In the first comprehensive survey of the world's mammals in more than a decade, scientists have found that one in four mammal species are threatened by extinction. Over half of mammalian species worldwide are showing declining trends in population.

The study, published this week in Science, concluded that its results "paint a bleak picture of the global status of mammals worldwide."

The study notes that the exact threat level is unknown, since the status of 836 species is currently undetermined due to a lack of data. Thus, the actual level of threat could lie between 21 percent and 36 percent. The conservation status of marine mammals is of particular concern, with 36 percent of known species threatened.

The study was headed by Jan Schipper from the International Union for Conservation of Nature (IUCN). This five-year collaborative effort between 1,700 experts in 130 countries compiled detailed information on species' taxonomy, distribution, habitats and population trends. The study also identified threats as well as recommended conservation measures for these species. The data has been made freely available to the general public.

The report updates the IUCN's Red List of Threatened Species, which overall includes 44,838 species, of which 16,928 are threatened by extinction. Of these, 3,246 are listed as critically endangered, 4,770 are considered endangered, and 8,912 are listed as vulnerable to extinction.

The IUCN estimates that 76 mammalian species, such as the sea mink, have gone extinct since 1500. Several species, such as the Scimitar-horned Oryx and Père David's Deer have been listed as extinct in the wild and continue to only exist in captivity.

The study also warned that those species that are currently not listed as threatened are not necessarily safe, citing that over 323 species are "near threatened." Over a fifth of species that have been classified as of "least concern" are currently exhibiting declines in population.