Natural variation in female mating frequency in a polyandrous butterfly: effects of size and age

In the polyandrous gift-giving butterfly Pieris napi, females mature at a smaller size than males under poor food conditions, so it has been suggested that females can compensate for their smaller size through nuptial feeding. We tested this hypothesis by assessing female polyandry in relation to female size in a study of a wild bivoltine population and in a laboratory experiment. Contrary to expectation, larger females had a higher mating frequency. In the wild population female polyandry was positively correlated with size and in the laboratory experiment larger females were also more polyandrous. Hence, smaller females cannot compensate for their size by increasing their mating frequency, perhaps because the rate of spermatophore breakdown sets the limit for polyandry and larger females are able to break down the spermatophore faster. In addition, the lifetime number of matings for wild females varied between one and five. As previous studies indicate that female fitness in P. napi appears to increase monotonically with number of mates as a result of the increase in spermatophore material received, it is surprising that 12% of the wild females in the oldest age classes had mated only once and that another 35-40% had mated only twice. This apparently maladaptive behaviour is discussed in terms of sexually antagonistic coevolution and whether environmental conditions influence optimal mating frequency. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.