Social rank is a key phenotypic trait in human and many animal societies. High-ranking animals have preferential access to resources such as food, mating partners or shelter. As a result, high-ranking individuals usually survive better and produce more offspring than lower-ranking individuals.
In group-living societies such as that of spotted hyenas and many primates, offspring usually acquire a social rank just below that held by their mother (‘rank inheritance’) and obtain the fitness benefits associated with it. As a result, daughters and sons of high-ranking mothers live a similarly privileged and long life as their mothers.
How is the social rank transferred to the offspring?
Three main hypotheses have been proposed:
- Genes: The offspring inherit genes from their mother that make them as competitive as their mother (direct genetic effects).
- Hormones: The social rank of the mother determines the concentration of sexual hormones (testosterone) that a foetus is exposed to, and this exposure in turn makes offspring as competitive as their mother.
- Social support: The mother supports her offspring during interactions with other clan members. During these interactions, the offspring ‘learn’ which clan members their mother – and they themselves – can dominate.
Adoptions offer an ideal natural experiment to test these hypotheses. Using rare cases of offspring adoption in spotted hyenas of the Ngorongoro Crater and the Serengeti, we and our colleagues from the Serengeti demonstrated that the rank of adopted offspring was very similar to that of their surrogate mother and quite different from the rank of their genetic mother. This demonstrated that rank inheritance is mainly driven by postnatal maternal behavioural support and that direct genetic effects and pre-natal hormonal effects may only play a minor role.
Mothers support their offspring during encounters with other hyenas. Young hyenas start initiating interactions with adult clan members from a very young age. If they approach a lower-ranking clan member, the mother supports them and shows dominant behaviour. If the opponent is higher-ranking, the mother does not interfere or shows submissive behaviour. Young cubs thereby quickly learn by observation who is above and who is below their mother (and themselves) within the clan hierarchy. This mechanism also works for siblings from different litters of the same mother.
Mothers always provide most support to the cubs of their most recent litter (‘youngest ascendancy’). As a result, the youngest cubs dominate their older siblings right from the beginning. When they grow older, hyenas defend the rank acquired during adolescence by building close relationships and coalitions with their mother, and older and younger siblings.
Hofer H & East ML (2003) Behavioral processes and costs of co-existence in female spotted hyenas: a life history perspective. Evolutionary Ecology 17(4): 315-331.
East ML, Höner OP, Wachter B, Wilhelm K, Burke T, Hofer H (2009) Maternal effects on offspring social status in spotted hyenas. Behavioral Ecology 20: 478-483.