Biologist Explores Strange Physiology Of The Hyena

Feeding a fistful of pills to a ravenous hyena without losing a finger, or wrestling a trapped wild hyena to the ground in the trackless Namibian desert -- these are only two of the "adventures" Duke biologist Christine Drea has experienced in her effort to understand the exotic physiology and behavior of the wild hyena.

Drea's research exemplifies one of the central precepts of science -- that good scientists must be willing to pursue their studies wherever the quest for understanding leads. In Drea's case, her pursuit has led her to explore the bizarre reproductive physiology of the spotted hyena.

The female spotted hyena is significantly larger and more aggressive than the male, as well as possessing a long clitoris that resembles a penis so closely that early naturalists believed that there existed no female spotted hyenas.

The female's masculine characteristics do offer advantages in competing for food and protecting cubs, said Drea, who is an assistant professor of biological anthropology and anatomy. However the penis-like clitoris exacts a severe penalty during a mother's first birth, because its structure constitutes a severely constricting birth canal. Thus, cubs in a female's first litter often die from suffocation while passing through the birth canal.

The female's unique genital physiology also presents major mating problems for the male, who must awkwardly insert his penis into the clitoris positioned well forward beneath the female's body, said Drea. To help overcome the mechanical problems in mating, said Drea, the male has evolved adaptations to aid successful copulation, including a long, angled penis with small spines at the tip.

A central biological question for Drea and her colleagues is how male hormones, called androgens, act to generate such extreme adaptations in both the female and the male.

So, Drea and her colleagues, decided to explore experimentally what would happen if both males and female hyenas were deprived of androgens in the womb. Answering that question required many years of research, the latest results of which were reported by Drea and her colleagues in a scientific article in the October 7, 2002, Proceedings of the Royal Society.

In their experimental study, Drea and her colleagues at the University of California at Berkeley fed pregnant female hyenas anti-androgen drugs and studied the effects on the reproductive organs of the offspring. The logistics of studying the hulking 160-pound hyenas -- kept in a colony at Berkeley -- were daunting, said Drea.

"Imagine hand-feeding over twenty pills per day to a ravenous hyena, without losing your fingers; or worse, coaxing a finicky eater to take medicated food," she said. "Either way, it's an ordeal.

"It wouldn't be so bad if every pregnancy was guaranteed to produce live cubs, but with the naturally high rate of complicated deliveries in this species, losing the cubs of a 'treated' pregnancy was exceptionally demoralizing.

"Researching such animals demands unwavering patience," said Drea. "A molecular geneticist might run an experiment in a matter of days, but we had to wait over five years for each female subject to give birth a first time." However, the payoff was dramatic. The effects on the offspring of the anti-androgen treatment were biologically striking, said Drea.

"The treated female offspring displayed subtle but critical changes," said Drea. "Their phallic clitoris was shorter, thicker, and with an even wider, more elastic opening than normal female spotted hyenas." Said Drea, "These changes were enough so that such females could give birth to live cubs the first time." However, reported the scientists in the Proceedings paper, the corresponding changes in the treated male offspring had major consequences.

"Treated males now had a penis that was slightly shorter, thicker, more rounded at the tip, and with a larger, more elastic opening," said Drea. "Although fairly subtle to the untrained eye, these changes were sufficient to prevent males from impregnating females." According to Drea, the study of such exotic species, while interesting in themselves, are more broadly important because they offer a perspective on biology.

"By studying exceptions to the rule, you can learn about the 'usual' process," she said. "Spotted hyenas challenge our normal way of thinking about both sexual differentiation and evolution.

"For instance, regarding mammalian development, we have a pretty good understanding of how the hormonal process gets us from a basic female form to a differentiated male form. However, we've learned from the spotted hyena that we lack a complete understanding of how to get the basic female form."

Thus, said Drea, she and her colleagues need much further study of the complex interplay between hormones and other developmental factors to understand the puzzle of the spotted hyena's reproductive machinery. What's more, she said, the spotted hyena offers important lessons about the complementary nature of evolution.

"This study highlights the necessary evolutionary relationship between the sexes, specifically male and female genital development," said Drea. "When a change occurs in one sex, a complementary change must also occur in the other sex -- a kind of lock-and-key scenario if you will."

Drea's work has also taken her into the depths of the Namibian desert in Africa , where she and colleagues have launched expeditions to study the brown hyena. This animal, cousin to the spotted hyena, does not show the same striking female masculinization. By taking blood samples of briefly captured animals, and observing their mating and social habits, Drea hopes to further understand the contributions of hormones to physiology and behavior in both species.