Environmental Variance in Lifetime Mating Success, Mate Choice, and Sexual Selection

Abstract

This paper analyzes the role of chance, expressed in terms of the likelihood of survival and mate encounters, in mate-choice strategies and variances in lifetime mating success (LMS) for males and females. Using Markovian mating models, we derive analytical expressions for the mean and variance for LMS as a function of the probabilities of survival and mate encounters per unit of time and of the duration of postmating latency, when individuals do not search for mates. If males have a similar survival rate but shorter postmating latencies than females, then males always exhibit higher variances in LMS than do females, given equal LMS means for both sexes. Therefore, simply observing a higher LMS variance for males than for females is not sufficient proof of the existence or operation of sexual selection or male-male competition, in accord with Sutherland''s (1985a) result. Absolute LMS variances generally increase with increased survival, decreased duration of postmating latency, and increased mate availability. However, the ratio of male to female LMS variances first increases and then decreases with increasing survival, and this ratio can become quite large if the length of postmating latency differs greatly between the sexes. Mate choice is favored by a sexual asymmetry in postmating latencies. This finding is not inconsistent with the LMS variance results because none of the LMS variance in the model results from mate choice. It was possible to derive a criterion for when choosiness should evolve in a mating environment consisting of two qualities in potential mates. Choosiness is expressed in terms of the differences in mate quality at which a choosy mating strategy outperforms an indiscriminate mating strategy. As a general rule, males have shorter postmating latencies than females; this implies that females should be choosier in their selection of mates at all levels of survival and of availability of potential mates. In the model, this is expressed by the smaller difference in potential mate quality required for females to be choosy. These results are relevant to the question of how to measure the opportunity for sexual selection on males. In this analysis, all variance in lifetime mating success results from chance and not from genotypic differences in viability or the ability to find and win mates. Because the variation in male LMS due to chance can be large and because this is not heritable variation, we suggest that the opportunity for sexual selection on males be redefined as the residual variance in male LMS that cannot be ascribed to known and quantifiable nongenetic life history variation.