Optimal copula duration in yellow dung flies: effects of female size and egg content

We examine data on copula duration in dung flies, Scatophaga stercoraria, i n relation to female phenotype. We use a marginal value theorem approach ba sed on the plausible mechanisms of sperm competition to predict the effect of female variation on optimal copula duration, t*, from the male perspecti ve. Future fertilizations are expected to have a trivial effect on t* with fully gravid females, but an increasing relative effect on t* towards compl etion of oviposition, t* is expected to be affected by female size because of variation in (1) a female's egg content, which increases the maximum egg gain available from a mating, and (2) the female reproductive tract, which affects the rate at which sperm are displaced. In fully gravid females, t* was not dependent on egg number variation, but showed a positive relation with egg content in females that had laid a varying proportion of their mat ure egg load at the time of mating, and were therefore not fully gravid. Ou r models predict that if a male can estimate egg content only by the disten sion of a female's abdomen, t* should increase in a similar way to that see n with 'take-over' females. We predict t* for fully gravid females by assum ing that males can monitor female size. The data showed that sperm displace ment rate decreased, and average egg content increased, with female size. U nder two models for a sperm displacement mechanism, one (which assumes indi rect displacement at a rate proportional to the increase in spermathecal vo lume) predicts the observed relation between t* and female size almost exac tly. Small males copulated for longer than large males (as predicted and re ported previously). (C) 1999 The Association for the Study of Animal Behavi our.