Thousands of mammals and counting: The planet’s hidden diversity keeps growing
A rock pocket mouse navigates the rocky terrain of Arizona's Santa Catalina Mountains in April 2025. This genus of pocket mice is now recognized to have 23 species, but only had 17 species in 2005 when the previous authoritative taxonomy was released. Photo by Nathan Upham/ASU
After hundreds of years of discovering and cataloging thousands of mammal species, it would be easy to assume that by now, science has found and named every kind of mammal that exists in nature. But a sweeping new study published in the Journal of Mammalogy reveals that our understanding of mammal diversity is still expanding, and at a rate faster than most people realize.
This study, led by Arizona State University researchers in collaboration with the American Society of Mammalogists, outlines which mammal species exist, where they live and what efforts are needed to better protect our biological heritage.
According to a major update of the Mammal Diversity Database, scientists now recognize 6,759 living and recently extinct mammal species, a nearly 25% increase from the last major reference taxonomy in 2005.
Since that earlier edition, known as Mammal Species of the World, Volume 3, researchers have identified 1,579 new distinct species, including 805 newly described and 774 that were split from previously known species. Over the same period, 226 species were combined or removed, leading to a net gain of 1,353 species, representing an average of about 65 species of mammals newly recognized per year.
For Nathan Upham, assistant professor in the School of Life Sciences and one of the scientists leading this database, the effort is not about discovering every mammal but about creating a system that keeps up with the ongoing flood of new research.
“Every week, new papers come out that change what we know about mammal diversity,” he said. “Sometimes it’s a brand-new species to science, and sometimes it’s realizing that what we thought was one species is actually two, or five.”
The Mammal Diversity Database tracks the taxonomic ebb and flow of the entire class Mammalia, a kind of living record of how humans make sense of nature’s family tree. The database team found that in the past 20 years, new species have been recognized in 20 of the 27 mammal orders, and in 98 of the 167 families. That’s nearly every major group of mammals, from rodents to whales.
The orders driving most of the growth are Rodentia (595 new species, a 21.7% increase), Chiroptera, or bats (410, up 27.6%), Eulipotyphla, which includes shrews and moles (166, up 27.7%) and Primates (161, up 30.8%). These numbers reveal not just the scope of biodiversity but also how much remains to be explored, even among familiar animals.
“Rodents and bats together make up almost two-thirds of mammal diversity,” Upham said, about 41% rodents and 22% bats. “They’re everywhere, but they’re also some of the least understood animals, partly because they’re small, nocturnal and elusive. There’s still an incredible amount left to discover about small mammals.”
The Mammal Diversity Database's work helps bring order to a field that used to rely on printed volumes published only once every decade.
“Before this database existed, mammal taxonomy was kind of stuck in books,” Upham said. “If you were studying a species, you might not realize that its name or classification had already changed years earlier. Now, all the literature, citations and updates are centralized, and we even track the differences between versions so researchers can more directly see how our understanding is evolving through time.”
One of the most valuable aspects of the database is its record of synonyms, which are old scientific names that once referred to the same species.
“It’s like a translation table,” Upham said. “If you’re reading a paper from the 1800s or looking at a museum specimen with a name no one uses anymore, the database tells you what that species is called today. It keeps the language of biodiversity consistent across time.”
That consistency, he said, is what makes taxonomy more than just paperwork.
“Species names are how we communicate about life on Earth,” Upham said. “It’s the language that connects scientists across centuries. I can be part of the same conversation Linnaeus started in 1758. When I say Peromyscus maniculatus, the deer mouse, people can know what I mean. The (database) helps that conversation by clarifying that this species is now considered to live only east of the Mississippi River, refuting earlier ideas that it inhabits most of North America, and defining the related western deer mice as separate.”
The team’s study also shows where most new discoveries are coming from: mountainous, tropical regions such as Madagascar, the Andes Cordillera, Indonesia and the Philippines. These areas are rich in biodiversity, with complex landscapes that isolate populations and allow them to evolve separately over time.
But other regions remain understudied, including West and Central Africa, the Amazon Basin, Central Asia, most of India and some Indonesian islands, like Java, Sumatra or Borneo. Limited access to genetic sequencing tools and research funding has restricted biodiversity studies in these areas, most of which are also threatened by logging and land conversion.
“A lot of what’s driving new discoveries now is technology,” Upham said. “Genomic methods, CT scanning and better computational tools let us see evolutionary relationships in much greater detail. But it’s also about global collaboration that is sorely needed to study and protect biodiversity before it is lost. In South America, for example, most of the new taxonomic work is being led by scientists in Brazil, Argentina and Peru rather than from Global North countries, which is also starting to happen in Africa as well.”
The new version of the database reflects 267 years of scientific history, providing a record of changing philosophies about what defines a species. Early taxonomists often divided animals into countless categories based on color or shape. Later generations, the so-called “lumpers,” merged many of those names once newer methods revealed they weren’t distinct.
“In the early 1900s, there was this explosion of species names based on fur color and other superficial traits,” Upham said. “Then in the 1960s, scientists realized they needed to clean up that mess. Today we’re in another wave of discovery, but it’s much more evidence-based, integrating genomics, morphology, ecology, behavior and geography.”
If current trends continue, the researchers project that by 2050, scientists will have recognized more than 8,000 currently living species of mammals. That steady increase reflects both better tools and the growing involvement of scientists around the world. However, with that growth in recognized diversity comes an increasing need to protect it.
According to the study, the International Union for the Conservation of Nature, which is charged with assessing conservation threats globally, is lagging behind with 25% of mammal species allocated to the “understudied” threat categories of Data Deficient (11%) or Not Evaluated (14%). This underscores the need for greater integration of Mammal Diversity Database data with conservation organizations like IUCN, as well as NatureServe and iNaturalist.
For Upham, the ongoing growth of the Mammal Diversity Database is less about chasing a final number and more about deepening our collective understanding.
“It’s easy to think we’ve already found everything,” he said. “But we’re still learning about the animals that share this planet with us. Every new name is a reminder that the story of life on Earth is still being written, and that we’re just beginning to understand how much there really is.”
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