With a last burst of energy, Rocky tried to cross a fallen log bridging a stream. Halfway across he faltered and, turning around to go back, collapsed. As an afternoon downpour thundered out a final farewell, Rocky died.

We placed Rocky's body on a crude wood-and-wire frame and hoisted him up for his last trip into the canopy. I hoped that this would keep scavengers, like Borneo's bearded pigs, from scattering his bones, which I wanted to study for signs of disease and injury. Two weeks later we were amazed to see Rocky's clean white skeleton laid out on a bed of stringy red hair. Later I finished the job that the insects had started.

So far as I know, such an orangutan death had never before been seen in the wild. Given the apes' low population densities and long lives, the probability of witnessing a death is extremely low. Even so, Rocky's remains were joined a few months later by those of another adult male, found in a streambed.

These rare finds were most likely the result of an increase in male conflicts arising from the influx of orangutans drawn to feast on the abundant fruit. We had seen as many as six large—and mutually intolerant—adult males ranging within a small region. Here was natural selection at work: Countless contests between adult males during orangutan evolution likely contributed to males being twice the size of females.

The battle between males to mate has reproductive payoffs but is not without cost. Testing urine samples, I have detected significant signs of infection resulting from wounds that can, as in Rocky's case, lead to death.

Field Data

When the apes turn in early, our open stilt house provides a retreat to review the day's data. At dawn I am back on the forest floor, collecting urine from an orangutan just awakening in her nest above. I can test the urine on site for signs of menstruation, infection, and weight loss.

I save some samples for later genetic analysis, so I can learn who is fathering offspring and how the animals are related. I'll also analyze the hormones in the urine back at the lab.

By measuring hormones for the first time in wild orangutans, I've found that estrogen levels increase when nutritional status improves. Pregnant Beth conceived her baby Bekti during the mast fruiting, when estrogen levels were high.