Heat tolerance in ectotherms scales predictably with body size

Abstract

Recent studies suggest that animals are decreasing in size as a general response to global warming, for reasons that remain unclear. Here, by analysing ectotherm death time curves that take into consideration the intensity and duration of a thermal challenge, we show that heat tolerance varies predictably with size. Smaller animals can maintain higher body temperatures than larger ones during short periods, but cannot maintain higher body temperatures over long periods as their endurance declines more rapidly with time. Body size effects and adaptive variation in heat tolerance may have been obscured in the past by these unaccounted for temporal effects. With increasing size, thermal death occurs at relatively lower metabolic rates with respect to rest at a non-stressful temperature, which might partly explain the reported reductions in organism size with climate warming and shed light on the mechanisms that underlie scaling. Analysis of ectotherm thermal death curves in the context of both challenge intensity and duration shows that smaller animals exhibit higher tolerance to acute stress, but lower tolerance to chronic stress. The size-dependent impact provides one explanation for warming-related reductions in animal size.