THE COST OF MALE SECONDARY SEXUAL TRAITS - DEVELOPMENTAL CONSTRAINTS DURING ONTOGENY IN A SEXUALLY DIMORPHIC WATER STRIDER

Natural selection is generally thought to constrain the effects of sexual selection on secondary sexual traits. In hemimetabolous arthropods, the evolution of complex secondary sexual morphologies may be constrained by interference during the molting process. This study assesses the effects of phenotypic variation in secondary sexual traits during ontogeny in the water strider Gerris odontogaster. In this species, males are provided with a grasping apparatus consisting of two abdominal, ventral processes. It is demonstrated that the length of the processes is positively related to the duration of the ultimate larval molt, presumably because the processes constitute a mechanical hindrance during molting. It is further shown that larvae experience a high risk of mortality from cannibalism during their ultimate molt and that this risk is density-dependent. On the basis of the assumption that cannibalistic events occur at random during molting, males with long processes will suffer higher risk of mortality during molting than will males with short processes. It is concluded that this form of developmental constraint can affect the evolution of male abdominal processes, and quantitative cost curves for the length of male abdominal processes are presented. Simulations of natural selection due to larval mortality show positive density dependence, in contrast to sexual selection in this species. It is suggested that the net effect of selection on secondary sexual traits in this species varies between negative and positive values and that net selection is zero only within a narrow range of environmental conditions. A comparison between natural populations showed that mean trait value co-varies negatively with environmental variables related to the cost of the trait in accordance with this suggestion.