Tough toad ahead: ontogenetic variation in thermal tolerance defines life-stage vulnerability in Duttaphrynus melanostictus

Abstract

Environmental temperature can critically impact the performance and survival of ectotherms. Assessing the ability to tolerate increasing temperatures and determining coping strategies, especially in vulnerable taxa such as amphibians, is therefore crucial. Yet, little is known about the thermal traits of amphibians in megadiverse regions such as Asia. Here, we studied the widespread Asian Common Toad, Duttaphrynus melanostictus, and examined the role of behaviour in coping with extreme heat in adults and tadpoles. When provided with a thermal gradient, tadpoles avoided temperatures close to their CT_max, spending ∼96% of their time in the cooler water at the bottom of the water column, whereas adults selected lower temperatures under dry conditions compared to wet conditions. We also determined thermal tolerance limits across three ontogenetic stages: tadpoles, juveniles, and adults. The thermal tolerance limits of this species ranged from 6 to 43 °C. Critical thermal maxima (CT_max) varied across life stages, with the highest values observed in tadpoles, followed by adults and juveniles. Critical thermal minima (CT_min) were lowest in adults, followed by juveniles and tadpoles. Juveniles had the narrowest thermal breadth compared to adults and tadpoles. Thermal tolerance limits were independent of sex and body size in adults. Our work provides a detailed understanding of the thermal envelope for the widespread yet understudied tropical amphibian, D. melanostictus. By also examining thermoregulatory behaviours, our work presents insight into how thermal traits shift across life stages, with implications for forecasting species responses to future warming.