Summary
Basking is a common thermoregulatory behaviour in many ectotherms, including reptiles. Because the key physiological processes affecting net energy retention (NER) are temperature dependent, ectotherms have the potential to modulate their energy budget by using basking behaviour. Many aquatic chelonians bask extensively. The energetic significance of basking is, however, largely unknown. We used biologging to measure the body temperature of free-ranging juvenile northern map turtles in Ontario, Canada. We measured the contribution of basking behaviour to the ability of turtles to reach their optimal body temperature for NER. We also used the predicted standard metabolic rate as a proxy to estimate the effects of basking on NER. Our results show that basking is essential for turtles to reach the optimal temperature for NER and suggest that basking behaviour allows turtles to increase their metabolic rate by 17.2 to 30.1%, which should translate into an even greater increase in NER. In addition, our results show that basking behaviour allows turtles to buffer the effects of climatic variations on their Tb and thus potentially on their energy budget. Collectively, our results suggest that basking behaviour has important ramifications for the energy budget, and by extension the fitness, of temperate-zone turtles.
Methodology
We studied northern map turtles in Lake Opinicon, a small mesotrophic lake at the Queen’s University Biological Station 100 km south of Ottawa, Ontario, Canada. From mid April to early May 2005 and 2006, we captured northern map turtles at a communal hibernation site. We surgically implanted temperature data loggers (Thermocron iButton DS 2422; Dallas Semiconductor, Sunnyvale, California, USA) in the abdominal cavity of 14 juvenile females (9 in 2005 and 5 in 2006). Details of the anaesthetic and surgical procedures are provided by Edwards and Blouin-Demers (2007). The loggers recorded Tb every 25 min between May and October. Each turtle with an implanted logger was also equipped with a radio-transmitter (SI-2FT or SB-2FT; Holohil Systems, Carp, Ontario, Canada). The radio-transmitters were bolted to the rear marginal scutes using stainless steel bolts. The combination of logger and radio-transmitter did not exceed 5% of the turtle’s mass. The fall or spring following logger implantation, we recaptured turtles to remove the transmitters and the loggers. Analyses were performed on Tb collected between May 15 and August 15 of each year.
The turtles used in this study were small and thus rapidly reached thermal equilibrium with water when submerged. For instance, painted turtles of equal size to our juvenile northern map turtles (ca 400 g) need approximately 11 min to cool 10 °C when submerged in water (Costanzo, 1982). We thus assumed that any time a turtle’s Tb was above the maximum surface temperature of the water (Smax), the turtle was emerged and thus basking. We calculated hourly mean Tb for each individual. From the hourly means, we calculated the percentage of time spent basking for each individual as the percentage of Tb measurements exceeding Smax. Thus, we obtained a single measurement of time spent basking per individual per year. In our calculation of the percentage of time spent basking, we excluded nighttime Tb measurements (1900 to 0700) because basking only occurs during the day. To obtain hourly measurements of Smax, we measured water surface temperature at four locations in the lake with temperature loggers (Thermocron iButton DL 1922; Dallas Semiconductor, Sunnyvale, California, USA). We calculated hourly Smax as the maximum surface temperature of the 4 locations. The 4 locations were: 1 deep (7 m) open water site, 2 shallow open water sites (2–3 m), and 1 shallow (1 m) marsh. These sites were selected to capture the range of possible surface temperatures in the lake.