CSIRO PUBLISHING www.publish.csiro.au/journals/ajz Australian Journal of Zoology , 2004, 52, 229–236 © CSIRO 2004 10.1071/ZO03043 0004-959X/04/030229 Behavioural thermoregulation in butterflies: the interacting effects of body size and basking posture in Hypolimnas bolina (L.) (Lepidoptera:Nymphalidae) Darrell J. Kemp A,B,C and Andrew K. Krockenberger A A School of Tropical Biology, James Cook University, PO Box 6811, Cairns, Qld 4870, Australia. Present address: Department of Zoology, Stockholm University, S-106 91, Stockholm, Sweden. B Department of Biology, Arizona State University, Tempe, AZ 85287-1501, USA. C To whom correspondence should be addressed. Email: darrell.kemp@zoologi.su.se Abstract 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. ZO03043 Bodysi zeand thermoregulationi nbut erflies D.J.KempandA.K.Krockenberger Introduction Body size at maturity is a key life-history trait that is closely tied to fitness in many organisms (Stearns 1992). Among the insects, most research into the reproductive consequences of adult body size has focused on readily quantifiable aspects of female fitness, such as fecundity, egg size and longevity (e.g. Wickman and Karlsson 1989; Honek 1993; Braby and Jones 1995). Although the situation is complex (e.g. Klingenberg and Spence 1997), and there is clearly much more to learn, the general principle emerging from these studies is that, due primarily to effects upon fecundity, adult fitness increases with increasing body size (although this advantage can be opposed, for example, by juvenile viability selection, resulting in stabilising net selection upon body size). This general adult size–fitness relationship, however, does not appear to extend to male insects. Empirical investigations into the reproductive consequences of variation in adult body size in this sex have routinely produced conflicting findings (see Rosenberg and Enquist 1991 v. Hernández and Benson 1998 v. Kemp 2000), and sometimes, failed to find any effect (Lederhouse 1982; Strohm and Lechner 2000). Whereas the fitness of female insects is generally prescribed by their ability to produce and lay eggs, the fitness of males depends upon their success in locating and mating with sexually receptive females, and in fertilising female gametes (Thornhill and Alcock 1983). Numbers of receptive females are in most cases greatly outweighed by sexually receptive