The baiji dolphin is functionally extinct, orangutans are disappearing and even some species of bats—the most numerous of mammals—are dying out. A new survey of the world's 5,487 mammal species—from rodents to humans—reveals that one in four are facing imminent extinction.

"Mammal species that are just declining, not necessarily near extinction, that's 50 percent," says conservation biologist Jan Schipper of the International Union for Conservation of Nature (IUCN), which keeps the Red List of Threatened Species. "And 836 species—especially rodents and bats—we determined they are threatened but we don't know how threatened, because we don't know enough about them."

Schipper and more than 1,700 scientific colleagues spent the past five years surveying the state of the world's mammals. The results, published in Science to coincide with IUCN's conference on biodiversity this week, reveal that 1,139 mammals around the globe are threatened with extinction and the populations of 52 percent of all mammal species are declining.

South and Southeast Asia are home to the most threatened mammals, from monkeys to rare rats. And many mammals in the species-rich tropical Andes Mountains of South America, Africa's Cameroonian highlands and Albertine Rift as well as the northern Atlantic and Pacific Oceans are also in trouble. Deforestation, along with hunting or gathering food are the prime causes of the rapid declines in land mammals, such as elephants in Asia; most endangered marine mammals, like the vaquita in Mexico's Gulf of California, are killed by fishing nets, ship strikes or pollution.

"Overall conservation status of mammals will likely deteriorate further unless appropriate conservation actions are put in place," the researchers warn in the report.

But the news isn't all grim: Some mammals, such as the black-footed ferret of western North America and the Hainan black-crested gibbon (found only on China's Hainan Island), have been able to rebound as the result of conservation efforts. "These are the kinds of success stories that we need to clasp onto and find out what worked," Schipper says. "Usually, it takes a lot of money."

But he cautions that any conservation success is likely temporary unless the root problems of, for example, deforestation are addressed. In the case of the Hainan gibbon, for instance, "there's not enough room for that species to go back to having a thousand individuals unless we stop deforestation and hunting," Schipper says.

There's also the clash between saving animals and curing other environmental ills such as global warming. Vast tracts of tropical rainforest have been replaced by palm oil plantations for food and biofuels, satellite imagery reveals.

But addressing climate change could also help lessen this extinction crisis as well; the loss of sea ice as a result of a warming world threatens to make life impossible for those mammals such as the polar bear and harp seal that rely on it to survive.

The "general trend is that many more mammal species are rapidly declining than we had suspected," Schipper says. "Fifty percent of species are declining and 5 percent of species are in an upward recovery—that's just not enough."