Do contest-related traits evolve faster? A test of the exaggerated evolution hypothesis in a clade of neotropical arachnids

The exaggerated evolution hypothesis proposes that intrasexual competition accelerates the evolution of contest-related traits. This hypothesis is supported by a simulation model and empirical data from beetles and mammals using horns or antlers as true weapons. However, it has never been tested for threat devices, which are expected to be under directional selection. Using a clade of neotropical harvestmen where males employ their fourth leg pair as a threat device, we test whether intrasexual competition accelerates the evolutionary rates of these legs. We used a time-calibrated molecular phylogeny and morphometric data to estimate evolutionary rates of each leg pair in males and females of 23 species. We found that the fourth legs in males evolved at rates 5.5-44 times higher than the other legs and 6.5 times higher than the homologous legs in females. In conclusion, threat devices also show accelerated evolutionary rates, supporting the exaggerated evolution hypothesis.