Mating opportunity and the evolution of sex-specific mortality rates in a butterfly

Life history theory predicts that organisms should only invest resources in to intrinsic components of life span to the degree that it pays off in term s of reproductive success. Here, we investigate if the temporal distributio n of mating opportunities may have influenced the evolution of intrinsic mo rtality rates in the butterfly Pararge aegeria (Satyrinae). In this species , females mate only once and the frequency of male mating opportunities dep ends on the temporal emergence pattern of virgin females. As expected, in a population from Madeira where females emerge continuously throughout the y ear, there was no sex difference in adult life span, while in a Swedish pop ulation with synchronised female emergence, males had significantly shorter life spans compared to females. A logistic mortality model provided the be st fit to the observed change in age-specific mortality and all categories reached an asymptotic mortality rate of a similar magnitude. However, the S wedish males reached this mortality plateau more rapidly than the other cat egories. External mortality, due to water and food limitation, affected the pattern of sex-specific mortality but males from Sweden still had higher r ates of mortality compared to all other categories. Mie argue that selectio n on male longevity is likely to be weaker in Sweden because under synchron ised emergence, all females emerge and mate within a short period of time, after which male reproductive value will quickly approach zero. On Madeira, however, male reproductive value decrease more slowly with age since the p robability of finding a receptive female is constant over the year.