The simple but very effective rules that female hyenas employ to prevent incest with closely related males are now known. The females go for familiar, younger males, a new study states.

The research, detailed in the Aug. 16 issue of the journal Nature, shows that a young female hyena prefers to mate with younger males in her clan—that is, males born into, or who joined, the group after she was born.

Older females also apply this rule and have an additional requirement: They prefer males that have built friendly relationships with them for several years.

The female preferences mean that most male hyenas end up leaving the group they were born into for other groups where females are less picky and more receptive.

"This is the first time a study has shown that in mammal species the system is driven by females using very simple rules to avoid breeding incestuously," said study team member Terry Burke of the University of Sheffield in the United Kingdom.

Inbreeding can be hazardous to a population because it weakens the gene pool by increasing the probability of inheriting harmful traits. Animals generally avoid inbreeding by either moving away from home or, like humans, learning who their relatives are and avoid mating with them.