Half of the world's mammals are declining in population and more than a third probably face extinction, according to a report by the International Union for Conservation of Nature (IUCN).
A survey of mammals included in the organisation's Red List, the most respected inventory of biodiversity, indicates that a quarter of the planet's 5,487 known mammals are clearly at risk of disappearing forever.
But the actual situation may be worse because researchers have been unable to classify the threat level for another 836 mammals due to lack of data.
"In reality, the number of threatened mammals could be as high as 36 per cent," Jan Schipper an IUCN scientist and the lead author of the mammal survey, said in remarks published separately in the US-based journal Science.
The most vulnerable groups are primates, a human's nearest relatives on the evolutionary ladder, and marine mammals, including several species of whales, dolphins and porpoises.
"Our results paint a bleak picture of the global status of mammals worldwide," said Schipper.
The revised Red List, unveiled at the IUCN's world conservation congress in Barcelona on Monday, is further evidence that Earth is undergoing the first wave of mass extinction since dinosaurs died out 65 million years ago, many experts say.
Over the last half a billion years, there have only been five other periods of mass extinction.
The list covers more than 44,000 animal and plant species and classifies plants and animals in one of which half-a-dozen categories depending on their survival status.
Nearly 40 per cent of 44,838 species cataloged are listed as "threatened" with extinction, with 3,000 of them classified as "critically endangered", meaning they face a very high probability of dying out.
'Extinct' to 'endangered'
Researchers noted, however, that that conservation efforts can prevent a species from being put into the category from which there is no return: "extinct".
The black-footed Ferret, native to the US, was moved from "extinct in the wild" to "endangered" after it was successfully introduced into seven US states and Mexico.
The European bison and the wild horse of Mongolia made similar comebacks from the brink starting in the early 1990s.
|Fishing nets and vessel strikes kill up to 1,000 sea mammals every day [AFP]
But these remain exceptions that highlight the need to act before other species populations dwindle beyond the threshold of viability, experts say.
"The longer we wait, the more expensive it will be to prevent future extinctions," Jane Smart, the head of the IUCN's species programme, said.
"We now know what species are threatened, what the threats are and where."
Michael Hoffman, a mammal expert at Conservation International who helped compile the Red List, said he was "blown away" when seeing the results.
"Nearly 80 per cent of primates in Asia are threatened with extinction, overwhelmingly because of hunting and habitat loss," he said.
An appetite in China for traditional medicines and prestige foods is the main driver of primate loss in southeast Asia, Hoffman said.
Vulnerable sea mammals
Sea mammals are also highly vulnerable. "The situation is particularly serious ... for marine species, victims of our increasingly intensive use of the oceans," said Schipper.
Previous research has shown that mile-wide fishing nets, vessel strikes, toxic waste and sound pollution from military sonar kill up to 1,000 air-breathing, ocean-dwelling mammals every day.
There are many drivers of species extinction and all of them stem either directly or indirectly from human activity, scientists say.