CHICAGO, Aug 15 (Reuters) - When it comes to picking a mate, female hyenas prefer the mysterious stranger to the boy next door, researchers said on Wednesday, in a finding that explains why male hyenas who leave the home clan are luckiest in love.

Researchers, writing in the journal Nature, said the preference for the outsider helps female spotted hyenas avoid inbreeding.

"Males that responded to the observed female mate preferences had the highest long-term reproductive success," said Dr. Oliver Honer from the Liebniz Institute for Zoo and Wildlife Research in Berlin and colleagues writing in the journal Nature.

Honer and colleagues at the University of Sheffield monitored the reproductive careers of eight resident hyena groups on the floor of the Ngorongoro Crater in Tanzania, then genetically determined the paternity of their offspring over a 10-year period.

They believe the reason most males move away from the home clan is because young females prefer to mate with new arrivals in a group -- males that have immigrated from another group after the female was born.

Unlike humans, female hyenas are unlikely to know who their father is because -- like most male mammals -- male hyenas do not contribute to the rearing of their offspring, they said.

And female spotted hyenas -- who are larger than males and lead the pack -- mate with several mates over their lifetimes.

Male hyenas who move to groups containing the highest number of young females ultimately sired the highest number of offspring, the researchers said.

Older females apply the same rules in mate selection, but with a twist. They tend to choose immigrants with whom they have built friendly relationships over several years.

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