The population of Sumatran orangutans is two times higher than previously thought, but the great ape remains still critically endangered because of forest loss and poaching, according to a new, landmark survey released last week.

An international team of researchers, spearheaded by Serge Wich at Liverpool John Moores University in the U.K., now estimates that the total number of Sumatran orangutans is close to 14,600, rather than the previously acknowledged count of 6,600. However, the new numbers are not the result of a growing population on the Indonesian island, instead reflecting more extensive surveying methods.

Tracking population numbers for these shy and illusive apes is a challenge, but orangutans regularly create nighttime sleeping nests at night that can be located even after the animal has moved on. For the study, the research team created a series of nest-decay surveys in order to approximate population densities across Sumatra. They discovered groups at higher elevations than expected, located individuals in regions that were thought to be devoid of orangutans, and encountered populations in degraded forests that had not been included in earlier studies.

"The chance that there will be zero [Sumatran orangutans] in the near future is certainly less now," lead study author Serge Wich said.

However, earlier estimates suggested that the population had dropped by 80 percent through the past 75 years, and Wich explained that the new information would not significantly alter that figure. Beyond the actual numbers, the density of animals found in different types of forests is also important; by overlaying population estimates on current and future land-use maps, the study re-establishes that orangutan numbers will continue to rapidly diminish as forests are degraded. This could cause a 14-33 percent decrease over the next 15 years.

"The fact that we see large areas of forest being converted to oil palm plantations and other land uses means that we are losing orangutans in those areas," Wich said. While more individual orangutans exist than previously thought, he said the decline "is still very real."

Erik Meijaard, Ecologist and Director of Borneo Futures, confirmed this perspective. "Our data on habitat loss and killing rates indicates a population decline of about 25% per decade. If the population is 14,631 rather than 6,600, this means that in 100 years from now... there will be 823 orangutans left rather than 372," he said. "Either way the species is very close to extinction."

Sumatran orangutans are one of two different orangutan species. The population of Bornean orangutans, which tend to have shorter, darker fur and slightly smaller faces, is estimated to be around 50,000 or 60,000. Besides humans, Orangutans are the only great apes that live in Asia and the largest arboreal mammal in the world.

The research was published in the March 4 issue of the journal Science Advances.