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More than 100,000 Critically Endangered orangutans have been killed in Borneo since 1999, research has revealed.
Scientists who carried out a 16-year survey on the island described the figure as "mind-boggling".
Deforestation, driven by logging, oil palm, mining and paper mills, continues to be the main culprit.
But the research, published in the journal Current Biology, also revealed that animals were "disappearing" from areas that remained forested.
This implied large numbers of orangutans were simply being slaughtered, said lead researcher Maria Voigt of the Max Planck Institute for Evolutionary Anthropology in Germany.
Dr Voigt and her colleagues say the animals are being targeted by hunters and are being killed in retaliation for crop-raiding - a threat that has been previously underestimated.
Prof Serge Wich from Liverpool John Moores University, UK, also part of the team, told BBC News: "We didn't expect the losses to be so large in standing forest, so these [studies] confirm that hunting is a major issue."
"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.
"Just last week, we had a report of an orangutan that had 130 pellets in its body, after being shot at in Borneo.
"It's shocking and it's unnecessary. Orangutans might eat farmers' fruit, but they are not dangerous."
Prof Wich called for leaders in Malaysia and Indonesia to speak out against this deliberate targeting of the apes. But the research also showed that natural resources were still being exploited in Borneo "at an unsustainable rate".
Deforestation alone, the researchers predict, could wipe out a further 45,000 orangutans over the next 35 years.