Claim: Mammals are a clade.

Mammals are any vertebrates within the class Mammalia -LRB- -LSB- məˈmeɪli.ə -RSB- from Latin mamma #Latin `` breast '' -RRB- , a clade of endothermic amniotes distinguished from reptiles -LRB- including birds -RRB- by the possession of a neocortex -LRB- a region of the brain -RRB- , hair , three middle ear bones and mammary glands . Females of all mammal species nurse their young with milk , secreted from the mammary glands .   Mammals include the biggest animals on the planet , the great whales . The basic body type is a terrestrial quadruped , but some mammals are adapted for life at sea , in the air , in trees , underground or on two legs . The largest group of mammals , the placentals , have a placenta , which enables the feeding of the fetus during gestation .  Mammals range in size from the 30 -- bumblebee bat to the 30 m blue whale . With the exception of the five species of monotreme -LRB- egg-laying mammals -RRB- , all modern mammals give birth to live young . Most mammals , including the six most species-rich orders , belong to the placental group . The largest orders are the rodents , bats and Soricomorpha -LRB- shrews and allies -RRB- . The next three biggest orders , depending on the biological classification scheme used , are the Primates -LRB- apes and monkeys -RRB- , the Cetartiodactyla -LRB- whales and even-toed ungulates -RRB- , and the Carnivora -LRB- cats , dogs , seals , and allies -RRB- .   Living mammals are divided into the Yinotheria -LRB- platypus and echidnas -RRB- and Theriiformes -LRB- all other mammals -RRB- . There are around 5450 species of mammal , depending on which authority is cited . In some classifications , extant mammals are divided into two subclasses : the Prototheria , that is , the order Monotremata ; and the Theria , or the infraclasses Metatheria and Eutheria . The marsupials constitute the crown group of the Metatheria , and include all living metatherians as well as many extinct ones ; the placentals are the crown group of the Eutheria . While mammal classification at the family level has been relatively stable , several contending classifications regarding the higher levels -- subclass , infraclass and order , especially of the marsupials -- appear in contemporaneous literature . Much of the changes reflect the advances of cladistic analysis and molecular genetics . Findings from molecular genetics , for example , have prompted adopting new groups , such as the Afrotheria , and abandoning traditional groups , such as the Insectivora .   The mammals represent the only living Synapsida , which together with the Sauropsida form the Amniota clade . The early synapsid mammalian ancestors were sphenacodont pelycosaurs , a group that produced the non-mammalian Dimetrodon . At the end of the Carboniferous period , this group diverged from the sauropsid line that led to today 's reptiles and birds . The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids -- sometimes referred to as mammal-like reptiles -- before giving rise to the proto-mammals -LRB- Therapsida -RRB- in the early Mesozoic era . The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era , after the extinction of non-avian dinosaurs , and have been among the dominant terrestrial animal groups from 66 million years ago to the present .   Some mammals are intelligent , with some possessing large brains , self-awareness and tool use . Mammals can communicate and vocalize in several different ways , including the production of ultrasound , scent-marking , alarm signals , singing , and echolocation . Mammals can organize 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 .   In human culture , domesticated mammals played a major role in the Neolithic revolution , causing farming to replace hunting and gathering , and leading to a major restructuring of human societies with the first civilizations . They provided , and continue to provide , power for transport and agriculture , as well as various commodities such as meat , dairy products , wool , and leather . Mammals are hunted or raced for sport , and are used as model organisms in science . Mammals have been depicted in art since Palaeolithic times , and appear in literature , film , mythology , and religion . Defaunation of mammals is primarily driven by anthropogenic factors , such as poaching and habitat destruction , though there are efforts to combat this .