Mammals (from Latin mamma "breast") are vertebrate animals constituting the class Mammalia , and characterized by the presence of mammary glands which in females (and sometimes males) produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201-227 million years ago. There are around 5,450 species of mammals. The largest orders are the rodents, bats and Soricomorpha (shrews and others). The next three are the Primates (apes, monkeys, and others), the Cetartiodactyla (cetaceans and even-toed ungulates), and the Carnivora (cats, dogs, seals, and others).
In cladistics, which reflect evolution, mammals--along with dinosaurs, and by extension, birds--are classified as endothermic amniotes. This trait evolved separately in both cases and is an example of convergent evolution. Mammals are the only living members of the clade Synapsida, which together with Sauropsida (reptiles and birds) 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 around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split into several diverse groups of non-mammalian synapsids--sometimes incorrectly referred to as mammal-like reptiles--before giving rise to Therapsida in the Early Permian period. 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.
The basic body type is quadruped, and most mammals use their four extremities for terrestrial locomotion; but in some, the extremities are adapted for life at sea, in the air, in trees, underground, or on two legs. Mammals range in size from the 30-40 mm (1.2-1.6 in) bumblebee bat to the 30-meter (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 are egg-laying mammals. The most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation.
Most 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.
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 civilizations. 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, 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. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation.
) is a large flippered marine mammal
with a discontinuous circumpolar distribution in the Arctic Ocean
seas of the Northern Hemisphere
. The walrus is the only living species in the Odobenidae
family. It is subdivided into two or three subspecies
: the Atlantic walrus
(O. rosmarus rosmarus
) found in the Atlantic Ocean
, the Pacific walrus
(O. rosmarus divergens
) found in the Pacific Ocean
, and a possible third subspecies, O. rosmarus laptevi
, found in the Laptev Sea
. Walruses are immediately recognizable due to their prominent tusks
and great bulk. Adult Pacific males can weigh up to 4,500 pounds
, and, among pinnipeds
, are exceeded in size only by the elephant seals
. They reside primarily in shallow oceanic shelf
habitat, spending a significant proportion of their lives on sea ice in pursuit of their preferred diet of benthic bivalve mollusks
. They are relatively long-lived, social animals and are considered a keystone species
in Arctic marine ecosystems
. Walruses have played a prominent role in the cultures of many indigenous Arctic peoples
, who have hunted walruses for their meat
, tusks and bone
. In the 19th and early 20th centuries, walruses were the objects of heavy commercial exploitation for blubber
and their numbers declined rapidly. Their global population has since rebounded, though the Atlantic and Laptev sub-population remain fragmented and at historically depressed levels.
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