There are 6,759 recognized species of mammals, according to the most current version of the Mammal Diversity Database maintained by the American Society of Mammalogists. That August 2024 count includes 6,629 wild living species, 113 that have gone extinct in recent centuries, and 17 domestic species. The number is higher than most people expect, and it keeps climbing.
Why the Count Keeps Growing
The total has jumped significantly in a short period. In 2005, the standard reference listed 5,416 mammal species. By 2018, that figure had risen to 6,495. The 2024 update pushed it to 6,759, a nearly 25% increase over the 2005 baseline. That doesn’t mean thousands of entirely unknown animals are wandering into camera traps. Most “new” species come from splitting what scientists previously considered a single species into two or more distinct ones.
DNA sequencing is the main driver. Over the past two decades, molecular studies have revealed that many animals once treated as one widespread species actually contain multiple genetically distinct lineages that look nearly identical to the naked eye. These are called cryptic species. When genetic evidence is strong enough, taxonomists formally split them, and the official count goes up. This process is ongoing, meaning the true number of mammal species is almost certainly higher than 6,759.
Which Groups Have the Most Species
Mammals are divided into roughly 27 orders, and the diversity is wildly uneven. Two orders dominate the count:
- Rodents (Rodentia): The largest order by far, with over 2,550 species. The two biggest families alone, the Old World mice and rats and the New World rats and mice, account for more than 1,600 species combined. Rodents also saw the largest jump during recent taxonomic revisions, gaining 371 species.
- Bats (Chiroptera): The second largest order, with roughly 1,386 species. The evening bat family alone contains nearly 500 species. Bats gained 304 species in recent updates, reflecting intense genetic research across the tropics.
Together, rodents and bats make up close to 60% of all mammal species. After them, the next largest group is the shrews, moles, and hedgehogs (Eulipotyphla), with about 527 species, followed by even-toed hoofed mammals like deer, antelope, and cattle (Artiodactyla). At the other extreme, a few orders contain just a handful of species. The aardvark is the sole living member of its entire order. The order containing the colugo, a gliding mammal from Southeast Asia, has only two species.
Marine Mammals
A small but notable slice of mammal diversity lives in the ocean. Roughly 130 species are classified as marine mammals, spanning whales, dolphins, porpoises, seals, sea lions, walruses, manatees, dugongs, sea otters, and polar bears. NOAA Fisheries alone has jurisdiction over about 119 of these species in U.S. waters. Marine mammals represent less than 2% of total mammal diversity, but they occupy an outsized role in conservation efforts because many are long-lived, slow to reproduce, and vulnerable to human activity.
How Many Are Threatened
About 27% of mammal species are classified as threatened with extinction on the IUCN Red List, meaning they fall into the vulnerable, endangered, or critically endangered categories. That percentage has a range of uncertainty (23% to 36%) because hundreds of species lack enough data for a confident assessment.
The extinction record puts this in longer perspective. At least 241 mammal species are known to have gone extinct during the Holocene, the geological period spanning roughly the last 11,700 years. Many of these losses occurred on islands, where mammals were especially vulnerable to human arrival, habitat loss, and introduced predators. The 113 recently extinct species cataloged in the Mammal Diversity Database reflect a narrower, more conservative window, typically counting extinctions since around 1500 AD.
How Scientists Track the Total
For decades, the standard reference was “Mammal Species of the World,” a book last updated in 2005 with its third edition listing 5,416 species. That resource became increasingly outdated as genetic tools accelerated species descriptions. In 2018, the American Society of Mammalogists launched the Mammal Diversity Database as a living, regularly updated digital replacement. The database tracks not just species counts but also geographic ranges, taxonomic changes, and the full history of scientific names, cataloging over 50,000 valid and synonymous names.
The shift to a continuously updated database matters because mammal taxonomy is no longer a slow-moving field. Between the 2005 book and the 2018 database launch, roughly 1,079 species were added in about 13 years. Between 2018 and 2024, another 264 net species were added. New descriptions now arrive steadily, driven by fieldwork in biodiversity hotspots across the tropics and by genetic reanalysis of museum specimens collected decades ago. If current trends hold, the count will likely pass 7,000 within the next several years.