We tested a common prediction of the thermal coadaptation hypothesis in the Northern Map Turtle (Graptemys geographica), an aquatic emydid with pronounced aerial basking. We measured the effect of body temperature on two locomotor performances (swimming and righting) to determine optimal temperature of performance (To) for each trait. According to the thermal coadaptation hypothesis, the preferred body temperature range (Tset) of ectotherms should match To of thermally sensitive traits that influence fitness. However, we predicted that preferred basking temperature and locomotor performance of Northern Map Turtles would not be coadapted, given that basking occurs on land and locomotion in water. We also tested for an ontogenetic shift in performance curves. We found that adult Northern Map Turtles have a wide Tset (19–30°C), both hatchlings and adults can achieve near-maximum performance over a wide range of temperatures, and an ontogenetic shift is present for swimming but not for righting. Although To for the two locomotor performances of adult turtles were within Tset, the large range of Tset coupled with the wide breadth in locomotor performance makes falsifying or supporting the coadaptation hypothesis difficult for these traits in Northern Map Turtles. Other metabolic and physiological processes need to be considered to understand more fully thermal coadaptation in aquatic emydids.
Study Site and Study Animals. - We conducted this study in 2005 and 2006 at the Queen's University Biological Station, approximately 100 km south of Ottawa, Ontario, Canada. Adult male Northern Map Turtles (N = 44) were obtained by snorkeling and from basking traps in Lake Opinicon. Males used in this study ranged from 87-135 mm in carapace length and from 72-244 g in mass. Turtles were housed in groups of 2-3 in a field laboratory in large (80 X 80 X 80 cm) basins filed with lake water (24- 27°C). We placed rocks in the basins to allow turtles to leave the water, and, although no radiant heat source was provided, turtles regularly exhibited typical basking behavior. The lab received natural sunlight from windows, and the ambient temperature was maintained at 24-26°C. Turtles were fed mealworms every other day and were released at their point of capture immediately after their trial was completed. Different individuals were used for the measurement of locomotor performance (2005) and the measurement of Tset (2006), but locomotor performance and Tset were measured in the same month (August) to ensure that individuals from each year had experienced a comparable thermal history. The hatchlings used in this experiment were obtained from the same population as the adult turtles. Gravid female Northern Map Turtles were captured during the 2005 nesting season (May to July) and induced to lay eggs with an intramuscular injection of oxytocin (20 USP units/ml, 0.5 ml/kg; Ewert and Legler, 1978). All clutches were incubated in moist vermicu- lite (1:1 ratio by mass of water: vermiculite) at 29°C. Twenty-four hatchlings from eight clutch- es were randomly selected for performance trials and were transferred to the laboratory at the University of Ottawa in September. Hatchling turtles used in this study ranged from 29.4r- 34.2 mm in carapace length and from 6.7-9.1 g in mass. The hatchlings were housed individu- ally in small (20 X 30 X 15 cm) plastic containers in an environmental chamber (Constant Temperature Control, Inc., Aurora, ON, Canada) and fed a diet of mealworms and bloodworms. The temperature of the chamber was set between 22°C and 25°C from September until the hatchlings were tested for performance in March. We selected this temperature such that the hatchlings would be acclimated (sensu Lagerspetz, 2006) to the same temperature as the adults (lake temperature during testing 21- 26°C), even though most of the evidence suggests that ectotherms acclimate poorly for locomotor performance (reviewed in Bennett, 1990; but see Glanville and Seebacher, 2006). All procedures were approved by the Animal Care Committee at the University of Ottawa (protocol BL-179).
Measurements of Preferred Basking Temperature. - Aquatic gradients, in which turtles can settle at one temperature, have been used traditionally to measure selected temperature for aquatic turtles (Nutting and Graham, 1993; Nebeker and Bury, 2000, 2001). However, Tset determined in such gradients is not likely representative of preferred basking temperature because the turtle cannot leave the water, and aquatic emydids predominantly use shut- tling between water and land to maintain their Tset in nature (Boyer, 1965). In addition, the upper and lower bounds of the Tset range determined with a gradient (e.g., central 50% or 80% of the Tb distribution) are arbitrary. To circumvent these limitations and to obtain a direct measure of preferred basking tempera- ture, we used a basking arena similar to the "shuttle box" proposed by Kingsbury (1993).