Sex differences in frequencies in a species with modest sexual size dimorphism

Sex differences in vocalizations are found across the animal kingdom, which may be due to different vocal apparatus, call function, and context. Rock hyraxes (Procavia capensis) of both sexes are vocal, but the sexes differ in repertoire size, call usage, and amplitude. In this study we examined sex differences in vocalization frequencies and predicted that they will be low and that frequency ranges will overlap since sexual size dimorphism in this species is modest (~ 15%). We utilized two datasets: recordings of captive hyraxes using a condenser microphone; and audio files of wild-living individuals recorded via a miniature recorder mounted on a collar. With the exclusion of two proximate call types recorded in the wild, all calls featured an ultrasonic component. However, in females there was an effect of duration on minimal frequency. Warning trills, which are heard by humans far away, featured the highest maximal visible harmonic in both datasets. No relationship was found between calling distance and the maximal harmonic in males, while in wild females, distant calls featured higher frequencies. Our results show sex differences in hyrax vocalization frequencies. Exploring the information encoded in frequencies beyond the human hearing range may expand our understanding of animal communication.