Evolutionary tradeoff between vocal tract and testes dimensions in howler monkeys
Halenar, Lauren B
Fitch, W Tecumseh
Knapp, Leslie A
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Dunn, J., Halenar, L. B., Davies, T., Cristobal-Azkarate, J., Reby, D., Sykes, D., Dengg, S., et al. (2015). Evolutionary tradeoff between vocal tract and testes dimensions in howler monkeys. Current Biology, 25 2839-2844. https://doi.org/10.1016/j.cub.2015.09.029
Males often face a trade-off between investments in precopulatory and postcopulatory traits, particularly when male-male contest competition determines access to mates. To date, studies of precopulatory strategies have largely focused on visual ornaments (e.g., coloration) or weapon morphology (e.g., antlers, horns, and canines). However, vocalizations can also play an important role in both male competition and female choice. We investigated variation in vocal tract dimensions among male howler monkeys (Alouatta spp.), which produce loud roars using a highly specialized and greatly enlarged hyoid bone and larynx. We examined the relative male investment in hyoids and testes among howler monkey species in relation to the level of male-male competition and analyzed the acoustic consequences of variation in hyoid morphology. Species characterized by single-male groups have large hyoids and small testes, suggesting high levels of vocally mediated competition. Larger hyoids lower formant frequencies, probably increasing the acoustic impression of male body size and playing a role analogous to investment in large body size or weaponry. Across species, as the number of males per group increases, testes volume also increases, indicating higher levels of postcopulatory sperm competition, while hyoid volume decreases. These results provide the first evidence of an evolutionary trade-off between investment in precopulatory vocal characteristics and postcopulatory sperm production.
We are grateful to Alexander Sliwa, Catalina Gomez, Robert Wallace, Michael Plavkan, Zelinda Braga Hirano, and Julio Cesar de Souza, Jr. for sharing data, Andrew Kitchener (National Museums Scotland) for loaning whole animal specimens, Michaela Gumpenberger and Jaap Saers for support with CT and MRI, Carolyn M. Crockett, Mariana Raño, and La Senda Verde Animal Refuge Bolivia for providing photographs and videos, Nadja Kavcik for help with the figures, and Dieter Lukas for help with statistical analyses. J.C.D. was funded by a Cambridge Humanities Research Grant. W.T.F. acknowledges support of ERC Advanced Grant SOMACCA (#230604) and Austrian Science Fund (FWF) grant W1234-G17.
External DOI: https://doi.org/10.1016/j.cub.2015.09.029
This record's URL: https://www.repository.cam.ac.uk/handle/1810/253207
Attribution 2.0 UK: England & Wales
Licence URL: http://creativecommons.org/licenses/by/2.0/uk/
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