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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