In terrestrial mammal species in which males are larger than females, males produce vocal signals that exaggerate their body size even further, shows a study published in Nature Communications this week. Sounding larger is thought to attract females with preferences for larger males and to deter potential competitors for mates.
Body size generally constrains the acoustic frequencies of vocalizations, with larger individuals producing lower frequencies due to their larger larynges and longer vocal tracts. However, some species have traits, such as descended larynges, that enable males to make atypically low frequency calls for their body size.
Benjamin Charlton and David Reby analyse the relationship between male vocal characteristics and body size across 72 terrestrial mammal species, such as koalas, lions, chimpanzees and humans. They find that, in a given species, vocal exaggeration (with atypically low frequency calls) is associated with sexual selection for large male body size. However, atypically high frequency calls occur in species with intense sperm competition, suggesting a trade-off between producing exaggerated vocalisations to gain mates and producing competitive sperm to win fertilization of eggs after mating.
In humans, although selection pressures related to speech production are thought to have led to the evolution of a short vocal tract relative to body size, adult males have a descended larynx that allows them to produce deeper sounds. The results of this study help put these traits into context and provide a comparison for further studies into human vocal communication.