Life history variation in the black swallowtail butterfly

The growth, development, and reproduction of black swallowtail butterflies (Papilio polyxenes) from Turrialba, Costa Rica and Brooktondale, New York are compared. When New York larvae are reared under a short photoperiod, they produce pupae that diapause and are smaller than those produced under long photoperiods. As a consequence, reproduction in the field is limited to two broods per summer and the adults of Brood I are smaller. Costa Rican larvae exhibit no response to photoperiod and breeding occurs throughout the year. Dispersal of adults in Costa Rica is thought to serve a function analogous to that of diapause in New York: to bridge temporal or spatial gaps between favorable habitats. Under favorable conditions, females from both locations grow rapidly, lay several hundred eggs soon after adult eclosion, and are short-lived, as expected for a colonizing species. However, differences occur in three specific life history traits: adult females from Brood I in New York are similar in size to Costa Rican females, but lay fewer eggs and at a slower rate; adult females from Brood II in New York are larger than Costa Rican females, but have a similar rate of egg production and total fecundity; the eggs of New York females are larger than those of Costa Rican females. It is unclear whether or not these differences reflect divergent adaptation of the life history to different environments. An adaptive hypothesis that emphasizes selection for early age at first reproduction in Costa Rica and for high fecundity in New York, and predicts disrupted latitudinal clines in size, is proposed to account in part for the observations reported here.