The study, published online recently in the journal Biology Letters, was led by Coren Apicella, a doctoral candidate in the Department of Anthropology in the Faculty of Arts and Sciences at Harvard University, with David Feinberg of McMaster University and Frank Marlowe of Florida State University.

"The results of this study have implications for the evolution of vocal dimorphism," says Apicella. "While we don't know the exact reason that these men with deeper voices have fathered more children, it may be that they have increased access to mates, begin reproducing at an earlier age or their wives have shorter inter-birth intervals because they provide more food to them."

"While we find in this new study that voice pitch is not related to offspring mortality rates," says Feinberg," we find that men with low voice pitch have higher reproductive success and more children born to them."

The anthropologists studied the reproductive patterns of the Hadza, a Tanzanian hunter-gatherer tribe that lives much the same way that human beings did 200,000 years ago. According to the Apicella, the Hadza were chosen because they provide a window to our past. The females gather berries and dig for tubers, while the males hunt animals and collect honey.

Marriages are not arranged, so that men and women choose their own spouses. The Hadza are monogamous, but extra-marital affairs are common, and the divorce rate is high.

For the study, voice recordings were collected from 49 men and 52 women between the ages of 18 and 55, in nine different Hadza camps. Participants provided the names of children born to them, whether surviving or deceased, and were then recorded speaking the Swahili word for "hello" into a microphone. These vocal recordings were analyzed for fundamental frequency.

The researchers found that, controlling for age, males with lower vocal pitch had more surviving children. Lower voice pitch did not have an effect on the number of deceased children; however, men with lower voice pitch were found to have fathered more total children, leading to a greater number of surviving children.

It was previously known that women find deeper male voices more attractive, especially during the more fertile phase of their menstrual cycle, but understanding the relationship between mate preferences and fertility is difficult in most modern populations, because of the widespread use of birth control methods. The Hadza, however, do not employ birth control methods, and therefore reproductive success corresponds directly with natural fertility.

Hadza females may choose mates with deeper voices because they are perceived to be better providers, according to the researchers. Previous studies have also shown a relationship between testosterone and deeper vocal pitch, and so increased testosterone may contribute to the male's ability to hunt. Because of their similarity to the hunter-gatherer lifestyle of our ancestors, the reproductive success of the Hadza could be indicative the way that human beings evolved.

"It's possible that vocal dimorphism has evolved over thousands of years, partly due to mate selection," says Apicella. "Perhaps at one time, men and women's voices were closer in pitch than they are today."

The research was funded by the National Science Foundation and the Department of Anthropology at Harvard University.

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