Introduction

Body size has profound consequences for animal life history and ecology (Bonner, 1965; Peters, 1983; Calder, 1984; Schmidt-Neilsen, 1984; Damuth, 1987), so it is very important to understand exactly how natural selection acts in the evolution of this character. Temperature is a crucial aspect of the environment that appears to influence body size in two ways. First, temperature may be an agent of natural selection in producing evolutionary (genetic) changes in the developmental mechanisms that control growth rate and adult size and, second, the thermal conditions during an individual's development may affect its final adult size. Thermal evolution of body size has been discussed mainly in the context of endotherms (principally mammals), where the relationships between the surface area and volume of the body are important in determining rates of heat production and dissipation, and hence in maintaining the standard body temperature. It has long been maintained that endotherms tend to evolve larger body size in colder conditions (Mayr, 1963): an idea traditionally termed ‘Bergmann's rule’ (Bergmann, 1847). This generalisation, however, has since been questioned (Rails & Harvey, 1985), and the relationships between body size and thermal ecology within and between endothermic species await rigorous comparative analysis.

Thermal evolution of body size is not restricted to endotherms, however, as several species of ectotherm show clear geographical clines in body size, with the larger individuals found in populations derived from higher latitudes, even when all are reared in standard conditions.