ECOLOGICAL FACTORS, PLEIOTROPY, AND THE EVOLUTION OF SEXUAL DIMORPHISM IN CHERNETID PSEUDOSCORPIONS (PHORESY, QUANTITATIVE GENETICS, SEXUAL SELECTION).

Abstract

The determinants of sexual dimorphism in a family of false scorpions (Pseudoscorpionida, Chernetidae) were investigated experimentally and with a literature analysis of comparative morphometric and habitat data. Species vary in the extent to which males and females differ in size of the conspicuous, prehensile pedipalps. Patterns within the Chernetidae suggest that dimorphism is a highly variable condition, relatively unconstrained by phylogenetic influences. The evolution of species with enlarged male pedipalps appears to be associated with a change from nonpairing to pairing sperm transfer behavior, and aggressive mate acquisition by males. Experiments with Dinocheirus arizonensis demonstrate a high correlation between male combat ability and chela size. Comparison of male and female life histories show prolonged development in males, and morphological comparisons implicate pedipalp dimorphism as a causative factor in this developmental rate difference. Prolonged development may be particularly costly to males, given the pattern of female sexual receptivity in this species. Females were found to become unreceptive soon after mating and remain so throughout a protected period of brood development. Experimental manipulations suggest that the male developmental rate cost is only outweighed under high density conditions when superior combat ability results in increased mating success. Repeated measures experiments failed to show any correlation between male pedipalp size and number of spermatophores accepted by a female. Parent-offspring regressions suggest the existence of additive genetic variance for male chela size and indicate a strong genetic correlation between this trait and cephalothorax length. Full-sib phenotypic correlations suggest that in D. arizonensis sexual divergence through sexual selection may be constrained by a high genetic correlation between males and females. Finally, the role of phoresy in the colonization of ephemeral, patchy habitats is investigated. Results support the hypothesis that attachment of pseudoscorpions to larger, more mobile arthropods represents a behavior functioning specifically for dispersal.