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

This blog post is provided by Vlad Demartsev, Michal Haddas-Sasson, Amiyaal Ilany, Lee Koren and Eli Geffen and tells the #StoryBehindthePaper for the paper ‘Male rock hyraxes that maintain an isochronous song rhythm achieve higher reproductive success‘, which was recently published in Journal of Animal Ecology. In their study, they explore whether singing ability in male hyraxes is linked to the number of offspring that they produce.

Rhythm is found in many biological processes including some of the basic physiological functions of living organisms. Firing of neurons, breathing, heartbeat, pace of movement; those are just few examples for processes in which rhythm stability is important and can indicate health and stamina of the organisms. On the other hand, irregularities in those rhythms could be a sign of illness or poor condition. When choosing mates, animals often identify cues indicating health and quality of the potential partner and various courtship displays advertising individual fitness have evolved. Some of those displays are broadcasted in the form of acoustic signals and bird songs are probably the best known example of this. Rhythmic performance can be related to underlying physiological processes like muscle control and breathing rate. As both humans and animals are quite good at spotting errors in rhythmic sequences, rhythm stability can be used for evaluating performers’ quality. Additionally, stable and predictable rhythm can help to synchronize performance of multiple individuals like musicians in a band or birds singing in a chorus.  

Charles Darwin, and several other scholars since him, have suggested that human music might have evolved with a similar function of courtship. Rhythmic structure is a fundamental feature of many musical styles and keeping the “right” rhythm is vital for a good musical performance. The rhythmic signatures existing in musical styles can also be found in animal songs supporting the idea of common functionality or at least common preconditioning for rhythm sensitivity in animals and humans.  Especially interesting are the relatively rare cases of singing mammals. Only a few mammalian taxonomic branches have been shown to perform songs – long and complex sequences of calls with clearly identifiable syntactic and also rhythmic structure. Whales are widely known for their songs, as well as gibbons for their long and elaborate great calls. A slightly less familiar singing mammal is the rock hyrax – a medium sized herbivore from sub-Saharan Africa, the Middle East and the Arabian Peninsula.

Male rock hyrax singing (Photo credit: Eran Gissis)

Rock hyraxes live in groups of up to 30 individuals. In a group there is usually one adult resident male, adult females, juveniles and pups of both sexes.  Hyraxes communicate mainly vocally and about 10 different call types have been identified in their repertoire. Males of this species emit long and complex songs that function as advertisement in courtship and competition with other males. These songs are individually unique and it is likely that hyraxes in the area can identify males by their singing. Mostly, male hyraxes sing by themselves, but in some cases two or three males sing in response to each other but they do not form choruses or sing in unison.  Rock hyraxes breed seasonally, with the mating season lasting two weeks in mid-summer. Both males and females can mate with multiple partners so it is not always clear who is the father of the pups. Females are pregnant for ~ 8 months and give birth to 1-5 pups. The pups are being taken care of by the whole group. Adult and juvenile hyraxes are seen babysitting mixed litters (pups from multiple mothers) and all the group takes part in predator defense.

In the past 25 years we have been studying the rock hyrax population at the Ein Gedi Natural Reserve, located on the coast of the Dead sea in Israel.  Over the course of this long term study we have collected DNA samples of hundreds of animals and reconstructed the family tree of our study population.  By performing genetic analysis, we identified who were the parents of each individual hyrax and calculated the number of surviving pups each male had in each year of its life. We also recorded hundreds of songs from identified males and linked the songs a male performed to the count of surviving sons and daughters that it fathered in the same year. We were interested in figuring out if the rhythmic structure of the songs was related to the male hyrax’s reproductive success and if males singing at a stable and consistent rhythm had more surviving offspring.

Song of a male rock hyrax (Amiyaal Ilany)

We found that male hyraxes who sang more frequently in a given year also fathered slightly more pups. We also found that males who managed to keep precise song rhythm, produce sequential song elements (calls) at nearly identical intervals, had more surviving offspring in comparison to males that made more “errors” or in general did not emit calls with equal intervals.  Interestingly the absolute tempo of the songs (fast or slow) was not important, only the ability to sing isochronally – with equal spacing between notes. We don’t yet know if the isochronous singing of hyrax males is attractive to females as an independent trait or if it makes the whole song more predictable and easier to follow and allows for a better or quicker evaluation of the singing male as a potential mate.

We only found one main rhythmic category (isochrony) in the songs of hyraxes while in other animal systems multiple rhythmic patterns can be present in a signal. Mixed rhythmic patterns could be a result of collective signalling. With multiple callers, while it is often important to stay generally in synch with the group, having some level of rhythmic flexibility could be useful for highlighting individual performance. So, unlike hyraxes that sing mostly solos, individuals in chorus singing species might strategically deviate from isochronous stability to stand out from the unison crowd.   But this is already a question for the next study.

Male rock hyrax
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