The Paradoxical Predator

"We hypothesized that they would need a more friendly behavioral repertoire in place before being transferred to the social communal den with other clan members. And so, we looked for more subtle effects of their behavior in litters. We found that after the first day, fighting declined precipitously, and in the second week of life they began vigorous rough-and-tumble play. So, we saw this behavioral flip-flop, where they go from high levels of aggression to high levels of play."

On the prowl: A spotted hyena in the wild. Getty Images.

The purpose of the early aggression, concluded Drea and Glickman, is to establish a dominance hierarchy. And the later play, as with primates such as humans, helps the group form social bonds. "I think that people hadn't really thought of hyenas as playing animals--animals that have a softer, gentler side," says Drea. In Glickman's studies, he and his colleagues have found fascinating differences between hyena-play and primate-play. Since female spotted hyenas are so masculinized--even possessing genitalia with a phallic clitoris that resembles a penis--hyena-play also shows this role-reversal, with females playing rougher than males.

Spotted hyena clans are remarkably social, says Drea. They cooperate closely in their hunting to bring down even the most formidable prey, such as the massive African buffalo. And in her experiments with the captive hyenas, Drea and her colleagues found that the animals would intelligently and readily cooperate in pulling two different ropes to obtain food. Intelligent cooperation evolved in the hyena for the same reason it did in primates, concludes Drea, to solve the problems of survival under pressure of their environment.

Drea and her Berkeley colleagues even explored the spotted hyenas' marked preference for "perfume." Hyenas and dogs seem to enjoy rolling in strong, animal-based odors, and the scientists set out to discover a possible social role of such scents. When the scientists applied plant perfumes to captive animals, they saw no effect once the scented animals were returned to their communal yard. But when they graced an animal with a rotten-meat odor, the perfumed hyena became extremely popular upon return to its colleagues--triggering a flurry of friendly activity that included grooming.

Spotted hyenas can even "lie" as adeptly as humans and other primates, the scientists have discovered. In experiments with the captive hyenas, Glickman and his colleagues found that when a dominant hyena accompanied by a "naïve" animal knows the location of hidden food, she will confidently approach it directly. But a subordinate hyena in the same situation will lead other animals astray, later sneaking back to the food to claim it.

Now at Duke, Drea has turned to studies of the brown hyena for comparative insights into the species. "Of the four extant hyena species, spotteds are the only ones to show masculinization in the female," she says. But since brown hyenas are both elusive and highly endangered, very little study had been done of them. Fortunately, Drea's graduate student Ingrid Wiesel, of the University of Hamburg in Germany, had gained the confidence of the Namibian government and the country's diamond industry in her earlier studies of the brown hyena in the so-called "diamond area" on the southwestern coast of Namibia. The Namibian Carnivore Monitoring Project is pleased to encourage such studies, says Drea, since they contribute to understanding and preserving the species. Nevertheless, few humans have been allowed into this highly protected desert conservation area, so Drea counts herself fortunate to have made three expeditions, the latest last fall.

"The brown hyenas have similarities in their social structure to the spotted hyenas, but they also have important differences," says Drea. "One is that they are not social group hunters, but are solitary foragers. However, the population of brown hyenas we're studying lives on the coast, where they hunt Cape fur seals. So, they are showing some behavioral plasticity."

One result of her latest expedition, sitting in a rack in Drea's office on campus, are nine test tubes of blood from the hyenas captured last summer. These samples represent the beginning of her effort to explore the biology and behavior of the mysterious animals. Hormonal analysis of the samples will yield clues to how the male hormones influence the females' development. And DNA analysis of both the male and female captives will reveal how the animals are related. By tracking such animals, the scientists can explore whether "cooperative breeding" does indeed occur.

"Reports have indicated that males were providing food to cubs they weren't related to, and that seems to go against common sense," Drea says. "It may, however, be an example of what is called 'reciprocal altruism,' in which one hyena helps others with an expectation of future help in return. But since there has never been any hard evidence to back up that claim, we're trying to get that." Before she can reach any conclusions, she and her colleagues will go through many more years of behavioral observation and extensive sampling of blood from both adults and cubs.

Drea recognizes that even with the best of data, she and her colleagues face a deep and complex biological mystery. "The whole theory behind masculinization in female spotted hyenas has been that androgens have to be responsible. But when we gave pregnant females drugs that blocked the source of androgens, their female cubs didn't look as masculine, but their sex organs were still masculinized. So, there's something else going on that we don't understand."

Closer to home, Drea also plans similar hormonal and behavioral studies of the lemurs at the Duke Primate Center. "Lemurs show the same female dominance and aggression as hyenas, and they are masculinized to some degree," she says. "But while lemur females aren't bigger than males, as in spotted hyenas, they are still dominant. So I want to find out what it is about females that makes them more aggressive than males." She hopes that tracking androgen levels during the pregnancies of ringtail and other lemurs will yield more clues to how hormones influence development and behavior.

Drea expects her studies with hyenas and lemurs to offer useful comparative insights into how the development of mammalian females in general, including humans. Like most basic biological studies, she says, exploring one species can yield insights into all other species.

"The broader lesson I see is that, for a long time, female sex differences have been accounted for in terms of how females differ from males, and how one gets from the female form to the male form. But while our current theory of sexual differentiation accounts for the differences between the sexes, it doesn't account for how the basic structures develop. And I think that current theory has seen female sexual development as a passive process--you do nothing, you get a female. You have to do something to that 'default' system in order to get a male. But I just don't think that theory gets at the heart of the issue--how do you get the basic form?"

Drea and her colleagues will continue to wrestle hyenas in forbidding African desert ghost towns--carefully capturing and releasing the animals in order to wrest from the resulting hard-won data some hard scientific truths.