Mammals (class Mammalia) are vertebrate animals characterized by the presence of sweat glands, including milk producing sweat glands, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in the brain. Mammals, other than the monotremes, give birth to live young instead of laying eggs. They also possess specialized teeth and use a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species, ranging in size from the Bumblebee Bat, (30-40mm), to the Blue Whale, (33,000mm), distributed in about 1,200 genera, 153 families, and 29 orders, though this varies by classification scheme.
Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales).
Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.
The mammalian line of descent diverged from the sauropsid line at the end of the Carboniferous period. The sauropsids would evolve into modern-day reptiles and birds, while the synapsid branch led to mammals. The first true mammals appeared in the Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.
) 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.