Competing for milk can be a stressful thing for hyena twin siblings

These findings were published in the science journal Biology Letters.

The researchers measured the concentration of metabolites of the 'stress' hormone cortisol in the faeces of young sibling and singleton spotted hyenas (Crocuta crocuta) in the Serengeti National Park, Tanzania, which depended on their mothers for milk. "It was not known whether sibling conflict over access to milk increased concentrations of cortisol metabolites in twin littermates -- we were now able to demonstrate that it significantly does," states Dr Sarah Benhaiem from the IZW, lead author of the study.

Surprisingly, hunger had little effect on the cortisol metabolite concentration of singletons, whether male or female. The picture was different for twin siblings -- both littermates had an elevated level when hungry. Interestingly, an even more important factor than hunger was the rivalry between twin littermates. In general, the less assertive (subordinate) cubs had higher cortisol metabolite levels than the more assertive (dominant) cubs.

More interestingly still, when hungry, subordinates competing against a sister were more assertive than subordinates competing against a brother. As a result, in such situations dominant sisters had higher cortisol metabolite levels than dominant brothers.

For the first time in a free-ranging wild mammal, the study shows how rivalry between twin siblings affects the cost of maintaining internal stability.