WEB EXCLUSIVE

Top Dogs

Steroid hormones give hyenas a head start.

By Jocelyn Selim|Friday, April 28, 2006
RELATED TAGS: SEX & REPRODUCTION

Gender roles in hyenas are curiously feminist. Unlike, say, wolves or lions, where alpha males rule the roost, females strictly dominate the hyena social hierarchy, with those who are most willing to fight getting prime access to both males and food.

It's a rough life—top hyena females typically exhibit robust coats and overall well-being, but those toward the bottom of the totem pole wither in ill health along with the boys. Now a team of zoologists led by Michigan State University's Kay Holekamp has figured out what lies behind top dogs' ability to leave the weak and meek behind.

The answer, says Holekamp, is prenatal exposure to androgens in the womb. For nearly a decade, Holekamp and her colleagues studied wild hyenas from a large, stable group in Kenya's Masai Mara National Reserve. After analyzing the droppings from pregnant, high-ranking females, they showed that during the last few weeks of gestation, blood levels of testosterone and similar androgenic male hormones rose sharply, while those of their less socially adept sisters remained stable.

Holekamp and her colleagues then correlated this androgen exposure to the behavior of their cubs, showing that high levels of prenatal hormones were closely tied to classic aggressive behaviors like fighting and mounting. And, as one would expect in a society that prizes meanness, the more aggressive cubs grew up into higher ranking adults.

While an extensive body of research demonstrates that prenatal hormone exposure influences the morphological and behavioral development of mammals, this is the first time a link between androgen exposure and inherited social rank has been conclusively demonstrated.

More interesting, the study should have connotations for human behavior. "We, like other primates, live in a hierarchal society, and I would be surprised if there wasn't some effect on humans' social position that was mediated by maternal hormones," Holekamp says, pointing out that a number of studies have demonstrated that aggressive play behavior and toy selection in girls has been linked to early elevated androgen levels.

But more isn't necessarily better—in hyenas too much androgen exposure can affect the ovaries, causing decreased fertility, while overly aggressive girls may be prone to unnecessarily risky behavior. "There's a balance," Holekamp notes.

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