Global patterns of sexual dimorphism in Amblypygi

Sexual dimorphism is a common feature of life. Researchers have noted that some species of Amblypygi (Class Arachnida) exhibit male-biased sexual dimorphism, but a broad overview of the prevalence of sexual dimorphism in the order does not exist. In order to provide such an overview, we imaged and measured morphological characteristics of 390 amblypygids from 36 species. Our aim was to gain insight into general patterns of sexual dimorphism across the order. We found that, generally, males have larger pedipalps, longer antenniform legs, but smaller carapace widths relative to conspecific females for nearly all species studied. Species ranged from no sexual dimorphism (Phrynus exsul and Charinus insularis) to male pedipalps almost double the length of females in some species (Charon grayi, Euphrynichus amanica). Phylogenetic patterns remain unclear in Amblypygi and our data better match geographic patterns than evolutionary history. We thus suggest that sexual dimorphism is ancestral for the order, but has been reduced or lost in some species.