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ANY teenage boy will confirm that older boys make it impossible to get the girls. Young male orang-utans with the same problem have a unique and unexpected solution: they don’t grow up until they are strong enough to challenge the dominant males.

Male orang-utans can reproduce from around age 15, but in order to attract a mate they also have to develop secondary sexual characteristics – the equivalent of men growing chest hair. These include conspicuous cheek flanges. Yet Sumatran orang-utans , sometimes for over 10 years. No other primates do this, not even .

Gauri Pradhan of the University of South Florida in Tampa and colleagues noted another difference between the species: unlike Bornean males, Sumatran males can monopolise females for weeks at a time. Pradhan built mathematical models of orang-utan populations from decades of field data, and varied the extent to which males could monopolise females. She found that males that could delay maturation did better when a few males controlled all the females. They gradually built up physical strength until they were capable of deposing the dominant males, at which point they matured (). The model is simple yet solid, says of the University of Amsterdam in the Netherlands.