In the Aggresive Hyena World, Male-Like Moms Rule
Berkeley scientists have found that the humans and spotted hyena share a surprising similarity: Females of both species often have difficulty giving birth because the newborns are unusually large and the birth passage is unusually small.
In humans, the evolution of high intelligence has produced a very large head to house a large brain, but the pelvis and birth canal have narrowed to permit upright walking.
In hyenas, the babies are large because they fight ferociously at birth, and the birth canal is narrow because it passes through the female's greatly enlarged clitoris, which is the size and shape of a male penis. She has no normal vagina and the clitoris is fully erectile.
In a recent paper in the British journal Nature, research associate Laurence Frank, project manager Mary Weldele and Professor Stephen Glickman report that up to 10 percent of first-time mothers, and over 60 percent of first-born young die during birth through this remarkable organ. The first birth stretches and tears the clitoris, so that subsequent births are uneventful.
The extraordinary male-like anatomy and aggressive behavior of the spotted hyena has been under intense study at the Berkeley Hyena Project, under the direction of Glickman, a professor of psychology.
Collaborative studies with Professor Paul Licht of the Department of Integrative Biology and endocrinologists at UCSF have shown that the masculinization syndrome is associated with massive quantities of the male hormone testosterone that is produced by the pregnant female, bathing her embryos and apparently masculinizing her daughters.
Similar genital masculinization sometimes affects human females due to a variety of hormonal disorders during gestation.
In spotted hyenas, however, female masculinization is the norm. The scientists believe that evolution has favored highly aggressive females as a response to the intensely competitive feeding that characterizes wild hyenas.
Contrary to popular belief, these animals are active predators. When they kill a wildebeest or zebra, the swarm of squabbling hyenas may reduce the carcass to a bloody patch on the ground within minutes.
In this severe competition for rapidly disappearing food, there is a great advantage in being more aggressive, and extensive studies of laboratory ani
mals have demonstrated a strong link between prenatal exposure to androgens (male hormones) and subsequent aggressiveness as adults. Prenatal androgens, however, are also responsible for the development of male genitalia. The hyena's unusual anatomy may thus be a side effect of natural selection for aggressive females.
Although the cost of childbirth through a clitoris may seem high, the benefits of aggressiveness are much higher. Frank has been studying the ecology of wild hyenas in Kenya for 17 years, with Kay Holekamp and Laura Smale of Michigan State University (both of whom earned their PhDs in Psychology at Berkeley). They have found that the most dominant female hyenas produce two to three times as many adult offspring as do all lower ranking females. Cubs of lower ranking females tend to die before reaching maturity, probably because they are unable to feed successfully in competition with higher ranking cubs and adults. Aggression maintains the social hierarchy over many generations, because females pass their social rank on to their daughters. Frank calculates that all the female hyenas in his wild study group will be descended from a single mother in about 40 years, or 15 generations.
The great evolutionary benefits of female aggressiveness thus seem to more than compensate for the loss of cubs and mothers in childbirth. The result is a most unusual animal which may provide a key to understanding the much more subtle interplay of hormones and behavior in other mammals, including humans.