The 21 Basic Mammal Groups

A large deer and two wild boards walk through the forest as silhouettes against a dawn sky.

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Classifying a family of vertebrates as broad and diverse as mammals is a notoriously difficult undertaking. Different people have different views about what constitutes orders, superorders, clades, cohorts, and all the other confusing terms biologists use when untangling the branches of the tree of life. 

01
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Aardvarks (Order Tubulidentata)

Aardvark walking through tall grass.

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The aardvark is the only living species in order Tubulidentata. This mammal is characterized by its long snout, arched back, and coarse fur. Its diet consists primarily of ants and termites, which it procures by tearing open insect nests with its long claws. Aardvarks live in the savannas, woodlands, and grasslands of sub-Saharan Africa. Their range extends from southern Egypt to the Cape of Good Hope, on the southernmost tip of the continent. The closest living relatives of the aardvark are even-toed hoofed mammals and (somewhat surprisingly) whales.

02
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Armadillos, Sloths, and Anteaters (Order Xenarthra)

Armadillo in profile standing on a rock.

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Originating in South America about 60 million years ago, only five million years after the dinosaurs went extinct, xenarthrans are characterized by their oddly-shaped vertebrae (hence their name, which is Greek for "strange joint"). The sloths, armadillos, and anteaters that belong to this order also have the most sluggish metabolisms of any extant mammals. The males have internal testicles. Today, xenarthrans lurk on the fringes of the mammalian mainstream, but during the Cenozoic Era, they were some of the largest animals on Earth. The five-ton prehistoric sloth Megatherium, as well as Glyptodon, the two-ton prehistoric armadillo, both lived during this time.

03
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Bats (Order Chiroptera)

Bat flying against a blue sky looking at the camera.

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The only mammals capable of powered flight, bats are represented by about a thousand species divided into two main families: megabats and microbats. Also known as flying foxes, megabats are about the size of squirrels and eat only fruit. Microbats are much smaller and enjoy more varied diets that range from the blood of grazing animals to insects to nectar. Most microbats, but very few megabats, have the ability to echolocate. This ability allows bats to bounce high-frequency sound waves off their surroundings to navigate dark caves and tunnels.

04
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Carnivores (Order Carnivora)

Lion with full mane looking into the distance.

Ltshears - Trisha M Shears/Wikimedia Commons/Public Domain

The order of mammals without which no TV nature documentary would be complete, carnivores are divided into two broad categories: feliforms and caniforms. Feliforms include not only obvious felines (like lions, tigers, cheetahs, and house cats), but also hyenas, civets, and mongooses. Caniforms extend beyond dogs and wolves to include bears, foxes, raccoons, and numerous other hungry critters, including the classic pinnipeds (seals, sea lions, and walruses). As you may already have surmised, carnivores are characterized by their sharp teeth and claws. They're also equipped with at least four toes on each foot.

05
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Colugos (Order Dermoptera)

Colugo clinging to a tree trunk looking at camera.

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Never heard of colugos? Well, there's a good reason: there are only two living colugo species in the world today, both residing in the dense jungles of southeast Asia. Colugos are characterized by the wide flaps of skin extending from their forelimbs, which enable them to glide 200 feet from tree to tree in a single journey. This is far beyond the capabilities of similarly-equipped flying squirrels, which are only distantly related to colugos. Oddly enough, while molecular analysis has demonstrated that colugos are the closest living relatives of our own mammalian order, the primates, their child-rearing behavior most closely resembles that of marsupials.

06
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Dugongs and Manatees (Order Sirenia)

Manatee and calf under the water.

Galen Rathbun/Wikimedia Commons/Public Domain

The semi-marine mammals known as pinnipeds (including seals, sea lions, and walruses) are lumped in the order Carnivora (see slide #5), but not dugongs and manatees, which belong to their own order, Sirenia. The name of this order derives from the mythical siren. Apparently, starving Greek sailors sometimes mistook dugongs for mermaids! Sirenians are characterized by their paddle-like tails, near-vestigial hind limbs, and muscular front limbs used to steer through the water. Modern dugongs and manatees are modestly sized, but a recently extinct sirenian, Steller's sea cow, may have weighed as much as 10 tons.

07
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Elephants (Order Proboscidea)

Two elephants winding their trunks together in a mating ritual.

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You might be surprised to learn that all of the world's elephants, order Proboscidea, belong to only two (or possibly three) species. They are the African elephant (Loxodonta africana), the Asian elephant (Elephas maximus), and, according to some experts, the African forest elephant (L. cyclotis). As rare as they are now, elephants have a rich evolutionary history that includes not only the familiar mammoths and mastodons of the Ice Age but distant ancestors like Gomphotherium and Deinotherium. Elephants are characterized by their large size, floppy ears, and long, prehensile trunks.

