Multimodal signalling in an antelope: fluctuating facemasks and knee-clicks reveal the social status of eland bulls

The evolution of status badges presents a challenge to animal communication theory because no obvious production costs prevent low-quality individuals from cheating. From mammals, evidence of visual status badges remains rare with most examples to date found in birds. Here we report that drastic fluctuations in the facial ornamentation of male eland antelope, Tragelaphus oryx, reliably indicated two aspects of social status: dominance in all-male herds and access to mates as the master bull in mixed-sex herds. The finding suggests that visual status badges may be more widespread in mammals than hitherto recognized, especially in taxa with complex social systems. The peak frequency of loud knee-clicks explained additional variation in male social status. Supporting a function as a body size indicator, the peak click frequency was in turn dependent on body depth and facemask darkness, the latter possibly related to testosterone levels and hence muscle development. Dewlap size reflected dominance in all-male herds but not master bull status in mixed-sex herds and, after controlling for the effect of age, no link with social status remained. Thus whether the primary function of the ungulate dewlap is in communication or rather in thermoregulation is still an open question.