Allometry and sexual dimorphism in three giant whip spider species (Amblypygi, Phrynidae, Heterophrynus)

Abstract

Amblypygids display high levels of sexual dimorphism, especially larger pedipalps in males compared to females. This study examines the difference in allometric rates as a potential cause of dimorphism in pedipalps and walking legs in three Heterophrynus species: Heterophrynus batesii (Butler, 1873); Heterophrynus longicornis (Butler, 1873), and Heterophrynus elaphus (Pocock, 1903). Dimorphism is observed in all studied appendages, but higher allometric rates are shown in males more than females for only the pedipalps (for all three species) and the first walking legs (for only H. elaphus), with higher dimorphism in the pedipalp for H. batesii and lower in H. elaphus. Dimorphism in pedipalps appears to be linked to species' mating system: larger dimorphism are observed in polygamous systems, whereas moderate and small dimorphisms are observed in monogamous and solitary species, respectively. Contrary to pedipalps, the evolution of walking legs seems to occur via natural selection rather than sexual selection.