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.
The Blue Whale
) is a marine mammal
belonging to the suborder of baleen whales
. At up to 33 metres
(110 ft) in length and 181 metric tonnes
(200 short tons
) or more in weight, it is believed to be the largest animal
to have ever lived, though some fragmentary dinosaur
discoveries such as the colossal Amphicoelias fragillimus
may challenge this long-held belief. Long and slender, the Blue Whale's body can be various shades of bluish-gray. There are at least three distinct subspecies: B. m. musculus
of the north Atlantic and north Pacific, B. m. intermedia
, of the Southern Ocean
and B. m. brevicauda
(also known as the Pygmy Blue Whale
) found in the Indian Ocean
and South Pacific Ocean
. B. m. indica
found in the Indian Ocean, may be another subspecies. As with other baleen whales, its diet consists mainly of small crustaceans
known as krill
, as well as small fish and squid. lue Whales were abundant in nearly all oceans until the beginning of the twentieth century. For over 40 years they were hunted almost to extinction
until protected by the international community in 1966
. A 2002 report estimated there were 5,000 to 12,000 Blue Whales worldwide located in at least five groups. More recent research into the Pygmy subspecies suggests this may be an underestimate. Before whaling the largest population was in the Antarctic, numbering approximately 239,000 (range 202,000 to 311,000). There remain only much smaller (around 2,000) concentrations in each of the North-East Pacific
, and Indian Ocean
groups. There are two more groups in the North Atlantic
and at least two in the Southern Hemisphere
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