Meeting Abstract

60.2  Wednesday, Jan. 6  Allometry of male fiddler crab genitalia varies with size relationships in mating pairs: a test of the one-size-fits-all hypothesis CHRISTY, J.H.*; VARGAS, L.E.; Smithsonian Tropical Research Institute; Universidad de Costa Rica christyj@si.edu

Sexual selection on male traits used as signals to court females or as weapons to fight other males typically results in their disproportionally rapid growth characterized by large (1.5 – 2) coefficients of allometry (hereafter “slopes”). Signal competition among males continues during copulatory courtship producing strong sexual selection on the morphology of male genitalia. However, unlike other sexually-selected traits, 196/207 (95%) of male genital structures of 117 arthropods have slopes less than 1 and less than those for non-sexually-selected body parts (Eberhard 2008). Eberhard has suggested that males are selected to fit and stimulate the average-size adult female; small males with relatively large, and large males with relatively small genitalia. We tested and found support for this “one-size-fits-all” hypothesis by examining the slopes of male fiddler crab genital traits in two species in which crabs do not pair by size and in one species in which they do because they mate only underground in males’ burrows the diameters of which limit the sizes of females with which males can mate. Male gonopod tip and female gonopore diameters in the burrow-mating species exhibited strong positive allometry with slopes of 1.5, larger than for any other known arthropod genital traits used in copulatory courtship. By contrast, the two species that do not pair by size had gonopod tip diameter slopes of .81 and .83 significantly less than slopes of female gonopores at 1.2 and 1.1 respectively, and less than for non-sexually selected traits. Female genital size and factors that affect the sizes of mating pairs may affect how selection based on morphological and stimulatory “fit” affects the allometry of male genital traits.