Gorillas, monkeys, lemurs and other primates are in danger of becoming extinct, and scientists say it's our fault our closest living relatives are in trouble, a new international study warns.

About 60 per cent of the more than 500 primate species are "now threatened with extinction" and three out of four primate species have shrinking populations, according to a study published Wednesday in the journal Science Advances.

This is a critical world problem. - Researcher Paul Garber

While scientists had tracked dwindling numbers of individuals and groups of primates in forests around the world, this is the first big-picture look. The result was "a bigger wake-up call" than previously thought, said researcher Paul Garber of the University of Illinois.

"The outlook is not very good," said Garber, who recently returned from studying marmosets in the jungles of Brazil..

Primate problems are recent

The decline has been blamed on human activities including hunting, mining and oil drilling. Logging, ranching and farming have also destroyed precious habitat in Africa, Asia and South America.

Primates, which include apes, monkeys and humans, have forward-facing eyes and grasping ability that set them apart from other mammals. Scientists study them to learn about human behaviour and evolution.

There are only about 14,000 Sumatran orangutans left in the world, according to a study by 31 primate experts published in the journal Science Advances. (Joshua Paul/Associated Press)

Many of the problems faced by primates are recent. For example, the Grauer's gorilla dropped from a population of 17,000 in 1995 to just about 3,800 now, mostly from bushmeat hunting and mineral mining, the study found.

There are only about 14,000 Sumatran orangutans left in the world. The Hainan gibbon in China is down to just 25 individuals, while 22 out of the 26 primate species in China are endangered, Garber said.

About 94 per cent of the lemur species in the world are endangered.

"We need to look at [population losses] almost as signals. They're telling us something about our future," Garber said. "This is a critical world problem."

The Grauer's gorilla population has dropped from 17,000 in 1995 to just about 3,800 now, mostly from bushmeat hunting and mineral mining, the study found. (Ben Curtis/Associated Press)

While there's hope that some species can be protected, many will disappear in the coming decades, said co-author Eduardo Fernandez-Duque of Yale University.

Emory University primate expert Frans de Waal, who wasn't part of the study, called the work "very detailed and timely and unfortunately correct."

"Primate populations are clearly moving in the wrong direction," he said.