TIME AND ENERGY CONSTRAINTS AND THE EVOLUTION OF SEXUAL SIZE DIMORPHISM - TO EAT OR TO MATE

We present an empirical test of the 'Ghiselin-Reiss small-male hypothe sis' for the evolution of sexual size dimorphism (SSD). In mating syst ems dominated by scramble competition, where male reproductive success is a function of encounter rate with females, small males may be favo ured when food is limiting because they require lower absolute amounts of food. Given a trade-off between time and energy devoted to foragin g and to mate acquisition, small males should be able to devote more t ime to the latter. If at the same time larger females are favoured, th is mechanism will contribute to the evolution of SSD and may be the ma jor determinant of the female-biased SSDs that characterize most anima l taxa. We tested this hypothesis using the water strider, Aquarius re migis (Heteroptera: Gerridae), a scramble competitor which mates many times over a prolonged mating season and which shows female-biased SSD . Laboratory experiments demonstrated that foraging success and giving up times (GUTs) are lower for males than for females during the repro ductive season and that male water striders flexibly alter their time budgets under conditions of energy limitation. Controlled feeding expe riments showed that male and female longevity, female fecundity and ma le mating success are positively related to food availability. As pred icted, male body size is negatively correlated with several indices of male fitness (longevity, number of mating attempts and mating success ), while female body size is positively correlated with longevity. The se results are consistent with the hypothesis that scramble competitio n for mates favours small males in this species and provides empirical support for the Ghiselin-Reiss small-male hypothesis.