Body weight of the two sexes determines the occurrence of polyandry in a moth

Polyandry has been a central topic in the study of animal behaviour. Yet, it is still not clear how the body weight of each sex affects the probability of female remating, and what each sex gains from polyandry. Here we aimed to shed some light on these questions using the Mediterranean flour moth, Ephestia kuehniella (Lepidoptera: Pyralidae). We found that virgin pairs mated regardless of their weight to ensure fertilization. However, only 58% of once-mated females remated. The probability of female remating increased when the female and/or the second male was heavier. Our findings indicate that fecundity, polygamy and longevity are functions of body weight in each sex. The nature of the last male sperm precedence in this species allows males and nonvirgin females to regulate their mating decisions. We propose that once-mated females would benefit from accepting heavier second males to replace most sperm from the lighter first mates and rejecting lighter second males to avoid displacement of sperm from the first heavier mates. Heavier males are more coercive, probably also preferred by females, and thus more likely to achieve mating with heavier nonvirgin females to replace most sperm from the previous males and dominate paternity. As body weight is heritable, we suggest that the fitness of both sexes rises when they pass on genes for this trait through female encouragement or avoidance of sperm displacement and male coercion of mated females into remating. We conclude that the two sexes play interactive roles in determining polyandrous patterns depending on their body weight.