Early male emergence and reproductive phenology of the adult overwintering butterfly Gonepteryx rhamni in Sweden

In insects males usually emerge before females. This is known as protandry and can be understood in terms of sexual selection acting on males to maximise the number of matings, or on females to minimise the prereproductive period. A corollary of protandry theory is that males should not emerge before females when the advantage of protandry is nullified, as is the case when diapause intervenes between adult eclosion and the mating season. The brimstone butterfly, Gonepteryx rhamni, has such a life cycle, and in Sweden it develops from egg to adult during April/May to July, and overwinters in the adult stage before the onset of the mating season in April the following year. The first objective of this paper was to test the predictions that (1) there should be no difference between the sexes in egg to adult development time, and that (2) males should emerge from hibernation earlier than females, i.e. protandry should be expected during the mating season. A second objective was to test what proximate mechanisms were responsible for the eventual sex difference in time of emergence from hibernation. Protandry theory shows that late emerging males are heavily penalized in terms of mating opportunities, and so male G. rhamni may benefit from being able to mate directly after emerging from hibernation, whereas females may not be in such a hurry to start laying eggs because the bud break of the host plant Frangula alnus occurs in late May. Hence male G. rhanmi may have to allocate their fat reserves both to winter survival and sperm production, whereas females can most likely allocate all their fat reserves to winter survival, and so the third objective was to document the reproductive phenology of males and females and to test if male G. rhamni suffered higher winter mortality than females. As expected from theory there was no sex difference in egg to adult development time, whereas three years of census data from the field showed that the first males emerged from hibernation on average 21 d earlier than the first females (with the peak of male abundance occurring on average 8 d before that of female peak abundance). The earlier male emergence from hibernation seemed to be the result of the combined effects of males having a slightly lower temperature threshold for flight, and males being ready to fly as soon as environmental conditions were suitable after 4 months of hibernation, whereas females seemed to require a higher temperature summation before being ready to fly. Dissections of males and females throughout the year showed that male sperm production started soon after eclosion with males entering hibernation with a substantial sperm complement, and experiments showed that both males and females mated within minutes after having spent at least 6 months in hibernation at 4°C. By contrast, female eggs were small and unyolked throughout winter, and egg development started only after females had emerged from hibernation. Females brought from hibernation, and mated, towards the end of March started to lay eggs only after spending between 7 and 14 d at room temperature. Although males divide their fat reserves between survival and reproduction during winter, there was no difference in winter survival between the sexes under laboratory conditions, and the sex ratio assessed in the field did not differ before versus after hibernation giving no support for the idea that males suffer higher winter mortality than females.