A new scientific study shows for the first time in spotted hyena (Crocuta crocuta) twin litters, that success in sibling competition for maternal milk is influenced by training effects, sex and hunger, and that dominant siblings exert incomplete control over their littermate's access to the resource.
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.
This expectation comes 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, 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 behaviour 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.