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A mammal (from Latin mamma 'breast') is a vertebrate animal of the class Mammalia (/məˈmeɪli.ə/). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 27 orders. The study of mammals is called mammalogy.

The largest orders of mammals, by number of species, are the rodents, bats, and eulipotyphlans (including hedgehogs, moles and shrews). The next three are the primates (including humans, monkeys and lemurs), the even-toed ungulates (including pigs, camels, and whales), and the Carnivora (including cats, dogs, and seals).

Mammals are the only living members of Synapsida; this clade, together with Sauropsida (reptiles and birds), constitutes the larger Amniota clade. Early synapsids are referred to as "pelycosaurs." The more advanced therapsids became dominant during the Guadalupian. Mammals originated from cynodonts, an advanced group of therapsids, during the Late Triassic to Early Jurassic. Mammals achieved their modern diversity in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present.

The basic mammalian body type is quadrupedal, with most mammals using four limbs for terrestrial locomotion; but in some, the limbs are adapted for life at sea, in the air, in trees or underground. The bipeds have adapted to move using only the two lower limbs, while the rear limbs of cetaceans and the sea cows are mere internal vestiges. Mammals range in size from the 30–40 millimetres (1.2–1.6 in) bumblebee bat to the 30 metres (98 ft) blue whale—possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which lay eggs. The most species-rich group is the viviparous placental mammals, so named for the temporary organ (placenta) used by offspring to draw nutrition from the mother during gestation.

Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Mammals can communicate and vocalise in several ways, including the production of ultrasound, scent marking, alarm signals, singing, echolocation; and, in the case of humans, complex language. Mammals can organise themselves into fission–fusion societies, harems, and hierarchies—but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.

Domestication of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in farming replacing hunting and gathering as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first civilisations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (meat and dairy products), fur, and leather. Mammals are also hunted and raced for sport, kept as pets and working animals of various types, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation. (Full article...)

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The Platypus (Ornithorhynchus anatinus) is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young. It is the sole living representative of its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a number of related species have been found in the fossil record. The bizarre appearance of this egg-laying, venomous, duck-billed, beaver-tailed, otter-footed mammal baffled European naturalists when they first encountered it, with some considering it an elaborate fraud. It is one of the few venomous mammals; the male Platypus has a spur on the hind foot that delivers a poison capable of causing severe pain to humans. The unique features of the Platypus make it an important subject in the study of evolutionary biology and a recognizable and iconic symbol of Australia; it has appeared as a mascot at national events and is featured on the reverse of the Australian 20 cent coin. Until the early 20th century it was hunted for its fur, but it is now protected throughout its range. Although captive breeding programs have had only limited success and the Platypus is vulnerable to the effects of pollution, it is not under any immediate threat.

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Did you know?

...that one burrow of the eastern chipmunk (pictured) was found to contain 390 acorns?
.. that the Hare Indian dog, now extinct, was not known to bark, but puppies learned to imitate the barking of other dogs when the breed was introduced to Europe?
... that the rodents Thomasomys ucucha from Ecuador and Oxymycterus hucucha from Bolivia were both named after the local Quechua word for "mouse"?
...that the incisors of blesmols are visible even when their mouths are closed?
... that the pebble-mound mice of northern Australia construct mounds of stones around their burrows up to 10 m² in area?
... that the stationmaster of the Kinokawa train station in Kinokawa, Japan was a cat named Tama?

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Image 1
Koala
Photo: JJ Harrison
A female koala (Phascolarctos cinereus), an arboreal herbivorous marsupial native to coastal regions of eastern and southern Australia. The word "koala" comes from the Dharuk word gula. English-speaking settlers from the late 18th century first called it "koala bear" due to its similarity in appearance to bears, although they are not at all related. Instead, its closest living relative is the wombat.
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Image 2
Impala
Photograph: Muhammad Mahdi Karim
A portrait of a male impala, showing its horns. Horns consist of bone cores surrounded by a covering of keratin and other proteins, and are often curved or spiral in shape. Most horned animal species have one pair, though some may have more.
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Image 3
Virginia Opossum
Photo credit: Cody Pope
The Virginia Opossum (Didelphis virginiana) is the only marsupial found in North America. A solitary and nocturnal animal about the size of a domestic cat, it is a successful opportunist and is found throughout North America from coast to coast (introduced to California in 1910), and from Central America and Mexico to southern Canada.
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Image 4
Red squirrel
Photograph: Peter Trimming
The red squirrel (Sciurus vulgaris) is a species of tree squirrel in the genus Sciurus common throughout Eurasia. This arboreal, omnivorous rodent feeds on seeds, nuts, berries, young shoots, and sap.
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Image 5
Steenbok
Photograph: Yathin S Krishnappa
The steenbok (Raphicerus campestris) is a species of small antelope, generally standing 45–60 centimeters (16–24 inches) at the shoulder, commonly found in Africa. They live in a variety of habitats, from semi-desert to open woodland and thickets, and feed on low-level vegetation and roots. These common animals are typically solitary, except during mating season.
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Image 6
Common wombat
Photo: JJ Harrison
The common wombat (Vombatus ursinus) is one of three species of wombat. It is native to south-eastern mainland Australia and Tasmania, and grows to an average of 98 cm (39 in) long and a weight of 26 kg (57 lb). It is solitary and lives in an underground burrow.
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Image 7
Lion
Photo credit: yaaaay
A male lion (Panthera leo) lying down in Namibia. One of the four "big cats" in the genus Panthera, the lion is the second largest cat, after the tiger. Males weigh between 150-250 kg (330-550 lb), and are easily recognizable by their manes. Though they were once found throughout much of Africa, Asia and Europe, lions presently exist in the wild only in Africa and India.
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Image 8
Lar gibbon
Photograph: JJ Harrison
The lar gibbon (Hylobates lar) is a primate found in South-East Asia. This endangered gibbon species is primarily a frugivore, but may also feed on leaves, flowers, and insects.

