Do honest signals or piercing weapons determine rank in a polygynous ungulate?

Abstract

Abstract In polygynous ungulates, males are often larger than females and bear more elaborate/larger weapons. Quantifying sexual dimorphism in different traits could provide insights into species-specific evolutionary pathways of sexual selection. Concerning the combination of secondary sexual traits, we found that Himalayan tahr (Hemitragus jemlahicus) is unique among the ~20 species in the tribe Caprini, as its body mass dimorphism is ~2-fold greater than the dimorphism in horn size, whereas horn shape appears to be near-monomorphic. Whilst horns show the same growth rate in both sexes, body mass increases faster in males. Considering age variation, dominant, golden-ruffed males are also heavier than brown-ruffed, lower-ranking males. Unlike most bovids, male–male competition in tahr does not seem to have influenced weapon development, suggesting a lower importance of horns in male–male competition compared to body mass, as their unritualized combat style also suggests. Our study highlights alternative evolutionary pathways occurring in the Caprinae, where intraspecific signals involve different traits, from weapons to pelage features. Accordingly, male tahr use their ruff colour as an ‘honest’ signal of rank.