Testosterone Pumps Up Hyrax Females
By Jennifer Viegas, Discovery News
May 22, 2006— Testosterone may be known as the "male hormone," but researchers have discovered that female rock hyraxes have significantly higher levels of the chemical than their male counterparts.
It is the first time an adult female mammal has been found to possess testosterone levels consistently equal to, or higher than, those found in males of the same species.
Since female members of the furry, badger-like species often outrank males, the find suggests hormone levels may help determine who's the boss, and who is not.
"Testosterone probably contributes to females’ aggression and social rank, along with other androgens as well," said Lee Koren, lead author of the study, which was recently published in the journal Hormones and Behavior.
Koren, a researcher in the Zoology Department at Tel Aviv University, Israel, added, "High hormonal levels may also contribute to females’ lack of choosiness in the mating season. Finally, testosterone also acts as a stress hormone."
"It is possible that females -- with their roles as mothers, sisters and group leaders -- are, understandably, very stressed," she said.
The researchers observed six hyrax social groups in Ein Gedi, Israel. The groups consisted of adult females, adult males, pups and a group of male bachelors that slept in separate burrows.
Some older males ranked among the most dominant members -- below a female or two -- but younger males were among the lowest-ranked.
Hair and blood tests revealed females had as much or more testosterone than males.
To eliminate the possibility they were observing an isolated phenomenon, the researchers studied other hyraxes from zoos around Israel and another group in the northern part of the country. The analysis yielded similar results.
Males are the dominant sex in most mammal species, but among rock hyraxes, low-ranked bachelors can often be seen sitting alone or huddling with females. Although female hyraxes are no larger than males, they are more likely to fight.
Previous studies suggest testosterone promotes dominance and aggression in human males, with levels rising in men during a challenge. Winners' testosterone levels rise after a fight, while losers experience a testosterone decline.
Research led by James Dabbs, a professor of psychology at Georgia State University in Atlanta, further supports the apparent testosterone-dominance connection.
Dabbs and his team measured the hormone levels of female prison inmates. Women who had committed violent crimes and those who demonstrated "aggressive dominance" in prison had higher than normal testosterone levels.
Neither Dabbs nor Koren, however, are sure which comes first -- the violent behavior or the testosterone. Aggression, whether evolved at the species level or developed over an individual's lifetime, may precede the body’s release of the hormone.
"It’s like the chicken and the egg," concluded Koren, "but hormones come to serve behavior, I think, and not the opposite."