Adaptive Versus Incidental Explanations for the Occurrence of Protandry in Butterfly, Leptidea sinapis L.

Abstract

In many butterflies males emerge before females, a phenomenon known as protandry. Protandry could either be incidental, resulting for example, from size differences between the sexes or adaptive in some sense. In the butterfly L. sinapis, 1 of the 2 annual generations develops without, and the other with, a pupal diapause. Because diapause pupae overwinter in an undifferentiated stage and are synchronized during hibernation, protandry in the diapausing generation is solely accomplished by sex differences in post-diapause development time of the pupa. In the non-diapausing generation development is not interrupted and protandry results from sex differences in time of egg-larval plus pupal development. The incidental explanations predict that the sex difference in pupal development time is equal in the 2 generations; the adaptive explanations predict that this difference increases in the diapausing generation so as to equal that of egg-larval plus pupal develpment in the non-diapausing generation. The sex difference in pupal development time is significantly longer in the diapausing generation. It is close to the accumulated differences during egg-larval and pupal development in the non-diapausing generation. These findings refute the incidental explanations and support the adaptive ones. Selection for timing other life history events may under some circumstances be in conflict with the achievement of an optimal degree of protandry. The data indicate that there is a trade off between protandry and commencement of activity early in the season in diapausing females after a long winter.