10 Facts About Mammals Everyone Should Know

Leopard - Panthera pardus
Leopard - Panthera pardus. Photo © Jonathan and Angela Scott / Getty Images.

Perhaps because it's the group that also includes human beings, mammals are often considered to be the most "advanced" animals on our planet. On the following slides, you'll discover 10 basic facts about mammals that every literate adult and child should know.

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There Are Approximately 5,000 Mammal Species

The Reindeer is also known as 'caribou' in North America. Alexandre Buisse / Wikimedia Commons / CC BY-SA 3.0

Definitive counts are hard to come by--since some mammals are on the verge of extinction, while others remain to be discovered--but there are currently about 5,500 identified mammal species, grouped into approximately 1,200 genera, 200 families and 25 orders. Do mammals really "rule the earth?" Well, compare that number to the roughly 10,000 species of birds, 30,000 species of fish, and five million species of insects alive today, and you can draw your own conclusions!

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All Mammals Nurture Their Young With Milk

Suckling pig
Scott Bauer, USDA / Wikimedia Commons / Public domain

As you can guess from the similarity of the words, all mammals possess mammary glands, which produce the milk with which mothers sustain their newborns. However, not all mammals are equipped with nipples: the exceptions are the monotremes, which nurture their young via mammary "patches" that slowly seep milk. Monotremes are also the only mammals that lay eggs; all other mammals give birth to live young, and females are equipped with placentas.

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All Mammals Have Hair (at Some Point in Their Life Cycles)

Musk Ox
Musk Oxen. Ben Cranke / Getty Images

All mammals have hair--which evolved during the Triassic period as a way to retain body heat--but some species are hairier than others. More technically, all mammals have hair at some stage in their life cycles; you don't see many hairy whales or porpoises, for the simple reason that whale and porpoise embryos only have hair, for only a brief period of time, while gestating in the womb. The title of World's Hairiest Mammal is a matter of debate: some tout the Musk Ox, while others insist sea lions pack more follicles per square inch of skin.

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Mammals Evolved From "Mammal-Like Reptiles"

Megazostrodon may have been the first true mammal. Theklan / Wikimedia Commons / CC BY-SA 4.0

About 230 million years ago, during the late Triassic period, a population of therapsids ("mammal-like reptiles") split off into the first true mammals (a good candidate for this honor is Megazostrodon). Ironically, the first mammals evolved at almost exactly the same time as the first dinosaurs; for the next 165 million years, mammals were banished to the periphery of evolution, living in trees or burrowing underground, until the extinction of the dinosaurs finally allowed them to take center stage.

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All Mammals Share the Same Basic Body Plan

Human inner ear
A diagram of the anatomy of the human ear. Chittka L, Brockmann / Wikimedia Commons / CC BY 2.5

As befits a family of vertebrates descended from a "last common ancestor," all mammals share some key anatomical quirks, ranging from the seemingly minor (the three tiny bones in the inner ear that carry sound from the eardrum) to the obviously not-so-minor (the neocortical area of the brain, which accounts for the relative intelligence of mammals compared to other types of animals, and the four-chambered hearts of mammals, which pump blood more efficiently through their bodies.)

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Some Scientists Divide Animals Into "Metatherians" and "Eutherians"

The Koala Bear, a typical marsupial. skeeze / Wikimedia Commons

Although the precise classification of mammals is still a subject of dispute, it's obvious that marsupials (mammals that incubate their young in pouches) are different from placentals (mammals that incubate their young entirely in the womb). One way to account for this fact is to divide mammals into two evolutionary clades: Eutherians, or "true beasts," which include all placental mammals, and "metatherians," "above the beasts," which diverged from eutherians some time during the Mesozoic Era and includes all living marsupials.

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Mammals Have Warm-Blooded Metabolisms

Polar Bear
A Polar Bear would freeze without its warm-blooded metabolism. Ansgar Walk / Wikimedia Commons / CC-BY-SA-3.0

The reason all mammals have hair (see slide #4) is that all mammals have endothermic, or warm-blooded, metabolisms. Endothermic animals generate their own body heat from internal physiological processes, as opposed to cold-blooded (ectothermic) animals, which warm up, or cool down, according to the temperature of the environment they live in. Hair serves the same function in warm-blooded animals as a coat of feathers does in warm-blooded birds: it helps to insulate the skin and keep vital heat from escaping.

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Mammals Are Capable of Advanced Social Behavior

A herd of Wildebeest. Winky from Oxford, UK / Wikimedia Commons / CC BY 2.0

Thanks in part to their bigger brains, mammals tend to be more socially advanced than other types of animals: witness the herd behavior of wildebeests, the hunting prowess of wolf packs, and the dominance structure of ape communities. However, you should bear in mind that this is a difference of degree, and not of kind: ants and termites also display social behavior (which, however, seems to be completely hard-wired and instinctual), and even some dinosaurs roamed the Mesozoic plains in herds.

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Mammals Display a High Level of Parental Care

Icelandic horse
Icelandic horse and its foal. Thomas Quine / Flickr / CC BY-SA 2.0

One major difference between mammals and other major vertebrate families--notably amphibians, reptiles and fish--is that newborns require at least some parental attention in order to thrive (if only for the simple fact that they have to suckle milk from their mothers!) That said, however, some mammal babies are more helpless than others: a human newborn would die without close parental care, while many plant-eating animals (like horses and giraffes) are capable of walking and foraging immediately after birth.

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Mammals Are Remarkably Adaptive Animals

whale shark
A Whale Shark. Justin Lewis / Getty Images

One of the most amazing things about mammals is the different evolutionary niches they've managed spread into over the last 50 million years: there are swimming mammals (whales and dolphins), flying mammals (bats), tree-climbing mammals (monkeys and squirrels), burrowing mammals (gophers and rabbits), and countless other varieties. As a class, in fact, mammals have conquered more habitats than any other family of vertebrates; by contrast, during their 165 million years on earth, dinosaurs never became fully aquatic or learned how to fly (except, that is, in the course of evolving into birds).