Parental investment and the control of sexual selection: can sperm competition affect the direction of sexual competition?

Abstract

We investigate the effect of sperm competition on potential reproductive rates (PRRS) of the sexes for the case where there is just one mating per female reproductive cycle: there may be no sperm competition if all the sperm are used from the immediate prior mating, or sperm competition if ejaculates overlap temporally across several clutches. Sperm competition per se cannot affect male PRR, though adaptive adjustments to the male time budget because of sperm competition can change male PRR. The most obvious effect of increased sperm competition in a species would be a general increase in male time out of mate searching activity because of greater time expenditure on gamete replenishment, arising from the heavier sperm demand under sperm competition. This would act to reduce male PRR, but this may not be a large effect because time out for sperm replenishment is often a low fraction of total male time budget. Where there is paternal care, this may be reduced in species were there is high sperm competition risk, which in turn may generally increase male PRR. Variations in the levels of sperm competition experienced by males with different females within a species may also promote facultative optima for times out (spent in sperm replenishment or in paternal care) which may affect male PRR, though less obviously, because the mean PRR may not be greatly affected by adaptive variation.