The evolution of hornedness in female ruminants

Abstract

Females of many ruminant species possess horns or hornlike organs, but their precise function remains largely unclear. In this paper, four previous explanations for female hornedness are compared with a new hypothesis, the Female Competition Hypothesis, which suggests that horns initially evolved for reasons of intrasexual competition for resources with con-specifics, the level of competition being correlated with female group size. Each hypothesis is first reviewed and necessary predictions arising from each are generated. In order to test between these hypotheses, the incidence of female hornedness across the ruminants is then examined using a comparative method which takes account of the evolutionary history of each species, in order to control for effects of phylogenetic correlation. Group size and body size are found to be the only variables which predict hornedness successfully; however, when the influence of the other predicting variable is removed, only group size remains as a significant predictor. This analysis is found to support the Female Competition Hypothesis and is shown to be robust both in relation to adjustments in phylogenetic construction (e.g. the position of Aepyceros and Boocerus) and to intraspecific variation in horned condition (e.g. horned and hornless races of Oreotragus).