Defining an intrasexual male weapon polymorphism in a New Zealand harvestman (Opiliones: Neopilionidae) using traditional and geometric morphometrics

In many species, competition for mates has led to exaggerated male sexually-selected traits. Sexually-selected male weapons are used in male-male combat and include structures like horns, antlers and enlarged teeth. Weapons often vary intraspecifically in size, resulting in either a continuum of weapon sizes or in discrete male polymorphisms. More rarely, complex weapon polymorphisms can also include variation in weapon shape; however, these are difficult to quantify. Here we first use traditional linear morphometrics to describe a weapon trimorphism in the endemic New Zealand harvestman, Forsteropsalis pureora Taylor, 2013. We identified three male morphs: a small-bodied gamma male with reduced chelicera, a large-bodied beta male with long, slender chelicerae, and a large-bodied alpha male with shorter, but very broad, robust chelicerae. Chelicera length alone failed to fully capture the variation in weapon investment. Using geometric morphometrics, we show that alpha males are different in weapon shape, whereas beta and gamma males have similar weapon shape, but vary in their body size and chelicera length. Additionally, we describe how the chelicerae function during male-male combat from observations of contests. This work demonstrates how combining linear and geometric morphometrics can help to elucidate complex polymorphism.