Mammals are a grouping of animals that have a familiar and similar set of basic characteristics, such as being warm blooded, maintaining their own body temperature (endothermic), and they all have some type of fur or hair covering.
All female mammals nurse their young by secreting milk through special gland called the mammary gland. This is where the word mammal comes from, mamma from the Latin meaning teat or pap.
Mammals (class Mammalia) are a clade (organism with a common ancestor) of endothermic amniotes distinguished from the reptiles and the birds by the possession of:
- three middle ear bones,
- mammary glands in females, and
- a neocortex (a region of the brain).
The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart.
The mammals include the largest animals on the planet, the rorquals (the grouping of baleen whales that include the blue whale) and some other whales, as well as some of the most intelligent, such as elephants, some primates and some cetaceans.
The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs.
The largest group of mammals, the placentals, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30–40 mm bumblebee bat to the 33-meter blue whale.
Origin of the word mammal
The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands.
Numbers of Mammals
According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.
In some classifications, the mammals are divided into two subclasses (not counting fossils):
- the Prototheria (order of Monotremata) and
- the Theria,
the latter composed of the infraclasses Metatheria and Eutheria.
The marsupials constitute the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.
Except for the five species of monotremes (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group.
Orders of mammals
The three largest orders, in descending order, are:
- Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals),
- Chiroptera (bats), and
- Soricomorpha (shrews, moles and solenodons).
The next three largest orders, depending on the classification scheme used, are:
- the primates, to which the human species belongs,
- the Cetartiodactyla (including the even-toed hoofed mammals, i.e. ungulates and the whales), and
- the Carnivora (cats, dogs, weasels, bears, seals, and their relatives).
While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in at the same time in literature, especially for the marsupials.
Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.
The origins of mammals
The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon.
At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds.
Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of the dinosaurs 66 million years ago.