picta marginata observed basking at 15 min intervals and averaged per day and then per month and (B) average hourly Te recorded simultaneously during the recorded basking periods.
Contributor Notes
Associate Editor: C. Bevier.