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New psychological research has revealed the lengths that some male mammals will go to in order to attract a mate, producing lower frequency vocal signals to sound bigger than they are and to deter potential competition from other males.

In a study published in Nature Communications, Dr Benjamin Charlton from University College Dublin and Dr David Reby from the University of Sussex found that terrestrial mammals have developed physical adaptations that enable them to make low-frequency (resonance) vocal sounds that usually only mammals of larger body sizes can produce, making themselves appear bigger to potential mates.

The researchers looked at male vocal characteristics and body size across 72 land-based mammal species, such as koalas, lions, chimpanzees and humans.

Usually, body size constrains the acoustic frequencies of vocalisations and larger individuals tend to produce 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 lower frequency calls than their body size would suggest.

Other mammals, such as howler monkeys and colobus monkeys possess additional resonators, while African elephants and southern elephant seals have snouts, which they use to attract mates.

In contrast, relatively high frequency calls occur in species where intense post-copulatory competition has instead selected for large testes, suggesting a trade-off between producing exaggerated vocalisations to gain reproductive partners before mating and producing large amounts of sperm to win fertilization of eggs after mating.

“This analysis provides a useful background for future investigations of animal vocalisations, by mapping the relationship between body size and frequency components across a very large number of species,” says Dr David Reby, commenting on the research.

“It confirms that large animals tend to produce calls with lower frequencies, but also shows that in species where having a large body size is advantageous in male competition, males tend to evolve anatomical specialisations that enables them to exaggerate the impression of body size conveyed by their sexual vocalisations.”

While human males also have a disproportionately descended larynx that allows them to produce deeper sounds, in the same way that other male mammals which exaggerate body size do, constraints linked to speech production or other functions have selected for a relatively short vocal tract relative to body size. The results of this research provide a background for further studies into the evolution of human vocal communication.

“Male mammals from mating systems with strong selection pressures for large male body size produce lower call frequencies than expected for their body size, i.e. they acoustically exaggerate the impression of their body size,” adds Dr Benjamin Charlton.

“The study sheds light on the evolution of acoustic diversity across mammals.” 

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