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Mammals are a class of vertebrate animal. The term mammal comes from the fact that female mammals have mammary glands that produce milk to feed their young offspring.

Mammals are warm-blooded, air-breathing animals that have hair or fur on their body. Mammals also have three middle-ear bones to provide hearing.

Most mammals give birth to live young using a placenta to carry the baby during gestation (i.e., placentals). Marsupials have a pouch in which females carry their young through early infancy. Monotremes, such as the platypus, are unusual mammals in that they lay eggs to give birth to their young.

There are approximately 5,400 species of mammals on Earth, divided into 153 families of animals. The largest family of mammals are the rodents, which include mice, rats, porcupines, beavers, capybaras, and other gnawing mammals.

The next largest family are the chiroptera or bat family, followed closely by the soricomorpha family including animals such as shrews and moles. The next three large orders are the carnivores (such as dogs, cats, bears and weasels), cetartiodactyla (hoffed animals such as cows, deers, pigs and hippopotamus), and primates (such as humans, monkeys and apes).

Mammals have several unique characteristics that differentiate them from other animals. Most mammals have hair, or fur, covering their body. They are also capable of regulating their body temperature.

The mammals metabolism controls heat production, and the sweat glands help cool the body. These allow the mammal to maintain a constant body temperature, regardless of the environmental temperature. One other difference is that mammals give birth to fully formed babies, and the female mammals produce milk to feed their young. 

Most mammals walk on 4 legs, with only the humans walking upright on 2 legs. Aquatic mammals have flippers, or fins, for swimming rather than legs.

Common mammals include: primates, such humans and monkeys; marsupials; rodents; whales dolphins and, seals.

Mammals are endothermic, that is, they maintain a constant body temperature independent of their environment. This characteristic has enabled mammals to occupy a wide range of habitats since they can remain active in extreme temperatures.

The hypothalamus, an area of the mammalian brain, maintains and monitors body temperature. By increasing metabolic rate, body temperature can be increased. By decreasing metabolic rate, body temperature can be decreased.

Additionally, the ability to widen or constrict blood vessels provides another means by which heat can be carried to or restricted from parts of a mammal's body. Behavior also can help to regulate body temperature. Groups of mammals can huddle together for warmth or descend into cool burrows to avoid extreme heat.

Mammals are the only animals that have hair covering their body, a rod of cells that are strengthened by the protein keratin. The different types of hair mammals have include fur, whiskers, spines, and horns. Fur provides protection for the skin and insulates the animal. Whiskers provide sensory information for animals. Spines (such as the quills of a porcupine) provide protection.

The ways in which mammals move from place to place is greatly varied. Some mammals use four limbs to walk (such animals are referred to as quadrapedal) while others use only two (such animals are referred to as bipedal).

The greatest differences in locomotion adaptation can be seen when comparing mammals from very different habitats. Dolphins, adapted for an aquatic lifestyle, have flippers to help them glide through the water. The jaguar, a terrestrial carnivore with explosive running power, is adapted for speed on land. The limbs of bats have evolved into wings enabling them to inhabit the skies.

A characteristic unique to mammals is that they all feed their young with milk produced by the female's mammary glands. Most mammals give birth to live young and almost all species have hair on their body.

Some types of mammals form complex social structures. A quick summary of mammalian characteristics include:
  • mammary glands
  • hair
  • diaphragm
  • three middle-ear ossicles
  • heterodont dentation
  • sweat, sebaceous, and scent glands
  • four-chambered heart
  • large cerebral cortex

Mammals are believed to have evolved from an ancient group of reptiles called the therapsids. Therapsids lived about 225 million years ago during the Triassic Period. As they evolved, therapsids developed a lighter, more flexible skeleton and the alignment of their limbs changed over time to be directly beneath their body (unlike reptiles whose limbs are at the sides of their body).

The first mammals are thought to have been relatively small creatures (less than 10 cm long) with light, delicate skeletons. The fossil record suggests that these creatures had teeth and skulls characteristic of herbivores or predators that fed on arthropods or other vertebrates. Well-developed senses of hearing and smell suggest they may have been nocturnal creatures. another important adaptation mammals displayed was endothermy or the ability to regulate their own body temperatures.

When climate change took place in the Mesozoic Era, temperatures fluctuated and dropped. The reptiles that once dominated the land suffered in the low temperatures, while mammals were able to better compete since their body temperatures were less effected by temperature fluctuation.

The Tertiary period, which began about 70 million years ago, brought with it the dawning of the "age of mammals", a time following the extinction of many reptile species. This is when mammals diversified significantly in number and kind.

Types of Mammal

Monotremata (egg-laying marsupials) 

Marsupialia (marsupials)

Insectivora (insectivores)

Chiroptera (bats)

Dermoptera (flying lemurs)

Macroscelidea (elephant shrews)

Scandentia (tree shrews)

Primates (primates)

Xenarthra (anteaters and relatives)

Pholidota (pangolins)

Lagomorpha (rabbits, hares, and pikas)

Rodentia (rodents)

Cetacea (cetaceans)

Carnivora (carnivores)

Pinnipedea (seals and sea lions)

Proboscidea (elephants)

Tubulidentata (aardvark)

Hyracoidea (hydraxes)

Sirenia (dugongs and manatees)

Perissodactyla (odd-toed hoofed mammals)

Artiodactyla (even-toed hoofed mammals)

Mammals Extremes

  • Fastest mammal (also the fastest land animal): the cheetah (60-70 mph = 97-110 kph) 
  • Slowest mammal - the sloth (less than 1 mph, or 2 kph) 
  • Biggest mammal, biggest animal that ever lived on Earth - the blue whale
  • Biggest land mammal- the African Elephant
  • Tallest mammal - the giraffe
  • Smallest mammals - the pygmy shrew (weighing 1.2-2.7 gm) and the bumblebee bat (weighing about 2 gm) 
  • Loudest mammal - the Blue Whale. The second loudest is the Howler Monkey. 
  • Smallest newborns - marsupials (pouched mammals, like the kangaroo) 
  • Smelliest mammal - the striped skunk (Mephitis mephitis) 
  • Fat - The blue whale has the thickest layer of blubber, but ringed seal pups have the greatest percentage of fat (about 50 %). 
  • The only venomous mammals - duckbilled platypus (males only), several species of shrews, and the Solenodon 



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