Behavioural thermoregulation in butterflies:: the interacting effects of body size and basking posture in Hypolimnas bolina (L.) (Lepidoptera: Nymphalidae)

The evolutionary significance of body size variation in male insects is often obscure. One way in which this parameter could affect reproductive success is via its relevance to thermoregulation. In this study we investigated the relevance of body size to heat exchange rates in a tropical nymphalid, the common eggfly (Hypolimnas bolina) (L.). Males of this territorial species elevate their body temperature above ambient levels via a series of basking postures coupled with strategic choice of perching microhabitat. In an experiment with dead butterfly models we found, as expected, heightened rates of heat exchange (heating and cooling rates) in smaller individuals. There was also a significant interaction between basking posture and body size, with smaller individuals exhibiting significantly greater variation in heating rate with increasingly open wing postures. This suggests that smaller males would have greater control over their rate of basking heat gain ( by having at their disposal a greater potential range of heating rates), but they would also radiate body heat at a higher rate than their larger conspecifics. Using 'grab and stab' techniques, we found no evidence that smaller individuals are closer to their putative thermal optimum under a range of ambient conditions in the field. However, a more substantive field program, incorporating a more precise characterisation of the ambient thermal environment, will be required to fully evaluate the thermal significance of body size variation in males of this territorial butterfly.