By Eve Davidian and Oliver Höner
Males that stay at home are not second-class males but can breed as successfully as their more adventurous competitors that leave home, our new study on spotted hyenas shows. The results were published in the open access journal Science Advances.
In most mammals, there are two kinds of males: those that stay at home and those that disperse from home to breed elsewhere. In the scientific literature, stay-home males were generally considered as ‘losers’ that failed to join another group and that father few offspring. Our new study now demonstrates, for the first time in a group-living mammal, that stay-home (philopatric) males and dispersers are not inherently different and have similar reproductive success. Our study further shows that their choice to stay at home or leave home is the outcome of a process during which all males answer the same question: which group will provide me with the best fitness prospects? Whether a spotted hyena male stays at home or hits the road simply depends on whether his birth clan or another clan contains the highest number of young females when he wants to start breeding.
For this study, we monitored the entire population of spotted hyenas inhabiting the Ngorongoro Crater in Tanzania. We combined two decades of demographic data from the eight clans with data on the behaviour, survival, and reproductive success of more than 250 males to investigate the causes and fitness consequences of male clan choice.
In natural populations, group sizes and the number of young females fluctuate due to chance and environmental effects. Thus, when there are more than two groups, the home group is less likely to contain the highest number of young females than a non-home group. As expected, most hyena males in the Ngorongoro Crater dispersed to another clan. Yet, we found that twice as many males stayed at home than we had expected based on the distribution of young females only, suggesting that staying at home comes with advantages.
The main advantage is provided by their mothers. In the matriarchal system of spotted hyenas, females can influence the competition among males. Mothers provide social support to their stay-home sons and ensure they acquire a high social rank among breeding males. This gives the mama’s boys privileged access to both food and females, allowing them to invest a lot of time consorting females. And this pays off. Genetic paternity analyses showed that stay-home males father their first cubs at a younger age than dispersers and father almost exclusively cubs with high-ranking females — females of high reproductive value because they are most successful at rearing offspring. This is the first empirical evidence in a group-living mammal, that stay-home males can be equally, if not more, successful as dispersers.
Dispersal is a key driver of ecological and evolutionary processes, yet scientists still do not fully understand why individuals of the same sex of a species differ in their propensity to disperse. By showing that dispersal patterns can be shaped by the distribution of breeding partners across groups, our study expands our understanding of the processes leading to the coexistence of philopatry and dispersal within a sex.
Davidian E, Courtiol A, Wachter B, Hofer H, Höner OP (2016) Why do some males choose to breed at home when most other males disperse? Science Advances 2 e1501236.