Are Locomotor Performances Coadapted to Preferred Basking Temperature in the Northern Map Turtle (Graptemys geographica)?

Abstract

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.