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Curr Biol. 2015 Nov 2;25(21):2839-2844. doi: 10.1016/j.cub.2015.09.029. Epub 2015 Oct 22.

Evolutionary trade-off between vocal tract and testes dimensions in howler monkeys.

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Division of Biological Anthropology, University of Cambridge, Pembroke Street, Cambridge CB2 3QG, UK. Electronic address:
Department of Biology, Farmingdale State College, State University of New York, 2350 Broadhollow Road, Farmingdale, NY 11735-1021, USA; New York Consortium in Evolutionary Primatology, American Museum of Natural History, Central Park West and 79(th) Street, New York, NY 10024, USA.
Division of Biological Anthropology, University of Cambridge, Pembroke Street, Cambridge CB2 3QG, UK.
Mammal Vocal Communication and Cognition Research Group, School of Psychology, University of Sussex, Falmer, Brighton BN1 9QH, UK.
Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
Clinic of Diagnostic Imaging, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210 Vienna, Austria.
Department of Cognitive Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria; Haidlhof Research Station, University of Vienna/University of Veterinary Medicine Vienna/Messerli Research Institute, Haidlhofer Strasse, 2540 Bad Vöslau, Austria. Electronic address:
Department of Anthropology, University of Utah, 270 1400 E, Salt Lake City, UT 84112, USA.


Males often face a trade-off between investments in precopulatory and postcopulatory traits [1], particularly when male-male contest competition determines access to mates [2]. 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 [3-5]. 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 [6]. 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.

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