Male rock hyraxes that maintain an isochronous song rhythm achieve higher reproductive success

Abstract

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.

Isochrony and rhythmic interaction in ape duetting

“…The production of isochronous rhythm in spontaneous animal vocalizations is rare and poorly studied [2,31]. Isochrony may have important biological functions: for instance, rock hyraxes producing more isochronous rhythmic patterns have higher fitness, in terms of reproductive success [8]. Here we provide evidence for above-chance occurrence of isochrony in an ape species.…”

“…Solo songs are, on average, longer [15] and show a higher call rate than duets. It is likely that a longer vocal display and a high call rate demand higher energy investment and depend on breathing constraints [19]: vocal fatigue may determine less regular rhythmic patterns in prolonged bouts [8,43]. At the same time, a costly display like the song is an honest signal of the emitter quality [44], making solo songs highly subjected to sexual selection [45].…”

“…The spontaneous production of isochronous vocal patterns seems relatively rare in other vertebrates; it has only been reported in two bird species [5,6], a bat [7], rock hyraxes [8] and one primate [9]. The empirical connection between isochrony and synchrony is even less clear because the two are often studied in isolation [10].…”

Striking pay dirt: Contemporary methods for studying animal sociality in the wild

“…Complex statistical models have allowed researchers to shed new light on social networks over the last decade (Allen et al, 2013; Hobaiter et al, 2014; Sosa et al, 2021), this being only one example of how statistical models can aid in the analysis of increasingly complex datasets. Both Barrett (2022) and Demartsev, Haddas‐Sasson, et al (2022) illustrate this in their articles. Barrett first tackles the question of having more than two options.…”

Robust rhythm reporting will advance ecological and evolutionary research

“…To unveil this mystery, a growing number of researchers are investigating the acoustic rhythms of different taxa across behavioural contexts and with various aims. Rhythm analyses are used to better understand species discrimination, physiological correlates between communication and movement, mating preferences, and arousal coding (Burchardt et al, 2019; David et al, 2003; Demartsev et al, 2022; Manser, 2001; McRae, 2020; Norton & Scharff, 2016). Other contexts in which temporal structures play a substantial role include during multi‐individual displays (e.g.…”