Sex pushes males to have deep voices, bigger balls but not both

Having a deep sexy voice doesn't necessarily mean you're virile, at least as far as mammals are concerned.

It's all about the tradeoffs. A study surveying 72 species of terrestrial mammals belonging to nine orders revealed that the males of species that exaggerate their body size by having lower-than-expected calls have smaller testes compared to males of species with similar body size that engage in sperm competition. The study was conducted by researchers from University College Dublin and the University of Sussex and published in the journal Nature Communications.

Deep voices: a matter of size

How deep or resonant an animal call is is largely a function of the animal's body size. As researchers Benjamin Charlton and David Reby describe it, "Larger animals tend to produce lower-pitched calls than smaller ones because they have larger larynges with longer vocal folds that can oscillate periodically at lower frequencies, [as well as] longer vocal tracts that produce lower resonances". 

Some exceptions to the rule do exist, typically in the males of the species. In these species, males have special physical adaptations that allow them to sound bigger than they actually are. Examples of these are the fleshy vocal pads of male lions and tigers that enable them to release ear-splitting roars.

Because these physical features are often only present or more pronounced in males and are often used for mating calls, scientists have assumed that these traits evolved because of mate selection pressures where females only mate with the larger—and therefore presumably stronger—males.

Are deep voices really an advantage?

Sounding bigger than they actually are gives males in this situation a distinct advantage. But while this assumption has been tested in a small number of species, the current study is the first one to see if this assumption is true across a broad range of mammalian groups.

The researchers used phylogenetic generalized least-squares (PGLS) regression models to simulate five different evolutionary scenarios. They started with a "global" model that included male body mass, habitat (arboreal [live in trees] or terrestrial), call type (sexual or non-sexual), and mating system (polygynous [male mates with several females], monogamous [male-female pair mates exclusively], polyandrous [female mates with several males], promiscuous [both males and females mate with multiple partners], or variable) of 72 species of terrestrial mammals. Sexual dimorphism [distinct difference in size or appearance between males and females] or relative testes size were included in the model depending on the hypothesis being tested. 

Their results showed that general acoustic allometry holds across the different mammalian orders: larger species produce calls with lower fundamental frequencies and lower formants. They also found that in the species where females actively preferred larger males, these males produced calls with lower resonance than expected for their body size. 

Bigger balls

In contrast, males with larger testes relative to body size but higher resonance calls were found in species where sperm competition occurs. Sperm competition occurs when a female can mate with multiple partners, which forces the sperm of different males to compete to fertilize the female's egg. This can happen through physical removal of a competing male's sperm, guarding the female so that she doesn't mate again, or simply producing more and stronger sperm. Larger testes produce more sperm and so would be an advantage in sperm competition. 

The researchers say that the current study provides a useful introduction for studying our own species' vocal communications.

What it means for humans

Previous research shows that the lower fundamental frequency and formants of the human male voice contribute to mate choice, and that the secondary descent of the larynx in human males at puberty (which allows them to make calls with even lower formant frequencies) also evolved to exaggerate body size. However, the resonance of the human male voice is actually higher than predicted for a terrestrial mammal weighing 75 kg (the average weight of a human male).

The researchers suggest that the "selection pressures to decrease facial size may have counter-balanced sexual selection pressures to exaggerate apparent body size, and resulted in the relative overall shortening of the human vocal tract". — TJD, GMA News