Sex specific vocal behaviour in white rhinoceros and its implication for mating

Vocal communication plays an important role for the coordination of mating behaviour in animals. Vocal exchanges between sexes can give important insights into their social relationship and can indicate the reproductive phase of females. In this project, we investigated sex-specific vocal behaviour in the captive white rhinoceros Ceratotherium simum simum by correlating vocal and social behaviour with hormonal data. Since males and female live separately, finding a mating partner at the right time is important for species survival. We conducted audio and video recordings of 7 males and 27 females from 10 zoological institutions (~10 hours per subject over a period of 1 to 2 months). Additionally, we collected faecal samples to analyse the level of testosterone metabolites in males and of oestrogen and progesterone metabolites in females. We obtained the call rate of the four most common call types Snort, Hiss, Grunt and Pant. Conducting a video analysis, we obtained the rate of affiliative, aggressive, defensive social interactions between group members as well as the amount of time the group members spent together. We compared these data between the receptive and non-receptive periods of a female to investigate signals of mating proceptivity. Using vocal network analysis, we found sex-specific differences in the vocal behaviour of white rhinoceroses. Females emitted more aggressive calls than they received and directed aggressive calls more often to males than to other females. Males emitted more affiliative calls than they received suggesting an active role in attracting females. Comparing vocal and social behaviour during the receptive and non-receptive period of female rhinoceroses indicated sex-specific signalling of mating proceptivity. Males increased the rate of affiliative and agonistic calls, whereas females increased presenting and marking behaviour during the receptive compared to the non-receptive periods. Overall, we found sex-specific vocal behaviour indicating a sexual power asymmetry in the white rhinoceros. We suggest that females can control mating by rejecting the male or leaving the territory, whereas males have to play an active role to keep females in their territory by increasing affiliative behaviour. Moreover, male vocalisations and female marking behaviour can serve as a real-time indicator for the receptive period of a female having the potential to improve reproductive management strategies in captive white rhinoceroses.