An organism’s capacity to cope with variable temperature will depend on its ability to balance physiological constraints against a suite of dynamic biological, ecological and environmental factors. Recent meta-analyses suggest physiological adaptation to local temperatures may largely be constrained by phylogeny, yet a paucity of thermal performance data from free-ranging animals is a major hindrance to predicting the response of various taxa to future increases in temperature.

We compared thermal performance (activity) and habitat selection in two species of wild stingrays that have different biogeographies (the temperate Trygonoptera testacea and sub-tropical Dasyatis fluviorum), inhabiting the same estuary. We predicted that regional adaptation would result in similar thermal thresholds, whereas divergent thresholds would indicate phylogenetic constraint.

For the temperate T. testacea, performance increased up to 23.9°C and declined thereafter. For the tropical D. fluviorum, performance continued to increase up to the maximum temperature recorded (26.4°C). We also found evidence for thermal-habitat selection, in which T. testacea exhibit avoidance of temperatures above their thermal optimum, compared to D. fluviorum that showed no avoidance. While D. fluviorum were diurnal, T. testacea were nocturnally active, with their degree of nocturnality linking to a temperature-dependent increase, followed by a potential loss of locomotive performance beyond the thermal optimum.

These data indicate that sympatric, ecologically similar species can have divergent physiological and behavioural responses to local temperature. This pattern highlights the constraints that phylogeny can impose on thermal performance thresholds, and suggests that future increases in temperature are likely to have differential impacts within taxonomic groups.