Image caption The Burmese long-tailed macaque has developed the ability to use stone tools to crack open shellfish

Macaque monkeys that have developed the ability to use stone tools to open shellfish are in danger of losing the skill because of human development.

Scientists found that illegal palm oil and rubber plantations in Thailand are disrupting the monkeys' feeding behaviour.

Dogs brought in to protect the farms are repelling the macaques from the shoreline, which inhibits their tool-using activities.

The study is published in Oryx.

The report in the international journal of conservation comes almost a decade after the first scientific description of tool-use among capuchin monkeys in South America. It is a rare skill set.

In addition to chimpanzees in Africa, the Burmese long-tailed macaques are the only primates known to use stone tools.

If the dogs repel them, the monkeys will use the shore less and less and they will stop using tools as much Dr Michael Gumert, Nanyang Technological University

Researchers have been monitoring these monkeys on the Laem Son National Park, on the Andaman sea coast of Thailand, since 2007.

On the island of Piak Nam Yai, they found that 88% of around 200 adults use stones to crack open hard-shelled invertebrate prey, including rock oysters, sea snails and crabs.

According to Dr Michael Gumert, from Nanyang Technological University in Singapore, who led this latest study, it is the development of the area in the wake of the devastating 2004 tsunami that is now threatening the macaques' rare ability.

"What's been happening is that over the past six years on the island, we've just seen more palm oil and rubber farms being developed in the forest. I've begun to notice that the groups that are closest to human activities, just aren't having kids anymore," he told BBC News.

Dog days

The research team argues that one critical element is the use of domestic dogs to protect the new farms.

The macaques are forced to constantly keep an eye out for dogs on the coast, and the scientists say they are paying less attention to learning how to use tools.

"The monkeys come down to the big rocky coasts and pick up rocks and crack things like oysters and crabs. But if the dogs repel them, the monkeys use the shore less and less and they will stop using tools as much," explained Dr Gumert.

Image caption Crab-eating macaques are in demand among animal traffickers

The scientist say that all across South East Asia, there is a growing problem of macaques becoming acclimatised to living around people and becoming dependant on humans for food. They are worried that these skilled monkeys could now go the same way.

"What we're looking at with these stone-tool-using monkeys is a rare case of truly wild long-tail macaques doing their original wild behaviour, unlike most of the other macaques that have had their behaviour destroyed by human development," said Dr Gumert.

"If we develop right next to them, they will stop going to the coast to feed and go to the local rubbish bin and find food there."

There is no danger that the macaques will die out but, argue the researchers, if the monkeys lose the opportunity and ability to use stone tools then the scientific opportunity to understand why some animals develop such skills will also be lost.

The palm oil and rubber farm developments on Paik Nam Yai are illegal, but efforts to evict the settlers have so far failed.

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