ABSENCE OF TRADE-OFFS BETWEEN SEXUAL SIZE DIMORPHISM AND EARLY MALE EMERGENCE IN A BUTTERFLY

Protandry, here defined as the earlier emergence of males, is a common feature in life histories and could be the result of sexual selection on males to maximize matings, or alternatively an incidental by-produ ct of other selection pressures on the sexes. If protandry is selected for per se, theory predicts that it should be associated with seasona l environments where there is little overlap between generations. The degree of protandry should be insensitive to environmental conditions. Moreover, on the assumption that males and females grow at the same r ate as larvae, a trade-off between development time and size is expect ed to result in a strong association between protandry and female-bias ed sexual size dimorphism. These predictions were tested by a combinat ion of comparative and experimental studies on five populations of the speckled wood butterfly, Pararge aegeria, from central and south Swed en, England, Spain, and the island of Madeira. Protandry was associate d with seasonal environments, as it was only exhibited in the three no rthernmost populations. Protandry in these populations remained largel y constant in a variety of temperatures, both under direct development , when protandry results from a sex difference in development time thr ough the egg, larval, and pupal stages, and under diapause development , when it results from a sex difference in pupal development time only . These results indicate that protandry is selected for per se through sexual selection in seasonal environments. Similar female-biased size dimorphism occurred in protandrous and non-protandrous populations al ike, and hence sexual size dimorphism in P. aegeria is not a result of selection for protandry, nor the causal factor behind protandry. Prot andry and sexual size dimorphism appear to be largely decoupled traits in the life history evolution of P. aegeria. This is achieved by mean s of variation in pupal developmental time and variation in the relati ve growth rates of the sexes. Variation in growth rates is likely to b e a general phenomenon and may make possible independent optimization of size and development time (age at sexual maturity), and accordingly influence expected patterns of size-related trade-offs.