Rhythmic stability (nonrandom temporal structure) is required for many neural and physiological functions, whereas rhythmic irregularities can indicate genetic or developmental deficiencies. Therefore, rhythmic courtship or contest signals are widespread in nature as honest advertisement displays. Examination of bird songs revealed the pervasiveness of categorical rhythmic patterns that can be described as small integer ratios between sequential inter-call intervals. As similar rhythmic profiles are prevalent in human music, it was suggested that a shared functionality could drive both animal songs and human musical rhythms, facilitating synchrony between signallers and enabling easy identification of performance errors. Here we examined whether the rhythmic structure and the rhythmic stability of vocal displays are related to reproductive success in male rock hyraxes (Procavia capensis), which presents an unusual case of a terrestrial singing mammal. We combined long-term parentage analysis of 13 male hyraxes (22 male/years) with an analysis of an audio library of 105 hyrax songs. Male annual reproductive success was determined by the number of offspring that survived to the age of 1 year. The frequency of singing events was used to determine the seasonal singing effort for each male. Songs were analysed for rhythmic structure, focusing on the presence of categorical rhythms and the contribution of rhythmic stability to annual reproductive success. We found that male hyraxes that sing more frequently tend to have more surviving offspring and that the rhythmic profile of hyrax songs is predominantly isochronous with sequential vocal element pairs nearly equally spaced. The ratio of isochronous vocal element transitions (on-integer) to element transitions that deviate from an isochronous pattern (off-integer) in hyrax songs is positively correlated with male reproductive success. Our findings support the notion that isochronous rhythmic stability can serve as an indication of quality in sexually selected signals and is not necessarily driven by the need for multiple caller synchronization. The relative scarcity of nonisochronous rhythmic categories in individually performed hyrax songs raises the question of whether such rhythmic categories could be a product of collective, coordinated signalling, while being selected against in individual performance.
Bibliographical noteFunding Information:
The authors are grateful for the logistic support provided by the staff of the Ein Gedi Nature Reserve and of Ein Gedi Field School, as well as to the Nature and Park Authority for permission to work at the site. The authors thank all the former students, field assistants and guests for their valuable help in the field and N. Paz for editorial services. Finally, the authors thank Roy Baethe for permission to use his outstanding hyrax illustrations in the graphical abstract. The study was supported by grants from the Israel Science Foundation (577/99, 488/05, 461/09, 550/14, 767/16, 244/19, 245/19) and from the U.S.‐Israel Binational Science Foundation (2015088, 2019156). Part of this work was carried out while V. Demartsev was an Alexander von Humboldt‐Stiftung postdoctoral fellow.
© 2022 The Authors. Journal of Animal Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.
- isochronous rhythms
- reproductive success
- vocal advertisement