No laughing matter

Natural History, July-August, 2006 by Nick W. Atkinson

To be top dog in a society of spotted hyenas, you've got to be a real bitch. Fiercely competitive scavengers, hyenas have no truck with the usual mammalian rules of dominance: females control the social hierarchy with an aggressiveness normally exerted only by males. Whether in females or males, aggression means androgens--male sex hormones, such as testosterone--and plenty of them. Biologists have known that prenatal exposure to androgens (independent of an individual's genetic endowment) fosters lifelong reproductive success in birds. Now, Stephanie M. Dloniak and Kay E. Holekamp, wildlife biologists with Michigan State University in East Lansing, and one of their colleagues have shown that prenatal androgen exposure in at least one mammal--the hyena--may translate into access to food and mates.

Androgens, like other hormones, pass from the blood of a pregnant female hyena through the placenta to her developing fetuses. By studying fluctuations in androgen levels during pregnancy, the biologists discovered that during the late stages, dominant females have higher androgen levels than do females further down the pecking order. That extra androgen boost in the womb leads to increased aggressiveness and mounting behavior in cubs of high-status females, traits that should give the cubs a competitive edge later in life. Practice at mounting is particularly important for male hyenas, say the biologists. The female mates and gives birth through her uniquely masculinized genitalia--an elongated clitoris that resembles a penis. Mating (not to mention birthing) is thus extremely difficult. So, the more mounting males do as cubs, the more successful at mating they're likely to be as adults. (Nature 440:1190-3)