Tuesday, March 05, 2013

Monopoly of the Male Orangutan: Comparative Field Observations On Sumatra and Borneo


The sexual development, mating habits and social hierarchy of the orangutans are more heavily dependent on their environment than had previously been assumed: where the rain forest supplies more food, the influence of the dominant male increases. In order to escape his attention, many other males remain "small."

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.

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