Why get big in the cold? Towards a solution to a life-history puzzle

The temperature-size rule (TSR), which states that body size increases at lower developmental temperatures, appears to be a near-universal law for ectotherms. Although recent studies seem to suggest that the TSR might be adaptive, the underlying developmental mechanisms are thus far largely unknown. Here, we investigate temperature effects on life-history traits, behaviour and physiology in the copper butterfly Lycaena tityrus in order to disentangle the mechanistic basis for the above rule. In L. tityrus the larger body size produced at a lower temperature was proximately due to a greater increase in mass, which was caused by both behavioural and physiological mechanisms: a much-increased food intake and a higher efficiency in converting ingested food into body matter. These mechanisms, combined with temperature-induced changes at the cellular level, may provide general explanations for the TSR. Body fat and protein content increased in butterflies reared at the higher temperature, indicating favourable growth conditions. As predicted from protandry theory, males showed reduced development times, caused by higher growth rates compared to females. The latter was itself related to a higher daily food consumption, while the total food consumption (due to the females' longer developmental period) and assimilation was higher in females and may underly the sexual body size dimorphism.