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Mammals range in size from the vast blue whale to tiny rodents. One of the six basic animal groups, mammals live in the sea, in the tropics, in the desert, and even in Antarctica. Different as they are from one another, however, mammals have a number of important physical and behavioral characteristics in common.01of 10
There Are Approximately 5,000 Mammal SpeciesAlexandre 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. Those numbers may appear large, but they're actually tiny in comparison to the roughly 10,000 species of birds, 30,000 species of fish, and five million species of insects alive today.02of 10
All Mammals Nurture Their Young With Milk
All mammals possess mammary glands, which produce the milk with which mothers sustain their newborns. However, not all mammals are equipped with nipples; the platypus and echidna are 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.03of 10
All Mammals Have HairBen 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; for example, whale and porpoise embryos only have hair for 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.
Mammals Evolved From "Mammal-Like Reptiles"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.05of 10
All Mammals Share the Same Basic Body Plan
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. Perhaps the most significant is 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 efficiently pump blood through their bodies.06of 10
Some Scientists Divide Animals Into "Metatherians" and "Eutherians"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 split is to divide mammals into two evolutionary clades: Eutherians ("true beasts") which include all placental mammals, and Metatherians ("above the beasts") which diverged from Eutherians sometime during the Mesozoic Era and includes all living marsupials.
Mammals Have Warm-Blooded MetabolismsAnsgar Walk / Wikimedia Commons / CC-BY-SA-3.0
The reason all mammals have hair 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.
Mammals Are Capable of Advanced Social BehaviorWinky 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. Examples of social behavior include the herd behavior of wildebeests, the hunting prowess of wolf packs, and the dominance structure of ape communities. However, you 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.09of 10
Mammals Display a High Level of Parental CareThomas Quine / Flickr / CC BY-SA 2.0
One major difference between mammals and other major vertebrate families such as amphibians, reptiles, and fish is that newborns require at least some parental attention in order to thrive. 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.10of 10
Mammals Are Remarkably Adaptive AnimalsJustin 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.