Assertiveness is the best form of defense, at least for hyena pups

These findings challenge the general expectation that in species with intense and potentially fatal sibling rivalry -- as the spotted hyena -, the dominant sibling has complete control over the share of food received by its sibling(s). The expectation derives from extensive theoretical and empirical research on sibling rivalry in birds. It was not known whether dominant siblings in mammalian litters exert such a despotic control over their siblings' access to the resource until this question was investigated for the first time in this study in the spotted hyena by researchers from the German Leibniz Institute for Zoo and Wildlife Research (IZW).

The authors studied sibling tactics during competition for access to maternal milk. The results, recently published in the scientific journal Proceedings of the Royal Society B, demonstrate that by using aggression, dominant siblings monopolise access to maternal milk and thereby benefit from higher growth rates than their subordinate littermate. However, subordinate siblings adjust their behavior according to their hunger level and increase their assertiveness when hungry. As a result, subordinate siblings enhance their own access to milk, suggesting that they can effectively adjust their submissiveness according to their likelihood of starvation.

But why does one offspring show submission to its sibling anyway? In contrast to bird species with asynchronous hatching, in the spotted hyena dominance does not depend on differences in body size or fighting ability. The study shows that young subordinates are intensively trained as 'losers' by their dominant sibling, and as a consequence, respect dominance conventions as they become older.

The authors then tested the effect of the sex of the littermate on sibling success during competition for milk. Interestingly, in a species where adult females socially dominate adult males, the researchers found that sisters were more effective than brothers during competition for maternal milk. This result is in contrast to many studies according to which females were weaker competitors than males during conflicts.

These new insights into how the fundamental dynamics of sibling rivalry is modified by litter sex composition and hunger state offer opportunities for future research. One such topic could be how mothers affect sibling rivalry by chosing to intervene or not to intervene.