Hyenas' Top Dog Status Begins in the Womb
for National Geographic News
|April 26, 2006|
High levels of male hormones known as androgens might explain how higher-ranking female hyenas
pass on dominant status to their offspring,
according to a new study.
In spotted hyenas both male and female cubs exposed to an extra dose of androgen in the womb begin engaging in more aggressive behavior soon after birth.
Male cubs engage in more sexual play, such as mounting, than the females. But extra maternal androgens are associated with higher rates of aggression and mounting in both sexes, the study finds.
Cub aggression, however, is not necessarily higher among offspring of high-ranking mothers, the study says.
The study is to be published in tomorrow's issue of the journal Nature.
"Basically what the analysis indicates is that, while some of the differences among individuals in levels of aggressive and sexual play in cubs can be predicted solely by the mother's dominance status, there are still important differences in levels of these behaviors that can only be accounted for by the levels of androgen the cubs experienced while in utero," said lead author Stephanie M. Dloniak of Michigan State University.
The research was carried out on wild hyenas at the Masai Mara National Reserve () in Kenya from 1993 to 2002.
The researchers measured hormone concentrations in the animals' fecal samples. The work revealed that higher ranking females had higher androgen levels than low-ranking females. The effect was apparent in their cubs.
Hyena society is rigidly hierarchical, and higher ranking members are the ones with first access to food.
Within a pack, females vary widely in rank, but all adult females are socially dominant to all adult males. Females are larger than the males and more aggressive.
Unlike the females of most social carnivore species, all the females among spotted hyenas produce offspring, but the higher ranking females have a longer reproductive life and more litters.
(See photos of spotted hyenas.)
The females are masculinized not only in their behavior, but also in their appearance.
In addition to being slightly larger than the males, their external genitalia look more like a penis than a vagina.
An elongated clitoris, as long as 7 inches (17 centimeters) in an adult, is the only visible sex organ, which makes telling a male from a female a challenge. In fact, the ancient Greeks thought hyenas were hermaphrodites.
"Female-dominated societies are few and far between," Dloniak said. "Spotted hyenas are even more weird, because of the extreme masculinization of both the external genitalia and the behavior of the females.
"The questions of why and how spotted hyenas are like this have been puzzles for a long time. We embarked on this study to test the hypothesis that maternal effects of androgens may be involved."
According to the researchers, this is the first time a relationship between maternal androgen levels and offspring behavior in wild mammals has ever been demonstrated.
The scientists also suggest that the female hyenas' masculine appearance and behavior might have evolved because of the importance of aggression wherever there is intense competition for food.
This kind of knowledge could only be gained by observing the animals in the wild, according to Dloniak.
"What is remarkable about our current findings is the observation that these are natural variations within the system," she said.
"It would be impossible to address this particular question in captive hyenas because the social environment would be completely different."
Researchers Weigh In
Other researchers are impressed with the work.
"This is one of few studies that has demonstrated an improved fitness of offspring resulting from maternal hormonal environment in mammals," said Micaela Szykman, an assistant professor in the department of wildlife at Humboldt State University in California.
"[This is also] the first [study] to demonstrate such a relationship reflecting a transfer of benefits of high rank from mother to offspring via prenatal hormone exposure," said Szykman, who was not involved in the study.
But not all experts are persuaded by the study's findings.
Wolfgang Goymann of the Max Planck Institute for Ornithology in Andechs, Germany, conducted his Ph.D. work on spotted hyenas.
"This is an interesting finding," he wrote in an email message, "but the authors miss the opportunity to discuss their results in the light of previous important findings, and they fail to discuss the issue of sibling rivalry and its implications for their findings.
"Hence I am not yet convinced whether the correlation between maternal androgen metabolites and mounting/aggressive behavior between non-littermates is meaningful."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
|© 1996-2008 National Geographic Society. All rights reserved.|