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.

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.

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.

However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematisation itself, partly through the new concept of cladistics.

The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; Chiroptera: bats; and Eulipotyphla: shrews, moles, and solenodons.

The next three biggest orders, depending on the biological classification scheme used, are the primates: apes, monkeys, and lemurs; Cetartiodactyla: whales and even-toed ungulates; and Carnivora which includes cats, dogs, weasels, bears, seals, and allies.

[8] The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap").

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial)[19] and varying interpretations of paleogeographic data.

[18] Mammal phylogeny according to Álvarez-Carretero et al., 2022:[20] Monotremata Paucituberculata Didelphimorphia Microbiotheria Notoryctemorphia Peramelemorphia Dasyuromorphia Diprotodontia Cingulata Pilosa Hyracoidea Sirenia Proboscidea Tubulidentata Macroscelidea Afrosoricida Eulipotyphla Chiroptera Pholidota Carnivora Perissodactyla Artiodactyla Scandentia Lagomorpha Rodentia Dermoptera Primates Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from the reptile lineage.

They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards;[34] the Jurassic Castorocauda, for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish.

These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.

[43] Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca)—as reptiles and birds also do—[44] and they lay eggs which are leathery and uncalcified.

The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.

[65] Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene.

Ingested food passes to the rumen and reticulum where cellulolytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants.

[112] Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger.

Marsupials have a short gestation period, typically shorter than its estrous cycle and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen.

[154] Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate (swing between trees).

To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture.

In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water.

[171] Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage;[170] the hind-flippers serve as stabilizers.

[174] Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures,[175] adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.

[200] Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with haemoglobin production, and as a laxative.

[210] Ruminants and most other herbivores partially submerge the tip of the mouth in order to draw in water by means of a plunging action with the tongue held straight.

In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.

[218] Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 cu in) and 7.8 kilograms (17 lb) in mature males.

The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes".

[273][274] Mammals serve a major role in science as experimental animals, both in fundamental biological research, such as in genetics,[276] and in the development of new medicines, which must be tested exhaustively to demonstrate their safety.

The deliberate or accidental hybridising of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes.

[297][298] The 2019 Global Assessment Report on Biodiversity and Ecosystem Services by IPBES states that the total biomass of wild mammals has declined by 82 per cent since the beginning of human civilisation.