The endangered yellow cheeked crested gibbon (Nomascus gabriellae) as seen from the Phnom Tamao wildlife sanctuary in Cambodia. (Sterling Zumbrunn/Conservation International)

Nearly half of the world's primate species and subspecies are in danger of extinction, according to a study released Tuesday by the International Union for Conservation of Nature.

The study — which examined 634 species and subspecies of man's closest relatives, including gorillas, monkeys and orangutans — found that 48 per cent of the distinct kinds of primates were listed as vulnerable, endangered or critically endangered.

The main threat to extinction was habitat loss from the burning and clearing of tropical rainforest, but hunting and the illegal wildlife trade were also cited as threats, according to the Switzerland-based conservation group.

"In many places, primates are quite literally being eaten to extinction," said Russell Mittermeier, chairman of the IUCN's Primate Specialist Group and president of Conservation International, in a statement.

"Tropical forest destruction has always been the main cause, but now it appears that hunting is just as serious a threat in some areas, even where the habitat is still quite intact."

More than 70 per cent of primates in Asia are now listed as endangered, the group found.

The review is part of a larger survey of the world's mammals, due to be released in October at the IUCN World Conservation Congress in Barcelona.

The findings are more alarming than a similar report issued last fall by the same group, then known as the World Conservation Union, which said just under a third of all primates were in danger of extinction.

The previous study looked at the 394 species of primates and did not break these species down into subspecies, as the IUCN study did.

As well, the IUCN study also considered primates listed as "vulnerable" to be in danger of extinction, while the previous study looked only at endangered and critically endangered species.

The distinction between the two conservation categories is a matter of degree. For example, if a decline in the animal's population is one of the measures used to assess the threat level, a vulnerable animal would be one whose population could be observed or estimated to have been reduced by 50 per cent over the last 10 years or three generations, whichever is longer.

Under a similar measure, an endangered animal would have a population reduction of 70 per cent or greater over the same time period.

The IUCN said non-human primates are important to the health of surrounding ecosystems through the dispersal of seeds and other interactions that help spread the range of plant and animal life.

Despite the gloomy outlook, the group found some success stories, with primates such as Brazil's black lion tamarin and golden lion tamarin — the rodent-sized monkeys native to South America — both upgraded from critically endangered to endangered due to conservation efforts.

In a separate report Tuesday, wildlife researchers said they've discovered 125,000 western lowland gorillas deep in the forests of the Republic of Congo, a major increased in the animal's estimated population.

The Wildlife Conservation Society, based at New York's Bronx Zoo, and the Republic of Congo said their new census puts the estimated number of the gorillas at between 175,000 and 225,000.