LORCH, P. D.; Univ. of North Carolina, Chapel Hill: Using upper limits on sexual selection to estimate the opportunity for sexual selection and to understand reversals in typical sex-roles

The widespread use of molecular markers to estimate parentage make possible a better index of the opportunity for sexual selection. I develop such an index. First, I describe what sets the upper limit on sexual selection by showing how maximum fecundity increases with number of mates. I consider how the amount of energy (or critical resources) available for reproduction, parental care patterns and sperm competition affect the upper limit on sexual selection for each sex. The upper limit on sexual selection is set for females by the value of the paternal investment that comes with each mating and for males by the fecundity of their mates (including any boost to female fecundity from paternal investment). Sex-roles are most likely to reverse (making males choosy and females competitive) when the amount of reproductive energy investment made by each sex is low, irrespective of the level of paternal investment. I propose that we use the difference between male and female upper limits on sexual selection to quantify sex differences in the opportunity for sexual selection. Finally, I show that sperm mixing, which can reduce the intensity of sexual selection on males below its maximum, when combined with reduced paternal investment, is not likely to cause sex-role reversal.