At least 100,000 critically endangered Bornean orangutans have been killed in the past 16 years, according to a new study carried out by an international team comprising of conservationists, biodiversity experts, and ecologists. The researchers who conducted this 16-year survey in the Borneo Island have described the decline in orangutan numbers as “mind-boggling.”

Bornean orangutans are the largest tree-dwelling animals in the world. These intelligent, four-foot-tall creatures are the closest living relatives to humans, thanks to their physical appearance as well as the genomes. Bornean orangutans differ in appearance from the Sumatran orangutans and are characterized by a darker color, shorter beard, and broader face. Because of their shyness, these animals prefer to live in dense forests, thereby making it difficult for the surveyors to determine their population by directly spotting them in the forests. Researchers estimate their numbers by monitoring their nests in the forests. Orangutans make these nests by bending long branches of trees. They are usually large in size and can be seen from helicopters.

Since 1999, researchers have been keeping an eye on orangutan nests in an area of 500 square miles in Borneo. When this survey began in 1999, researchers were finding 22 nests in every kilometer traveled, but the figure dropped by more than half to just 10 nests in 2015.

According to researchers, the forest habitat of Bornean orangutans is declining at an alarming rate due to deforestation, hunting, and many other reasons. In Borneo, forests are being cut continuously for oil palm, mining, logging, and the making of paper mills. Especially, cultivation of oil palm is a major cause of Bornean orangutans’ habitat destruction.

Surprisingly, orangutans are also “disappearing” from those areas that are still forested. Researchers believe these animals are being targeted by the hunters. Locals are also killing them to save their crops and fruits from these animals.

People are slaughtering orangutans and wiping them out https://t.co/cLmXgnuie3 pic.twitter.com/IUdMp8lCi3 — New Scientist (@newscientist) February 15, 2018

“We didn’t expect the losses to be so large in standing forest, so these [studies] confirm that hunting is a major issue,” Professor Serge Wich from Liverpool John Moores University, UK, told BBC News.

“When these animals come into conflict with people on the edge of a plantation, they are always on the losing end. People will kill them.”

2 workers cruelly killed an endangered orangutan with 17 shots but alleged their murder as self-defence https://t.co/4y1SbdknAy pic.twitter.com/DJBjSHpBwh — BastilleGlobal (@BastilleGlobal) February 6, 2018

Researchers estimate approximately 100,000–148,500 Bornean orangutans were killed during the period from 1999–2015, and now there are just 17,000 to 100,000 orangutans left in Borneo.

A mathematical model developed by the researchers also suggests that approximately 45,300 animals would be lost by 2050 due to habitat destruction.

According to the worldwildlife.org, northwest Bornean orangutans are the most endangered subspecies among Bornean orangutans, and just 1,500 of them exist in the world. Central Bornean orangutans have a comparatively larger population of approximately 35,000 individuals.

The detailed findings of the study were published in the journal Current Biology.