08
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Elephant Shrews (Order Macroscelidae)

Elephant shrew walking along the ground.

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Elephant shrews (order Macroscelidea) are small, long-nosed, insect-eating mammals native to Africa. There are about 20 named species of elephant shrew alive today, including the golden-rumped elephant shrew, the checkered elephant shrew, the four-toed elephant shrew, the short-eared elephant shrew, and the dusky elephant shrew. The classification of these small mammals has been a matter of debate. In the past, they've been classified as close relatives of hoofed mammals, hares and rabbits, insectivores, and tree shrews. The latest molecular evidence points to kinship with, appropriately enough, elephants!

09
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Even-Toed Hoofed Mammals (Order Artiodactyla)

Close up of white cows on a farm looking at the camera.

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Even-toed hoofed mammals, order Artiodactyla, also known as cloven-hoofed mammals or artiodactyls, possess feet structured so that the animal's weight is carried by its third and fourth toes. Artiodactyls include familiar animals such as cattle, goats, deer, sheep, antelope, camels, llamas, pigs, and hippopotamuses, amounting to about 200 species worldwide. Virtually all artiodactyls are herbivores. The exceptions are omnivorous pigs and peccaries. Some, like cows, goats, and sheep, are ruminants (cud-chewing mammals equipped with extra stomachs), and none of them are particularly bright.

10
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Golden Moles and Tenrecs (Order Afrosoricida)

Golden mole looking at camera.

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What used to be the mammalian order known as Insectivora ("insect-eaters") has undergone a big change recently, splitting into two new orders, Eulipotyphia (Greek for "truly fat and blind") and Afrosoricida ("looking like African shrews"). In the latter category are two very obscure creatures: the golden moles of southern Africa and the tenrecs of Africa and Madagascar. Just to show how complicated the business of taxonomy can be, various species of tenrecs, via the process of convergent evolution, closely resemble shrews, mice, possums, and hedgehogs, while golden moles are, appropriately enough, reminiscent of true moles.

11
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Hares, Rabbits, and Pikas (Order Lagomorpha)

Black rabbit in an autumn landscape.

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Even after centuries of study, naturalists still aren't sure what to make of hares, rabbits, and pikas, the only members of the order Lagomorpha. These small mammals are similar to rodents, with some important differences: lagomorphs have four, rather than two, incisor teeth in their upper jaws. They're also strict vegetarians, whereas mice, rats, and other rodents tend to be omnivorous. In general, lagomorphs can be distinguished by their short tails, their long ears, the slit-like nostrils on the sides of their snouts that they can shut tight, and (in some species) a pronounced inclination to hop and jump.

12
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Hedgehogs, Solenodons, and More (Order Eulipotyphia)

Hedgehog curled up on a brick walkway.

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As mentioned in slide #11, the too-broad order once known as Insectivora has since been cleaved in two by naturalists availing themselves of the latest DNA technology. The order Afrosoricida includes golden moles and tenrecs, while the order Eulipotyphia includes hedgehogs, gymnures (also known as moonrats or hairy hedgehogs), solenodons (venomous shrew-like mammals), and the strange creatures known as desmans, as well as moles, shrew-like moles, and true shrews. Confused yet? Suffice it to say that all Eulipotyphians (and most Afrosoricidans, for that matter) are wee, narrow-snouted, insect-eating balls of fur, and leave it at that.

13
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Hyraxes (Order Hyracoidea)

Hyrax eating grass and staring at the camera.

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Not the most familiar order of mammals, hyraxes are thick, stubby-legged, plant-eating mammals that look a bit like a cross between a house cat and a rabbit. There are only four species (the yellow-spotted hyrax, the rock hyrax, the western tree hyrax, and the southern tree hyrax), all of them native to Africa and the Middle East. One of the strangest things about hyraxes is their relative lack of internal temperature regulation. They're technically warm-blooded, like all mammals, but spend an inordinate amount of time huddling together in the cold or basking in the sun during the heat of midday.

14
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Marsupials (Order Marsupialia)

Two kangaroos fighting each other.

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Unlike the placental mammals featured elsewhere on this list — which gestate their fetuses in the womb, nourished by placentas — marsupials incubate their young in specialized pouches after an extremely short interval of internal gestation. Everyone is familiar with the kangaroos, koala bears, and wombats of Australia, but the possums of North America are also marsupials, and for millions of years the largest marsupials on Earth could be found in South America. In Australia, marsupials managed to displace placental mammals for most of the Cenozoic Era, the only exceptions being the "hopping mice" that made their way from southeast Asia, and the dogs, cats, and livestock introduced by European settlers.

