Mammals

Mammals are the group people think they already understand, which is exactly why the mistakes are so consistent. Pandas get called bears and red pandas get called pandas. Skunks get filed with badgers. Hyenas get treated as dogs when they sit closer to cats. Hippos get grouped with rhinos when their nearest living relatives are whales. This page is the corrected map, and underneath it every mammal article on this site, organised by what the animals actually are rather than what they look like.

What actually makes a mammal

Hair and milk are the two everyone knows. The diagnostic character, the one that separates mammals from every other vertebrate lineage in the fossil record, is the ear.

You have three bones in each middle ear: malleus, incus and stapes. Reptiles have one. The other two are not new inventions. They are the articular and quadrate bones, which in your ancestors formed the jaw joint and which migrated backwards into the ear across roughly a hundred million years. The transition is documented in successive fossils, and it is one of the best evidenced macroevolutionary sequences we have. If you want a single fact that makes the mammal group real rather than a convenience label, that is the one.

Add a single lower jaw bone, the dentary, where reptiles have several; a neocortex; a diaphragm; and a four chambered heart with a left aortic arch, where birds kept the right. Mammary glands give the group its name but leave almost nothing in the fossil record, so palaeontologists work from the jaw and the ear.

How many mammal species are there?

More than there were when you were at school, and the number is still climbing. The American Society of Mammalogists Mammal Diversity Database, version 2 as of August 2024, recognises:

6,759total species recognised
6,629wild and extant
113recently extinct
17domestic
1,353genera
167families

The count has risen 68.8 per cent since 1980, at roughly 65 species a year, mostly through splitting existing species once genetic data showed they were more than one thing, rather than through finding animals nobody had ever seen. Projections in the 2025 review in the Journal of Mammalogy put the total near 7,079 by 2030.

The figure that should worry you more is this one: 25 per cent of recognised mammal species are either Data Deficient or Not Evaluated. One mammal in four has never been properly assessed for extinction risk. We are describing the group faster than we are assessing it.

Four groupings that are wrong, and why

  • Red pandas are not pandas. The giant panda is a genuine bear, Ursidae. The red panda sits alone in Ailuridae, and its closest relatives are the mustelids, raccoons and skunks. Same name, different branch entirely.
  • Skunks are not mustelids. They were classified as such for a long time on appearance, then molecular work moved them into their own family, Mephitidae.
  • Hyenas are not dogs. They belong to Feliformia, the cat side of Carnivora, alongside mongooses and civets. The resemblance to a canid is convergence, not relatedness.
  • Hippos are not related to rhinos or to pigs. Hippos are even toed, Artiodactyla, and their closest living relatives are the whales and dolphins. Rhinos are odd toed, Perissodactyla, sitting with horses and tapirs. Large grey megafauna is a folk category, not a taxon.

If a term below is unfamiliar, the glossary of ecology terms defines more than 1,500 of them, and the wider concepts sit in the ecology section.

Every mammal on this site

185 articles, grouped by order and then by animal. Where a group has enough depth it has its own full guide; the rest are listed article by article. I am adding guides steadily, so groups will move from lists to links over the coming weeks.

Carnivora

The carnivores

Wolves

Canis lupus and its relatives, Canidae. Read the full guide to wolves, which pulls together all 20 articles with sources.

Coyotes

Canis latrans, Canidae.

Foxes

Vulpes and Urocyon, Canidae.

Big cats as a group

Panthera plus the cheetah, a popular grouping rather than a strict one.

Lions

Panthera leo, Felidae.

Tigers

Panthera tigris, Felidae.

Jaguars

Panthera onca, Felidae.

Leopards

Panthera pardus and the snow leopard P. uncia, Felidae.

Cheetahs

Acinonyx jubatus, Felidae. Not a Panthera cat, which is why it cannot roar.

Bobcats and lynx

Lynx, Felidae. The bobcat is Lynx rufus.

Bears

Ursidae. The giant panda is a genuine bear; the red panda is not.

Badgers

Taxidea, Meles and Mellivora, Mustelidae.

Otters

Lutrinae, Mustelidae.

Martens and fishers

Martes and Pekania, Mustelidae.

Raccoons

Procyonidae.

Hyenas

Hyaenidae, closer to cats than to dogs.

Mongooses and meerkats

Herpestidae.

Primates

The primates

Gorillas

Gorilla, Hominidae.

Orangutans

Pongo, Hominidae.

Baboons

Papio, Cercopithecidae.

Monkeys

various families across two infraorders.

Artiodactyla

Even toed hoofed mammals, and the whales

Hippos

Hippopotamidae. Their closest living relatives are the cetaceans.

Camels

Camelus, Camelidae. Read the full guide to camels, which pulls together all 6 articles with sources, plus hump biology, water physiology and the wild camel’s status.

Llamas and alpacas

Lama and Vicugna, Camelidae.

Moose

Alces alces, Cervidae.

Deer and elk

Cervidae.

Giraffes

Giraffidae.

Gazelles

Bovidae.

Wild pigs

Suidae.

Dolphins

Delphinidae.

Whales

Cetacea.

Perissodactyla

Odd toed hoofed mammals

Rhinos

Rhinocerotidae.

Proboscidea

Elephants

Elephants

Elephantidae.

Xenarthra

Armadillos and sloths, an ancient South American radiation

Sloths

Pilosa.

Armadillos

Cingulata.

Marsupialia

The marsupials

Kangaroos and quokkas

Macropodidae.

Koalas

Phascolarctidae.

Rodents, bats and small mammals

Beavers

Castoridae.

Squirrels and chipmunks

Sciuridae.

Capybaras

Caviidae.

Bats

Chiroptera, the only mammals capable of powered flight.

Written by Erzsebet Frey, MSc Ecology, University of Belgrade. Taxonomy follows the ASM Mammal Diversity Database version 2. Last reviewed September 2026.

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