The most vulnerable species are primates (excluding humans, of course) and marine mammals. The threatened mammal species are being killed off by a variety of human activities, as the Washington Post's Juliet Eilperin notes:
Land and marine mammals face different threats, the scientists said, and large mammals are more vulnerable than small ones. For land species, habitat loss and hunting represent the greatest danger, while marine mammals are more threatened by accidental killing through fishing bycatch, ship strikes, and pollution.
While large species such as primates (including the Sumatran orangutan and red colobus monkeys) and ungulates (hoofed animals) may seem more physically imposing, the researchers wrote that these animals are more imperiled than small creatures such as rodents or bats because they "tend to have lower population densities, slower life histories, and larger home ranges, and are more likely to be hunted."
Primates face some of the most intense pressures: according to the survey, 79 percent of primates in South and Southeast Asia – including the Hainan gibbon – are facing extinction.
Some species have made a recovery in recent years. The black-footed ferret, for instance, is no longer extinct in the wild after the US Fish and Wildlife Service mounted an extensive captive-breeding program and successfully reintroduced it into the wild in eight Western states. This ferret, which eats prairie dogs, now appears to have three small but self-sustaining populations in South Dakota and Wyoming.