Researchers have discovered that deeper male voices in primates, including humans, serve a purpose beyond sex appeal. These voices may have evolved as a means for males to assert dominance and repel competitors in polygynous mating systems, where males have multiple mates. This study, which examined vocal pitch differences between sexes across a wide range of primate species, has the potential to shed light on social behavior in humans and their closest relatives.
According to David Puts, a professor of anthropology at Penn State and co-author of the study, adult male humans have an average speaking pitch about an octave lower than that of adult females. This difference in pitch emerges at sexual maturity across species and likely influences mating success by either attracting mates or intimidating competitors. Puts believes that vocal pitch is a trait subjected to sexual selection, where mating opportunities influence the transmission of certain traits to offspring. Given the highly communicative nature of humans and other primates, vocal pitch becomes a relevant trait to consider when examining social behavior.
To investigate vocal pitch differences, the researchers utilized specialized computer software to analyze vocalizations and measure voice pitch in recordings from 37 primate species, including gorillas, chimpanzees, and 60 humans evenly divided by sex. They collected additional data on each species, such as body size differences, habitat type, adult sex ratios, mating competition intensity, and testes size, in order to identify correlations between male and female voice pitch and factors contributing to the evolution of this trait. The researchers also categorized each species by mating system—monogamous, polygynandrous, or polygynous.
Simultaneously testing five hypotheses, including the intensity of mating competition, large group size, multilevel social organization, trade-off against sperm competition, and poor acoustic habitats, the researchers aimed to identify the strongest factors driving sex differences in vocal pitch. Their findings revealed that fundamental frequency differences between males and females increased in larger groups and those with polygynous mating systems, particularly in groups with a higher female-to-male ratio.
The researchers concluded that deeper male voices may serve as a means to deter mating competitors without resorting to physical confrontation. A lower vocal pitch can make males sound larger, complementing other physical traits like height and muscle size. In the case of adult humans, males vocalize at an average of 120 hertz, while females vocalize at around 220 hertz, which aligns with the characteristics of polygynous societies.
Puts emphasized that although social monogamy is prevalent in humans, our ancestors predominantly lived in polygynous societies. Understanding the drastic differences in male and female voices in our species can be attributed to this evolutionary history, where living in large groups allowed certain males to reproduce with multiple females.
This study, published in Nature Communications, highlights the significance of sexual selection in shaping sex differences in vocal pitch across primate species. It provides insights into the vocal characteristics of our common ancestors from millions of years ago. The findings contribute to our understanding of the role played by male and female voices in social behavior and mate selection among primates.
Source: Pennsylvania State University