Should I stay or should I go? For a young male spotted hyena, the decision to stay or disperse from his clan depends on his mating prospects: If there are more young and willing females back home than elsewhere, he’ll stay. The findings, published in Science Advances this week, suggest that male hyenas regularly assess the reproductive potential of other clans.
Dispersal is one of the most important – yet least understood – drivers of ecological and evolutionary processes. There are variations even within the same sex of the same species. Some individuals leave the location where they were born to seek breeding opportunities, while others never leave home, or they might but not until later. Leaving generally reduces kin competition and inbreeding, but staying offers kin cooperation and the advantages of familiar territory.
Spotted hyenas (Crocuta crocuta) live in social groups structured by strict hierarchies: Females and their offspring are always dominant over newly settled males, and the sons of high-ranking females grow faster and start reproducing earlier. But any male moving into a new clan always starts at the bottom, and they move up with increasing tenure. Both sexes are promiscuous, and while female hyenas have complete control over mating (and apply rules to avoid incest), reproduction isn’t monopolized by only high-ranking individuals. A male who’s developed friendly relationships with females would have a better chance than a bigger male who can fight well. And with more access to food, high-ranking males can spend more energy developing these key relationships.
Neither high-ranking nor low-born males will be evicted from their natal clan. But before choosing to settle in a clan for breeding, both stay-home (or philopatric) males and dispersers trek out to other clan territories, likely to assess their prospects in potential breeding clans.
To understand why there’s both philopatry and dispersal among male spotted hyenas, a Leibniz Institute for Zoo and Wildlife Research team led by Oliver Höner studied data gathered from 1996 through to 2014 on a free-ranging population comprised of eight clans at Ngorongoro Crater in Tanzania. They combined demographic data with information on breeding-group choice, survival, reproductive success, and various traits like age and rank of 254 males: 41 philopatric males and 213 dispersers.
They found that hyena groups with lots of philopatric males also have more young females who are willing to mate with them. The breeding prospects were good at home. Meanwhile, groups that were left behind by many males also offered fewer breeding opportunities. Unpromising prospects likely prompted them to leave. Importantly, males who stayed home and males who hit the road enjoyed similar reproductive success.
The other hypothesis the researchers tested – but found no evidence for – suggests that only high-quality males would be able to go on prospecting trips, and the rest are left to make do at home. This just wasn’t the case for spotted hyenas: Not only were philopatric males and dispersers of varying ranks similarly successful, they all applied similar clan-choice rules, which are based on their own prospects.