Abstract
Thermoregulatory behaviour determines an organism's body temperature and therefore its physiological condition, and may differ for organisms situated across climate gradients. Species' preferred or selected temperatures may be higher in warmer locations-referred to as coadaptation-or lower in warmer temperatures-countergradient variation. Here, we tested if rainforest amphibians exhibited coadaptation or countergradient thermal selection across an underappreciated spatial climate gradient (vertical height from forest floor to canopy) and separating diel activity (diurnal versus nocturnal behaviour). We captured 2534 amphibians over 216 ground-to-canopy surveys, and conducted 282 thermal selection assays for 37 species while pairing microclimate measurements and mechanistic model predictions to understand vertical and daily thermal variation in the field. Amphibians exhibited countergradient thermal selection: species occupying cool nocturnal conditions in canopies selected warmer temperatures than species occupying hot diurnal conditions at the forest floor. Furthermore, amphibians selected warmer temperatures than the average conditions that they were exposed to when active, and this divergence was especially high for nocturnal arboreal species (8.68°C). This suggests that rainforest amphibians dramatically underfill the warm end of their thermal niches, a trend across local thermal gradients that reflects recent findings across elevational and latitudinal gradients. We show that considering multidimensional climate gradients is important to evaluate thermoregulatory behaviour, and its evolutionary underpinnings, for understanding species' niches and community assembly.