Group size and mating system predict sex differences in vocal fundamental frequency in anthropoid primates (original) (raw)

2023, Nature Communications

Vocalizations differ substantially between the sexes in many primates, and lowfrequency male vocalizations may be favored by sexual selection because they intimidate rivals and/or attract mates. Sexual dimorphism in fundamental frequency may be more pronounced in species with more intense male mating competition and in those with large group size, where social knowledge is limited and efficient judgment of potential mates and competitors is crucial. These non-mutually exclusive explanations have not been tested simultaneously across primate species. In a sample of vocalizations (n = 1914 recordings) across 37 anthropoid species, we investigated whether fundamental frequency dimorphism evolved in association with increased intensity of mating competition (H1), large group size (H2), multilevel social organization (H3), a trade-off against the intensity of sperm competition (H4), and/or poor acoustic habitats (H5), controlling for phylogeny and body size dimorphism. We show that fundamental frequency dimorphism increased in evolutionary transitions towards larger group size and polygyny. Findings suggest that lowfrequency male vocalizations in primates may have been driven by selection to win mating opportunities by avoiding costly fights and may be more important in larger groups, where limited social knowledge affords advantages to rapid assessment of status and threat potential via conspicuous secondary sexual characteristics. Determining why sex differences evolved and vary among primates is critical to understanding the evolution of mating systems and social organization 1. Sexual dimorphism in body size, skeletal size and shape, dentition, pelage coloration, and ornamentation have been studied extensively 2-6 , but little is known about the evolution of sex differences in primate vocalizations despite the importance of vocal communication 7-9. Comparative studies have focused on acoustic allometry, i.e. the link between an animal's body size and the acoustic properties of its vocalizations 10-13 , as well as adaptive explanations (e.g., acoustic variations in response to the degree of sperm competition 14 and the strength of social bonding 15). However, these studies have conducted analyses at the species level without distinguishing sex, or within one sex only. Primate larynges, including extralaryngeal appendices 16,17 , are also substantially larger in proportion to body size, more variable in size, and have evolved faster than carnivore