This specimen was photographed in Kaeng Krachan National Park, Phetchaburi, Thailand.
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Image 9
Yak
Photograph: travelwayoflife
A yak at Letdar on the Annapurna Circuit in the Annapurna mountain range of central Nepal. Mostly domesticated, these long-haired bovids are found throughout the Himalayan region of south Central Asia, the Tibetan Plateau and as far north as Mongolia and Russia.
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Image 10
Goat kid
Photo credit: Fir0002
A 2-month old domestic goat (Capra aegagrus hircus) kid in a field of capeweed. Goats are one of the oldest domesticated species. For thousands of years, goats have been used for their milk, meat, hair, and skins over much of the world. Female goats are referred to as does or nannies, intact males as bucks or billies; their offspring are kids.
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Image 11
Tamias rufus
Photo credit: Mdf
The Hopi Chipmunk (Tamias rufus) is a chipmunk found in Colorado, Utah and New Mexico, in the Southwestern United States. Chipmunks are small squirrel-like rodents, native to North America except for one species in northeastern Asia. Though they are commonly depicted with their paws up to the mouth, eating peanuts, or with their cheeks bulging out on either side, chipmunks are actually omnivorous.
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Image 12
Javan slow loris
Photograph credit: Aprisonsan
The Javan slow loris (Nycticebus javanicus) is a species of primate native to western and central portions of the Indonesian island of Java. It is arboreal, feeding on fruit, gum, eggs, lizards, and the seeds of the cocoa tree. Weighing up to 680 g (24 oz), this small nocturnal mammal sleeps in the open on branches during the day and is easily poached for the exotic-pet trade. Populations are declining, and the animal is listed by the International Union for Conservation of Nature as a critically endangered species and included in a list of the world's 25 most endangered primates.
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Image 13
Impala
Photograph: Muhammad Mahdi Karim
A female impala (Aepyceros melampus) photographed at Mikumi National Park, Tanzania. These medium-sized African antelopes inhabit savanna grasslands and woodlands close to water sources, feeding on grasses, forbs, monocots, dicots and foliage. They are fast runners and known for their leaping ability, reaching heights up to 3 m (10 ft).
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Image 14
Leopard seal
Photograph: Godot13
The leopard seal (Hydrurga leptonyx), also known as the sea leopard, is the second largest species of seal in the Antarctic, after the southern elephant seal. Its only natural predators are the killer whale and possibly the elephant seal. It feeds on a wide range of prey including cephalopods, other pinnipeds, krill, birds and fish. Together with the Ross seal, the crabeater seal and the Weddell seal, it is part of the tribe of lobodontini seals.

This image shows a yawning leopard seal in the Antarctic Sound in 2016. See also the same seal in profile.
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Image 15
Rothschild's Giraffe
Photo credit: Fir0002
The head of a Rothschild's Giraffe (Giraffa camelopardalis rothschildi), a subspecies of giraffe found in Uganda and north-central Kenya. It has deep brown, blotched or rectangular spots with poorly defined cream lines and its hocks may be spotted.
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Scientific classification

Kingdom→ Animalia Phylum→ Chordata Subphylum→ Vertebrata Superclass→ Tetrapoda (unranked)→ Amniota Class→ Mammalia

Subclasses and Infraclasses

Subclass †Allotheria*
Subclass Prototheria
Subclass Theria
- Infraclass †Trituberculata
- Infraclass Metatheria
- Infraclass Eutheria

* Member of a haplogroup and not any of its subclades.
† Taxon is extinct.

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