Behaviour 2019
Does one size really fit all? Genital allometry of an Amazonian arachnid with male dimorphism
Bruna O. Cassettari, Glauco Machado. Universidade de São Paulo, São Paulo, SP, Brazil

Genital structures are often species-specific and evolve rapidly. According to the “one size fits all” hypothesis, male genital size is subject to stabilizing evolution. Thus, small males are expected to have disproportionately large genitalia, and large males should have disproportionately small genitalia. This pattern of negative allometry is observed in many arthropods with continuous variation in male size. However, species with male dimorphism are subject to disruptive selection in several morphological traits, so we asked if disruptive selection also leads to differences in genital allometry between male morphs. We predicted that if the “one size fits all” hypothesis applies to dimorphic males, we should find negative allometry in genital structures of males of both morphs and no differences in allometric coefficients between morphs. We studied males of the Amazonian harvestman Poecilaemula lavarrei to test these predictions. We found negative allometry for penis length of both morphs, but minors showed a greater allometric coefficient than majors. We discuss the results considering implications for genitalic mechanical and stimulatory functions, in male dimorphic arthropods.