Why males disperse
In most group-living mammals, dispersal is male-biased: most males leave their natal group shortly after reaching maturity and disperse to another group to breed while females usually remain at home.
Four main hypotheses have been proposed to explain male-biased dispersal:
- Male competitors: young males are chased away by more competitive (stronger, more experienced) males.
- Food: young males avoid competition for resources.
- Inbreeding: males avoid breeding with close female relatives.
- Female mate choice: males disperse as a response to female mate preferences.
We simultaneously tested all four hypotheses and measured the long-term fitness consequences of male dispersal decisions in our spotted hyena population. We found that male-biased dispersal resulted from an adaptive response by males to simple female mate-choice rules.
Spotted hyena females have very clear ideas about which mate to choose. Young females prefer males that were born into or that joined the group after the female was born. By applying this simple rule, females effectively avoid incestuous breeding with their father and older brothers. In addition, older females preferentially choose males that developed a friendly, long-term relationship with them.
Female mate preferences concert the dispersal behaviour of males. The two female mate-choice rules imply that young males that are about to start their reproductive career stand the best chances to find females willing to mate with them in the clan that contains the highest number of young females. And indeed, young males strongly preferred starting their reproductive career in the clan with the highest number of young females.
Winners are those that adjust best to female preferences. The tactic to choose the clan with the highest number of young females is very successful: males applying this tactic have access to more females and father more cubs on the long term than males that chose clans with relatively few young females. Thus, adjusting the behaviour and breeding clan choice to female mate preferences has a substantial selective advantage for spotted hyena males.
Female mate preferences drive male dispersal patterns and population dynamics. In the Crater, the number of young females in the 8 clans fluctuates due to chance and environmental effects. For example, a given clan may produce fewer daughters than sons – spotted hyena females cannot determine the sex of their offspring – during some periods, or suffer higher mortality during a disease outbreak than other clans. A clan that contained many young females during one period may thus not be the one containing the highest number of young females during another.
Because the Crater population contains eight hyena clans, the probability that the birth clan of a male contains the highest number of young females is about 1/8. This means that most of the time a clan other than a male’s birth clan contains the highest number of young females. Consistent with this, 85% of young males in the Crater disperse from their birth clan to breed in another clan. Furthermore, because males preferentially disperse to the clan with the highest number of young females, the sex ratio in these clans is constantly balanced. These findings illustrate nicely, what far-reaching consequences two simple female mate-choice rules can have.
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.
Höner OP, Wachter B, East ML, Streich WJ, Wilhelm K, Burke T, Hofer H (2007) Female mate-choice drives the evolution of male-biased dispersal in a social mammal. Nature 448: 798-801.
Höner OP, Wachter B, East ML, Streich WJ, Wilhelm K, Burke T, Hofer H (2008) Do female hyaenas choose mates based on tenure? Reply. Nature 454: E2.
Roll U (2014) Weibchen an der Macht. Bild der Wissenschaft 20. Mai.