Mature male orangutans have large flappy cheek-pads, known as flanges. As far as females are concerned, they prefer males with them, over those without

Fully mature males are also twice the size of females and grow large throat sacs, all of which are characteristics associated with dominance.

But these traits can take a while to show. Some wild male orangutans take 20 years to grow flanges. These males look more like females and are much smaller than their flanged counterparts.

A new study tries to explain why males take so long to grow their flanges.

To discover this, researchers collected poo from 17 wild Bornean orangutans. Ten were flanged, six were un-flanged and one was in the process of developing his flanges. They all lived on the Mawas Reserve in Central Kalimantan, Indonesia.

The researchers then analysed the hormone levels in the orangutans' poo. They found that those with flanges showed increased levels of testosterone. 

"Un-flanged males have relatively low levels of testosterone," says lead author Pascal Marty of the University of Zurich in Switzerland.

The research has been published in the American Journal of Primatology

As soon as males begin to develop bigger cheek pads, their testosterone levels peak. "The very high testosterone levels of the one developing male in the study was a bit surprising but indicates the need for high testosterone levels to develop secondary sexual characteristics [such as cheek-pads]," says Marty.

When a male becomes "fully-flanged", his testosterone levels out again.

A previous study of zoo-housed orangutans had suggested that competition between males could increase stress levels and therefore lead to larger flanges.

A previous study of zoo-housed orangutans had suggested that competition between males could increas stress levels and therefore suppress development of flanges in subordinate males. But Marty's study found no evidence to support this idea.

"At our study site, developing males were never seen to engage in physical fights with fully-flanged males. Thus, we do not have evidence that increased aggressive competition is associated with the observed androgen levels," the authors write. "The results of this study allow us reject social stress as a major factor responsible for the arrested development in wild Bornean orangutans."

As well as an advantage when it comes to finding mates, studies show that those with larger cheek pads are also healthier. Weaker and older males have shrunken flanges.

The development of flanges is clearly based on hormonal changes. But "the trigger leading to this change is still unknown," says Marty.

He believes the answers may come from studies that track orangutans throughout their lives.