Colony size as the main driver of the evolution of song diversity and composition in weaverbirds
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Abstract
Birdsong is a complex signal shaped by multiple factors and has been explored most widely through the lens of sexual selection, but with mixed results. Here, we focus on the evolution of two song parameters: diversity, which is widely studied, and composition, which is poorly understood. We assessed the potential role of mating system as a proxy for sexual selection, but in addition, investigated whether colony size, a proxy for sociality, and phylogenetic history influence the evolution of these parameters in weaverbirds family (Aves: Ploceidae). Using comparative and path analyses we find that, as expected, species living in larger colonies present greater song diversity and had similar song composition. However, contrary to expectations, polygamous species do not present higher song diversity, nor more similar acoustic composition than monogamous species. A relatively high effect of phylogeny was detected on both song variables. Our results thus suggest that, in this family, sociality is a stronger driver of song diversity and composition than sexual selection. These findings highlight the importance of testing multiple factors when studying bird song evolution and the relevance of sociality.
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It has now been over a century since the American linguist Edward Sapir observed that there are no ‘primitive’ languages, stating ‘When it comes to linguistic form, Plato walks with the Macedonian swineherd, Confucius with the head-hunting savage of Assam’ (Sapir 1921, p. 104). Nothing has been found among human groups since that would refute Sapir’s insight. However, in interspecific comparisons among non-human animals, such equality is most definitely not the case, and scientists have asked what might account for some species being blessed with rich and complex communication while others seem to make do with a relatively undeveloped repertoire of signals. Many of the investigations have focused on acoustic communications, as these are more easily identified as signals, measured, and demarcated than substrate vibrations, odors and …
It has now been over a century since the American linguist Edward Sapir observed that there are no ‘primitive’ languages, stating ‘When it comes to linguistic form, Plato walks with the Macedonian swineherd, Confucius with the head-hunting savage of Assam’ (Sapir 1921, p. 104). Nothing has been found among human groups since that would refute Sapir’s insight. However, in interspecific comparisons among non-human animals, such equality is most definitely not the case, and scientists have asked what might account for some species being blessed with rich and complex communication while others seem to make do with a relatively undeveloped repertoire of signals. Many of the investigations have focused on acoustic communications, as these are more easily identified as signals, measured, and demarcated than substrate vibrations, odors and optical displays. But no consensus has emerged explaining the ‘inequality among the lexicons’.
In the article ‘Colony size as the main driver of the evolution of song diversity and composition in weaverbirds’, Erwan Harscouet-Commecy and coauthors aimed to find definitive answers to the issue of interspecific differences in communication complexity. The article considers two aspects of communication complexity, basic diversity – essentially the number of different sounds made by a species – and ‘composition’, a higher level measure centered on acoustically recognizable ‘types’ of sounds. Sexual selection, social complexity, habitat and phylogeny are then examined as potential influences on communication. Weaverbirds, the monophyletic family Ploceidae, including 122 species in 15 genera in sub-Saharan Africa and tropical Asia, was an eminently appropriate choice for the study. The birds have diverse nesting and mating behavior and songs, and many have been investigated intensively. For their analyses, the authors were quite rigorous in evaluating the species’ songs and in assessing the potential influences on song complexity. Unexpectedly, they did not find that heightened sexual selection led to complex songs, but they note that the proxy used for sexual selection – a polygynous as opposed to a monogamous mating system – may be inaccurate. Additionally, the strong pair bonding in monogamy may have actually selected for song complexity.
What the authors did find was a very clear influence of social complexity, measured by colony size, and a secondary influence of phylogeny. Questions do remain, such as the specific pathway – common genetic or common cultural heritage – along which phylogeny influences song complexity. Another question one might raise is whether low song complexity in some species reflects reliance on other modalities of communication, e.g. plumage or movement displays. But at present, the article appears as a gold standard for investigating the complexity of communication among animal species.
References
Sapir, E. 1921. Language: An Introduction to the Study of Speech. New York: Harcourt, Brace. ISBN 978-1-108-06378-4
E. Harscouet-Commecy, N. Adenot, A. Thetiot, N. Bresciani, D. Oschadleus, R. Covas, F. Rybak, C. Doutrelant (2024) Colony size as the main driver of the evolution of song diversity and composition in weaverbirds. OSF, ver.2 peer-reviewed and recommended by PCI Evol Biol https://doi.org/10.31219/osf.io/z5bgy_v2
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