The effect of size and cheliped autotomy on sexual competition between males of the mud crab Cyrtograpsus angulatus Dana

Abstract

Size advantage in male–male competition over mates, combined with male preference over large females, is a common feature that can drive to size assortative mating and, eventually, sexual se lection. In crabs, appendage autotomy can a V ect assortative mating and opportunity for sexual selection by a V ecting size advantage in mating contests. In this work, we evaluate the e V ect of size and appendage autotomy in generating assortative mating in the mud crab Cyrtograpsus angulatus . Field observations of guarding pairs in two di V erent populations show a positive correlation between carapace width of males and females in both the populations. In one of the populations, incidence of appendage autotomy was low and the variability in the size of reproductive males was lower than the variability in the size of randomly collected males (i.e. only larger males were successful in getting a female), whereas there was no di V erences in the other population (i.e. most male sizes were successful) where the incidence of appendage autot- omy was very high, indicating that the importance of size is higher when the incidence of autotomy is low. In this con- text, experiments (in both populations) show that, in con- tests for a female, larger males outcompete smaller ones only when they had intact appendages. When males had missing chelipeds, winning or loosing against smaller males was random. This may lead to a decrease in the importance of male size in populations with high incidence of cheliped autotomy, a V ecting assortative mating and opportunity for selection and, thus, a V ecting selective pressures.