Size dependence of courtship effort may promote male choice and strong assortative mating in soldier beetles

Large males of the soldier beetle, Chauliognathus pennsylvanicus (Cantharidae), exercise choice for larger females in the field and laboratory. The length of copulation limits the maximum mating rate to once per day. This may afford more competitive males the opportunity to reject females early in the day without significantly reducing their mating rate. Males copulate if they secure evasive females. Body size correlates with the abilities of males to secure females, and of females to evade males. Thus, agonistic courtship gives larger males a mating advantage and may ensure the availability of larger females late into the daily courtship period. Also, larger males spend more time searching for mates and courting than do smaller males which could increase their likelihood of encountering and mating a large female, even after rejecting a smaller one. Stochastic simulations of agonistic courtship indicate that the benefits of male choice are limited to larger males, who are more likely to successfully court a female after having rejected one. Simulations also indicate that strong assortative mating, as observed in the field, requires male choice in combination with agonistic courtship. Both males and females benefit from larger mates, as the fecundity of females is a function of both their own size and the size of their mates. Thus, strong assortative mating magnifies the fecundity advantage of large females through mate effects on fecundity.