Why males have lower-pitched voices - study

A new study finds that large group size and mating systems in which males have multiple mates drove the evolution of lower-pitched voices in primates.

  (photo credit: USO / GETTY IMAGES)
(photo credit: USO / GETTY IMAGES)

Actors including Morgan Freeman, Sean Connery, Orson Welles, and James Earl Jones are among those regarded by women as being sexy because of their deep voices. 

Now, researchers at Pennsylvania State University have discovered that deeper male voices in primates may have evolved as another way for males to drive off competitors in large groups that favored polygyny (mating systems in which some males have multiple mates).

Published in the prestigious journal Nature Communications under the title “Group size and mating system predict sex differences in vocal fundamental frequency in anthropoid primates.”

Researchers analyzed vocalizations from various species to map evolutionary differences 

The researchers analyzed 1,914 vocalizations from 37 anthropoid primate species – including men – and looked to see which physical and social traits could have driven evolutionary differences in vocal pitch between males and females. They found that differences in vocal pitch between the sexes increased in large groups and in groups that favored polygynous mating systems, especially those with greater female-to-male ratios. 

The study is the most comprehensive investigation of differences in vocal pitch between sexes to date and can help to shed light on social behavior in humans and their closest living relatives. 

 A chimpanzee. One of the closest animals to humans on the evolutionary ladder (illustrative). (credit: Tambako The Jaguar/Flickr)
A chimpanzee. One of the closest animals to humans on the evolutionary ladder (illustrative). (credit: Tambako The Jaguar/Flickr)
The average speaking pitch of an adult male human is about half the average pitch – an octave lower – than that of an adult female human, said anthropology Prof. David Puts, a study co-author.  “It’s a sex difference that emerges at sexual maturity across species, and it probably influences mating success through attracting mates or by intimidating competitors,” he said. “I thought it has to be a trait that’s been subjected to sexual selection in which mating opportunities influence which traits are passed down to offspring. Humans and many other primates are highly communicative, especially through vocal communication. So it seems like a really relevant trait for thinking about social behavior in humans and primates in general.” 

Researchers utilized specialized software to simultaneously test five hypotheses on sex differences in vocal pitch

The researchers used specialized computer software to visualize vocalizations and measure voice pitch in recordings from the primate species most closely related to humans including gorillas and chimpanzees and recordings of 60 men evenly divided by sex. Samples for each species included at least two male and two female vocal recordings, for a total of 1,914 vocalizations. The team then calculated average male and female vocal fundamental frequency for each species to see how pronounced the difference was between the genders.  

The scientists collected additional information for each species to help identify correlations between male versus female voice pitch and factors that could have contributed to the trait’s evolution. Additional variables included body size and body mass differences between males and females, habitat type, adult sex ratios, mating competition intensity and testes size. They also categorized each species by mating system — monogamous, in which males and females have one mate at a time; polygynandrous, in which males and females have multiple mating partners; and polygynous.

The researchers used these data to test five hypotheses simultaneously to identify which factors may have played the strongest roles in driving sex differences in vocal pitch. These were the intensity of mating competition, large group size, multilevel social organization, trade-off against the intensity of sperm competition and poor acoustic habitats. 

Puts said that previous research has looked at one or two of these hypotheses at a time, but the new study was the first to test multiple theories simultaneously for vocal pitch differences using a robust dataset, ensuring data consistency and garnering convincing results. 

Deeper male voices may have evolved to make males sound bigger, fending off mating competitors 

The team found that fundamental frequency differences by sex increased in larger groups and those with polygynous mating systems, especially in groups with a higher female-to-male ratio. 

Deeper male voices may act as an additional way to fend off mating competitors without having to engage in costly fighting by making males sound bigger, in addition to other physical traits like height and muscle size, according to the researchers. In adult humans, for instance, males vocalize at an average of 120 hertz whereas females vocalize at an average of about 220 hertz, putting humans right in the middle of polygynous societies, the researchers reported. 

“Although social monogamy is really common in humans, mating and reproduction in our ancestors was substantially polygynous,” Puts concluded. “Our findings help us to understand why male and female voices of our species differ so drastically. It may be a product of our evolutionary history, particularly our history of living in large groups in which some males reproduced with multiple females.”