Using upper limits of "bateman gradients" to estimate the opportunity for sexual selection

The widespread use of molecular markers to estimate parentagemakes possible a new index of the opportunity for sexual selection.After demonstrating the need for a new measure, I develop onebased on the upper limit on sexual selection. I describe whatsets the upper limit for each sex by showing how maximum fecundityincreases with number of mates, accounting for the amount ofenergy (or critical resources) available for reproduction andlevels of parental care. For females the upper limit on sexualselection is set by the value of paternal investment that comeswith each mating. For males, the upper limit on sexual selectionis set by the fecundity of their mates (including any boostto female fecundity from paternal investment). Sex-roles aremost likely to reverse (making males choosy and females competitive)when the amount of reproductive energy investment made by eachsex is low, irrespective of the level of paternal investment.Finally, I propose that we use the difference between male andfemale upper limits on sexual selection to quantify sex differencesin the opportunity for sexual selection. Using upper limitsto estimate the opportunity for sexual selection is more intuitivethan older methods (e.g., standardized variance in mating success),it is experimentally measurable, and it is valuable in understandingthe evolution of mating systems.