By Mark Kinver
Science and nature reporter, BBC News
A global review of the world's primates says 48% of species face extinction, an outlook described as "depressing" by conservationists.
The IUCN Red List of Threatened Species says the main threat is habitat loss, primarily through the burning and clearing of tropical forests.
More than 70% of primates in Asia are now listed as Endangered, it adds.
The findings form part of the most detailed survey of the Earth's mammals, which will be published in October.
PRIMATES IN PERIL
Nations with the highest percentage of threatened species:
Cambodia - 90%
Vietnam - 86%
Indonesia - 84%
Laos - 83%
China - 79%
(Source: IUCN Red List)
Other threats include hunting of primates for food and the illegal wildlife trade, explained Russell Mittermeier, chairman of global conservation group IUCN's Primate Specialist Group and president of Conservation International.
"In many places, primates are quite literally being eaten to extinction," he warned.
"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."
The survey, involving hundreds of experts, showed that out of 634 recognised species and subspecies, 11% were Critically Endangered, 22% were Endangered, while a further 15% were listed as Vulnerable.
Asia had the greatest proportion of threatened primates, with 71% considered at risk of extinction. The five nations with the highest percentage of endangered species were all within Asia.
"It is quite spectacular; we are just wiping out primates," said Jean-Christophe Vie, deputy head of the IUCN Species Programme.
RED LIST DEFINITIONS
The Tonkin snub-nosed monkey is listed as Critically Endangered
Extinct - Surveys suggest last known individual has died
Critically Endangered - Extreme high risk of extinction - this means some Critically Endangered species are also tagged Possibly Extinct
Endangered - Species at very high risk of extinction
Vulnerable - Species at high risk of extinction
Near Threatened - May soon move into above categories
Least Concern - Species is widespread and abundant
Data Deficient - not enough data to assess
He added that the data was probably the worst assessment for any group of species on record.
"The problem with these species is that they have long lives, so it takes time to reverse the decline. It is quite depressing."
Although habitat loss and deforestation were deemed to be the main threats globally, Dr Vie explained how human encroachment into forests was also creating favourable conditions for hunters.
"This creates access, allowing people to go to places that they could not go in the past," he told BBC News.
"Primates are relatively easy to hunt because they are diurnal, live in groups and are noisy - they are really easy targets.
"Many of the Asian primates, like langurs, are 5-10kg, so they are a good target. Generally, you find that what is big and easy to get disappears very quickly."
In Africa, 11 of the 13 kinds of red colobus monkeys assessed were listed as Critically Endangered or Endangered.
Conservationists fear that two may already be extinct. The Bouvier's red colobus has not been seen for 25 years, and no living Miss Waldron red colobus has been recorded since 1978.
The authors of the primate Red List did consider downlisting mountain gorillas to Endangered from Critically Endangered because the great apes had recorded a population increase.
But they decided to delay reclassification as a result of five of the gorillas being killed in July 2007 by gunmen in the DR Congo's Virunga National Park, which is still at the centre of a conflict between rebel forces and government troops.
During 2007, wildlife rangers in the park recorded a total of 10 gorilla killings. The rangers have been documenting their struggles in a regular diary on the BBC News website over the past year.
"If you kill seven, 10 or 20 mountain gorillas, it has a devastating impact on the entire population," Dr Vie explained.
"Within the Red List criteria, you are allowed to anticipate what will happen in the future as well as look at what has happened in the past.
"So it was decided not to change the mountain gorillas' listing because of the sudden deaths, and we do not know when it is going to stop."
Dr Emmanuel de Merode, chief executive of Gorilla.cd - an EU-funded programme working in Virunga National Park - said the gorillas' long-term survival was still far from assured.
"Militias have been in control of the Gorilla Sector since September last year, which means the Congolese wildlife authority has been unable to manage the area and protect the gorillas," he told BBC News.
"Until the war ends and the rangers are able to get back in and patrol the area, we have no idea as to the fate of almost a third of the mountain gorillas left in the world."
Golden glimmer of hope
Despite the gloomy outlook, the Red List did record a number of conservation successes.
The re-introduction of golden lion tamarins is one of the few successes
Brazil's populations of golden lion tamarins and black lion tamarins were downlisted from Critically Endangered to Endangered.
"It is the result of decades of effort," said Dr Vie. "The lion tamarins were almost extinct in the wild, but they were very popular in zoos so there was a large captive population.
"So zoos around the world decided to join forces to introduce a captive breeding programme to reintroduce the tamarins in Brazil."
However the first attempts were not successful and the released population quickly crashed because the animals were ill-prepared for life in the wild, he recalled.
"They were not exposed to eagles or snakes and they did not know how to find food, so a lot of them died. But some did survive and, slowly, the numbers began to increase."
Ultimately, the success was a combination of ex-situ conservation in zoos and in-situ conservation by protecting and reforesting small areas around Rio de Janeiro and Sao Paulo.
"It took time, money and effort at all levels, from the politicians to scientists and volunteers on the ground, for just two species."
The findings, issued at the International Primatological Society Congress in Edinburgh, Scotland, will be included in a survey described as an "unprecedented examination of the state of the world's mammals", which will be presented at the IUCN World Conservation Congress in October.