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Old Math Counts More Apes

Scientists trying to count critically endangered Bornean orangutans in the Malaysian state of Sarawak are getting some help from a mathematical theorem formulated in the 1700s.

The scientific team, led by WCS’s Malaysia Program and supported by Sarawak Forestry Corporation and partners, used Bayesian analyses, a theorem created by Thomas Bayes in 1763, which says probability can change as new information is gathered.

The standard survey protocol to estimate orangutan density consists of counting all nests visible from a line transect or plot and generating nest density within the area surveyed. Orangutans build nests of branches and leaves at the end of each day for sleeping.

The problem with this technique is that the rate of decay of the nests is extremely variable. In some situations, nests can disappear in less than two months, but in other instances the decayed leaves may remain as long as 18 months.

The team adapted Bayesian analysis to measure the precision of the orangutan count estimates. Thus, the assumption is that removing decay rate gives greater precision.The Bayesian framework allowed stronger and more reliable estimates to be generated with a measure of precision to more accurately count – and subsequently protect – this imperiled primate.

Using Bayesian analyses, the team estimates that there are as many as 355 orangutans in their study sites, with many living outside of protected areas. As a result of the team’s preliminary data, the government of Sarawak, led by the Forest Department, has extended the Lanjak-Entimau Wildlife Sanctuary for orangutan conservation.

The team is grateful to the Sarawak Forestry Corporation and the management of Lanjak-Entimau Wildlife Sanctuary, who supported the surveys and allowed the orangutan research to be conducted in the protected area.