A study of Bornean orangutan encounters with humans over the past 150 years suggests that the population of orangutans has declined over time. Potential explanations for the loss include habitat loss and degradation, hunting, and disease, all of which are examined in this paper.

Historic orangutan encounter rates appear to have been substantially higher than recent rates, suggesting that in the past, orangutans lived at much higher population densities than presently.

Possible explanations for lower encounter rates in recent years include the fact that orangutans are now more elusive and have learned to avoid people, particularly where they have learned to consider humans a serious threat. However, a more likely explanation is that local densities of orangutans have actually declined. Possible causes of decline include habitat loss, degradation, hunting, and disease.

Although there are gaps in the data, after examining several possible explanations, researchers concluded that a high level of hunting was the most likely cause of the reduced encounter rates over time. Orangutans are hunted for a variety of reasons, including food, as agricultural pests, and to obtain young individuals for the pet trade.

Other unpublished studies in Indonesian Borneo suggest that more than 1,000 orangutans are killed annually.