15
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Monotremes (Order Monotremata)

Short-beaked echidna walking across the ground.

Gunjan Pandey/Wikimedia Commons/CC BY 4.0

Hands-down the most bizarre mammals on the face of the earth, monotremes —consisting of one species of platypus and four species of echidna — lay soft-shelled eggs, rather than giving birth to live young. And that's not the end of the monotreme weirdness: these mammals are also equipped with cloacas (a single orifice for urinating, defecating, and reproducing), they're completely toothless as adults, and they have a talent for electroreception (sensing faint electrical currents from a distance). According to current thinking, monotremes evolved from a Mesozoic ancestor that predated the split between placental and marsupial mammals, hence their extreme weirdness.

16
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Odd-Toed Hoofed Mammals (Order Perissodactyla)

Zebra standing in the grass in profile.

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Compared to their even-toed artiodactyl cousins (see slide #10), odd-toed perissodactyls are a sparse lot, consisting entirely of horses, zebras, rhinoceroses, and tapirs — only about 20 species in all. Besides the unique structure of their feet, perissodactyls are characterized by a pouch called a "caecum" that extends from their large intestines. It contains specialized bacteria that aids in the digestion of tough plant matter. According to molecular analysis, odd-toed mammals may be more closely related to carnivores (order Carnivora) than they are to even-toed mammals (order Artiodactyla).

17
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Pangolins (Order Pholidota)

Pangolin in the grass walking near a road.

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Also known as scaly anteaters, pangolins are characterized by the large, plate-like scales (made of keratin, the same protein found in human hair) covering their bodies. When these creatures are threatened by predators, they curl up into tight balls with sharp-edged scales pointing outward. For good measure, they can also expel a smelly, skunk-like excretion from a specialized gland near the anus. All that said, you may be relieved to learn that pangolins are native to Africa and Asia, and are practically never seen in the western hemisphere (except in zoos).

18
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Primates (Order Primates)

Two young monkeys playing on a branch.

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Comprising prosimians, monkeys, apes, and human beings — about 400 species in all — primates in many ways can be considered the most "advanced" mammals on the planet, especially as pertains to their larger-than-average brains. Non-human primates often form complex social units and are capable of rudimentary tool use. Some species are equipped with dexterous hands and prehensile tails. There's no single trait that defines all primates as a group, but these mammals do share some general features, such as eye sockets encircled by bone and binocular vision (an excellent adaptation for spotting prey, and predators, from a long way off).

19
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Rodents (Order Rodentia)

Mouse sitting on the ground.

 Alexas_Fotos/Pixabay

The most diverse mammal group, consisting of over 2000 species, order Rodentia includes squirrels, dormice, mice, rats, gerbils, beavers, gophers, kangaroo rats, porcupines, pocket mice, springhares, and many others. What all of these tiny, furry critters have in common are their teeth: one pair of incisors in the upper and lower jaw and a large gap (called a diastema) located between the incisors and the molars. The "buck-toothed" incisors of rodents grow continuously and are maintained by constant use. The grinding and gnawing of rodents ensure that their incisors always remain sharp and stay at the correct length.

20
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Tree Shrews (Order Scandentia)

Tree shrew standing on a branch.

Anthony Cramp/Flickr/CC BY 2.0

If you made it through the Afrosoricida (slide #11) and the Eulipotyphia (slide #13), you know that classifying small, insect-eating mammals can be a wearisome affair. Once lumped in the now-discarded order Insectivora, tree shrews aren't true shrews, and not all of them live in trees. The 20 or so extant species are native to the tropical forests of southeast Asia. Members of the order Scandentia are omnivorous, feasting on everything from insects to small animals to the "corpse flower" Rafflesia. Oddly enough, they have the highest brain-to-body-size ratio of any living mammal (including humans).

21
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Whales, Dolphins, and Porpoises (Order Cetacea)

Two orca whales in the ocean.

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Comprising close to a hundred species, cetaceans are divided into two main groups: toothed whales (which includes sperm whales, beaked whales, and killer whales, as well as dolphins and porpoises), and baleen whales, which includes right whales, bowhead whales, and the biggest cetacean of them all, the 200-ton blue whale. These mammals are characterized by their flipper-like forelimbs, reduced back limbs, nearly hairless bodies, and the single blowhole on top of their heads. The blood of cetaceans is unusually rich in hemoglobin, an adaptation that allows them to stay underwater for long periods of time.