March 16, 2013

Food, sex and the male orangutan

The sexual development, mating habits and social hierarchy of the rainforest dwelling orangutans are more heavily dependent on their environment and local biodiversity than scientists had assumed. Now, according to a new study supported by supported by the Swiss National Science Foundation (SNSF), there is strong evidence that where a rain forest supplies more food, the influence of the dominant male orangutan increases. What’s more, in order to keep a low profile and escape the attention of the dominant male, many of the other males remain “small.”

While the Malay language word “orangutan” means man of the woods to them, in reality forest dwellers are our most distant relatives within the great ape family. Furthermore, the male orangutan is unique among us apes in that it can go through two different phases of development. This means 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.

On Sumatra, 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. Here, males engage in disputes for the favour of fertile females much more often than those on Sumatra where a single dominant male tended to monopolise sexual relations with the females. With better food supply in the rainforest of Sumatra than in those of Borneo, the dominant male has sufficient time to keep a close watch over the females and drive out any other males with cheek pads before they can approach a female.

The strategy of “smaller” males (those without secondary sexual characteristics) is to make themselves much less conspicuous. In Sumatra at least, this makes it easier for them to copulate with a female, even though the females frequently put up resistance. In Borneo, on the other hand, the males are constantly engaged in fighting.

According to one of the authors of the study: “it goes to show 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”.


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