Monopoly of the male orangutan: Comparative field observations on Sumatra and Borneo

This is the conclusion of a study supported by the Swiss National Science Foundation (SNSF).

In Malay, the word "orangutan" means man of the woods. In fact, however, these rain forest dwellers clad in a reddish-brown coat are our most distant relatives within the great ape family. The orangutan differs from all of the others because the male can go through two different phases of development. It is for this reason that there are two types of sexually mature males, the smaller appearing externally like the female and the larger developing secondary sexual characteristics such as cheek pads and throat pouches.

Arrested development

Certain small males may remain at an arrested stage of development for years or even throughout their lives without the final spurt of growth ever arriving. As Lynda Dunkel, holder of a Marie Heim-Vögtlin scholarship of the SNSF, and her colleagues at the Anthropological Institute and Museum of Zurich University have now shown, this developmental arrest occurs more frequently on Sumatra than on Borneo, the other south-east Asian island which is home to the orangutans.

On Sumatra, the researchers observed twice as many small males as adults with cheek pads. During a five-year period of observation in the rain forest, only a single male was seen to develop secondary sexual characteristics. By way of contrast, on the island of Borneo, there are twice as many males with cheek pads as without.

Forced copulation

These males engage in disputes for the favour of fertile females much more often than those on Sumatra where, within the area under observation, a single dominant male monopolises sexual relations with the females. As there is more food available in the jungle on Sumatra than in the forests of Borneo, the dominant male has sufficient time to keep a close watch over the females in his environment and he drives out any other males with cheek pads before they can mate with a female.

However, smaller males without secondary sexual characteristics are less conspicuous. On Sumatra, this makes it easier for them to copulate with a female, even though the females put up resistance in 60% of the cases observed. Forced copulation also takes place on Borneo. There, however, as the males are constantly engaged in fighting in which the smaller ones never prevail, the advantages of developmental arrest disappear.

The fact that food supply in the forest has such a strong impact on the mating behaviour of the orangutan came as a surprise to Dunkel. "It goes to show," she says, "that the organisation of these great apes -- and perhaps that of our ancestors as well -- is more variable than we had hitherto assumed. Apparently, natural selection not only moulds appearance but also adapts social behaviour to the conditions of the local environment."