All the better to bite you with! Striking intrasexual differences in cheliceral size define two male morphs in an Amazonian arachnid

Abstract

When two mating tactics can be successfully used by different-sized males, disruptive selection may favour morphological divergence between males, resulting in intrasexual dimorphism. Here we characterize intrasexual dimorphism based on cheliceral size in males of the harvestman Paecilaemula lavarrei. We also describe how males of the two morphs use their chelicerae in intra- and intersexual interactions, and investigate microhabitat use to infer their mating tactics. The distribution of body and cheliceral traits was markedly bimodal, indicating the existence of a major morph (majors) with large body and cheliceral size, and a minor morph (minors) with small body and cheliceral size. Male-female interactions were similar between morphs, but only majors initiated male-male contests by biting the legs of rivals using their chelicerae. Males and females preferred tree trunks with large perimeters and containing burrows that could be used as shelters. Female spatial distribution was aggregated, whereas males of both morphs were randomly distributed. Majors seem to fight for reproductive territories that concentrate females, and minors probably rely on territory invasion to sneak copulations. Little overlap in the distribution of body and cheliceral size between morphs suggests that male dimorphism in P. lavarrei may not be a polyphenism, and it is associated with different mating tactics.