Growth Strategies and Optimal Body Size in Temperate Pararginii Butterflies

In temperate insects the evolution of growth strategies and the optimal age and size at maturity will depend strongly on seasonal variation in temperature and other resources. However, compared to photoperiod, temperature itself is a relatively poor predictor of seasonal change and timing decisions in insects are often most strongly influenced by the photoperiod. Here I review the evolution of seasonal growth strategies in the butterfly tribe Pararginii (Satyrinae: Nymphalidae) and relate it to life history theory. The results indicate that individual larvae may adjust their growth trajectories in relation to information on time horizons obtained from the photoperiod. The growth strategies can be characterized by a set of state-dependent decision rules that specify how an individual should respond to its internal state and external circumstances. These decision rules may also influence how individual growth change with a rise in temperature, showing that the standard expectation of increased growth rates with increasing temperatures may not always be true. With less time available individual larvae increase growth rates and thereby achieve shorter development times, most often without any effects on final sizes. One reason for the apparent optimization of growth rate seems to be that growing fast may incur costs that larvae developing under lower time limitations chose to avoid. The patterns of growth found in these and many other studies are difficult to reconcile with common assumptions of what typically determines optimal body size in insects. In particular it seems as if there should be some costs of a large body size that, so far, have been poorly documented.