Hyenas provide clues to sexual differentiation

Study of this unusual animal could help scientists separate biological from environmental influences on sex-specific behaviors.

By Beth Azar
Monitor staff

For researchers interested in how sex hormones affect the development of the sexes, spotted hyenas are interesting not because of their differences, but because of their similarities, according to psychologist Stephen Glickman, PhD, of the University of California at Berkeley.

Not only are female spotted hyenas extremely aggressive, but they have genitalia that make them difficult to distinguish from males: They sport an overdeveloped clitoris that can become erect, looking much like an erect penis. Females urinate, copulate and give birth through this enlarged clitoris.

Glickman discussed the unusual genitalia of the spotted hyena and theories of how they develop at a symposium on sex hormones that was part of the 'Mind, brain and behavior' symposium series sponsored in part by the National Science Foundation and organized by APA divisions 3 (Experimental) and 6 (Behavioral Neuroscience and Comparative Psychology) at APA?s 1998 Annual Convention.

Psychologists study sex hormones because they play such a large role in behavior. Better understanding of how the hormones control sexual differentiation may someday help them separate biological from environmental influences on sex-specific behaviors.

The general theory of how animals develop into males and females states that the only way to make male genitalia is to have male hormones?called androgens and including testosterone?circulating during fetal life, said Glickman. The spotted hyena presented a nice test for this theory: Are there naturally circulating androgens around during female development? And, if so, are they necessary for growth of the enlarged clitoris?

The answers are twofold, said Glickman. There are plenty of male hormones circulating during fetal development of female pups. And these hormones likely modify the nervous system and adult behavior. But the hormones are not fully responsible for the male-like genitalia some other, as yet unknown mechanism is, he said.

Looking for hormones

Although male-like in looks and behavior, adult female spotted hyenas don?t have high testosterone circulating in their blood, Glickman and his colleagues find. But they do have high levels of a steroid hormone called andro-stenedione, which is produced in the ovaries. This is an interesting hormone, said Glickman, because it has no known function, but the body can convert it into either testosterone or estrogen?the largely female hormone.

Androstenedione levels become extremely high in pregnant hyenas. And, Glickman and his colleagues find, the placenta converts it to testosterone and passes it on to the fetus?male or female. In humans, the placenta normally converts andro-stenedione to estrogen, which binds to other proteins and never reaches the fetus. However, in several cases in humans, the placenta was found to work much like that of the hyena, converting androstene-dione to testosterone and passing it onto the fetus. In such cases, genetically female babies are born with male genitalia. In a recent set of studies, Glickman and postdoctoral student Christine Drea, PhD, tested whether the testosterone provided to the hyena fetus is responsible for the females? male-like genitalia. They injected pregnant hyenas with anti-androgens?substances that block the function of androgens, including testosterone?to see if they could change the course of fetal development. (In dogs, anti-androgens produce male pups with a very small penis and a blind vaginal pouch near the tail.)

In hyenas, it didn?t completely work. Male pups became 'feminized' but only to the point of looking more like a female spotted hyena, with a somewhat shorter and wider penis than normal. And the female clitoris was feminized even more, becoming exceptionally short and thick. But the structures of the genitalia remained in both males and females.

These data imply that, at least in the spotted hyena, testosterone is not responsible for the initial development of male-like genitalia, said Glickman. However, the hormone is necessary for full maturation of the structures and may also play a role in nervous system and behavioral differences between male and female spotted hyenas.