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The year before, we had made the trip upriver to the research site in the middle of an unusually severe dry season. The water was too low to use a boat with a motor, so we had to drag our dugout canoes, burdened with more than a year's worth of research supplies, across the sandy river bottom. As we penetrated farther and farther into the forest, a steady stream of white and pink flowers floated down to greet us as if we were in a ticker tape parade. The flowers heralded a so-called mast fruiting—an event in Southeast Asia rain forests when a large proportion of trees bear fruit at the same time. The mast provides a boon for orangutans, like Roman, who gorge themselves on high-calorie fruits.

The sun's last rays turn mist into a flaming yellow blanket, tucking the rain forest in for the night. Lush and teeming with life, this botanical wonder leads many to think of it as a virtual Garden of Eden that produces an unceasing cornucopia of succulent fruits. In reality, though, fruit abundance varies greatly even if temperature varies little.

Periods of high fruit production happen only at odd intervals in Borneo, about once every four to seven years, although smaller fruit peaks occur every year. Being there during a mast fruiting proved ideal for studying how changes in food abundance influence orangutan reproduction and behavior.

Unlike chimpanzees, for example, orangutans do not live in groups. Adult males travel alone, and mothers are usually accompanied only by an infant and sometimes by an older juvenile.

This is partly because the fruits that the animals prefer are widely dispersed and can’t support large gatherings of the apes.

But orangutans are not wholly unsociable. During the mast we've seen as many as eight individuals feeding together in the huge dipterocarp trees that dominate the rain forest. Clearly orangutans get together when the food supply permits.

I spend countless hours sitting on soggy ground observing the animals. One day I watch Roman eat one after another of the pineapple-size durian fruits. Hearing a rustle, I turn and see Rob, a subadult male, throw together a nest in a tree behind me and dive in. Rob lacks the cheek pads and throat sac of fully mature males like Roman and doesn't announce his presence with long, bellowing calls. He eyes Roman and the durian tree. When Roman has eaten his fill, he quits the tree, ignoring Rob as the smaller ape approaches to feed. After gorging on durians, Rob pauses at a small Baccaurea tree to grab a handful of its glossy red fruit.

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