Hyenas Encourage Sex With Strangers to Prevent Incest

James Owen
for National Geographic News

August 15, 2007

Female hyenas avoid incestuous mating by encouraging male relatives to look elsewhere for sex, new research shows.

The females use their dominant status in hyena society to spurn males in their clan, thereby avoiding the risk of inbreeding, the study suggests.

This tactic has never been demonstrated before in mammals but may be widespread among other species that live in groups, the scientists added.

The ten-year study was based on eight groups, or clans, of spotted hyenas living in Ngorongoro Crater, the world's largest intact volcanic caldera, in Tanzania.

A team led by Oliver Höner at the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, investigated the dispersal of male hyenas using DNA samples and field observations of more than 400 individuals.

The findings, reported in the latest issue of the journal Nature, indicate that young female hyenas prefer mating with males that immigrate from other clans, or with males that were born after they were.

Older females were also found to mate with immigrants, favoring those that had courted them for several years.

As a result of these preferences, 89 percent of young males left their clans to take their sexual exploits elsewhere, the researchers found.

Höner said this pattern is the result of females following an instinctual mating rule that prevents incestuous encounters.

"This rule requires males to have entered the group after the females were born," he said.

"The older females also have an additional rule: They don't particularly like new, young males that they don't know well," Höner added.

Pseudo Penis

Inbred offspring are vulnerable to disease and other handicaps and are less likely to survive than normal young.

It's especially in the female's interests to avoid inbreeding, the team argues, because female spotted hyenas give their offspring exceptionally lengthy care, lasting 15 to 18 months.

Males, on the other hand, are largely absent fathers.

"Females invest so much more in their young," Höner said. "If males breed with a close relative, they don't lose very much because they have other females they can produce offspring with."

However, male hyenas have no choice but to go along with the mating preferences of the socially dominant females, whose bizarre genitalia make forced sex almost impossible.

"Females have a pseudo penis—an enlarged clitoris—which points forward," Höner explained.

"This makes it difficult for the male to mate. He has to balance precariously and needs the full cooperation of the female."

Female Power

Despite this peculiarity, female mate choice may help explain male dispersal patterns seen in many other mammals, the study team said.

"In the vast majority of mammals the typical pattern is that males disperse and females stay, or that males disperse over a greater distance than females," Höner said.

"We think that in many group-living mammals where females have a choice of different mates, it's very difficult for females to recognize their fathers," he added.

"The female mate choice we found here is likely to play an important role in other species," Höner said, primates included.

Laurent Lehmann of Stanford University commented that the female mate choice rule proposed by the study team "is very simple and very plausible, and so might apply to other social or nonsocial mammals as well."

But, he added, the extent to which the rule explains "the level of male sex-biased dispersal in natural populations [of other species] is not yet clear."

Spotted hyena expert Kay Holekamp from Michigan State University said the new research probably could only have been conducted in Ngorongoro Crater, "where so many clans live in close proximity to one another and where visibility for observers is excellent."

Holekamp added that the study "supports a hypothesis many of us have favored for many years—that female mate choice is all-important in this species."

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