Mammal

Mammals Temporal range: Upper Triassic–Recent
Raccoon (Procyon lotor )
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Amniota
Superclass:	Tetrapoda
(unranked):	Synapsida
Class:	Mammalia Linnaeus, 1758
Subclasses & Infraclasses
Subclass †Allotheria Subclass Prototheria Subclass Theria Infraclass †Trituberculata Infraclass Metatheria Infraclass Eutheria

Mammals are a group of vertebrate animals which form the class Mammalia.

Characteristics

Reproduction

Most mammals (but not quite all) bear live young. All mammals feed their young on milk, which is made by glands in the skin or the mammary glands. All mammals protect and look after their young, though details differ greatly.

Cycles

Most marsupial and eutherian mammals have a reproductive cycle known as the oestrous cycle (U.S: estrous cycle). Females are sexually active only during the oestrous stage, when they are 'on heat' for a few days each month. If an ovum is not fertilised, the endometrium (uterus lining) is resorbed. Oestrus cycles may occur once or twice a year, or many times a year. Each group of mammals has its own frequency.

Humans, and many primates, are quite different. They have a menstrual cycle. In this case, females are sexually receptive at any time, but only fertile when an ovum is released from an ovary. In this case, the endometrium (if not needed for an fertilised egg) is discarded. The endometrium is shed, and takes with it a certain amount of blood. In this system, eggs are released from the ovaries mostly in the middle of the cycle, away from the mentrual period. This ovulation is 'concealed', meaning, it is not obvious when it occurs.

Homeothermy

Mammals are warm-blooded; they regulate their body temperature. The heat they need to live is made chemically inside their body. With this goes a higher rate of metabolism, and heat-retaining fur.

Skeleton

One diagnostic feature is the lower jaw which, unlike earlier forms, is composed of a single bone, the dentary.

Neocortex

Another diagnostic feature is the neocortex of the brain, which no other vertebrate has.

Main groups

Almost all mammals give birth to live young. There are only two mammals that lay eggs, called Monotremes, the Duck-billed Platypus Ornithorhynchus, and the Spiny Anteater Echidna, with four species. All are confined to Australia and New Guinea, and are the sole survivors of an earlier group of mammals. However, like other mammals, they feed milk to their young, and protect and look after them.

Other mammals are divided into the Marsupials and the Eutheria, the placental mammals. Marsupials are mammals with pouches to carry young in, like the kangaroo. All other mammals are Eutherian.

Modes of life

Mammals are found on land and in water, and also in the air, where they compete with lots of other animals. Their ability to move from place to place and adapt has made them a most efficient group. Many mammals live in cold places. These mammals have thick hair or blubber to keep them warm. Others may live in rainforests. Some live in deserts, and still others are in rivers, lakes, and seas all around the world.

Sea-going mammals, the Cetacea and the pinnipeds, are very successful and significant predators. This includes the whales, seals, walrus, dolphins and others. In the air, the bats (Chiroptera) are the mammalian order with the most species. They 'own' the nighttime, since birds are largely diurnal (daytime) animals.

On land the rodents (rats, mice) are hugely successful, more common in numbers than any other mammals. Large mammals on land have been hunted to extinction in many parts of the world. The ones which remain are now being better protected.

Last, but certainly not least, are the primates. Their natural habitat, with few exceptions are the forests. Most live in the trees, with hands that grasp, good colour vision, and intelligence. In the Pliocene period some moved out onto the savannahs as grassland began to replace forests.

Taxonomy

The evolutionary relationships among land vertebrates is as follows:

• Tetrapoda (land vertebrates)
  • Amphibia (lay eggs in water)
  • Amniota (lay cleidoic eggs)
    • Sauropsida (all true 'reptiles')
    • Synapsida (all 'mammal-like amniotes')
      • Pelycosauria
      • Therapsida
        • Mammalia (bear live young; milk; raise their young etc.)

This sort of classification is not traditional, but it does reflect our knowledge of palaeontology and evolution.

Standardized textbook classification

A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. It is based on living animals. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. 2000.

Class Mammalia

• Subclass Prototheria: monotremes: platypuses and echidnas
• Subclass Theria: live-bearing mammals
  • Infraclass Metatheria: marsupials
  • Infraclass Eutheria: placentals

List of orders

Over ¾ of mammal species are in the orders Rodentia (blue), Chiroptera (red), Soricomorpha (yellow) and Primates (green).

Mammals can be divided in a number of orders:

• Monotremes (monotremata)
• Marsupials (marsupialia)
• Eutheria
  • Superorder Xenarthra
    • Order Pilosa
    • Order Cingulata
  • Superorder Afrotheria
    • Order Proboscidea
  • Superorder Laurasiatheria
    • Insectivora (no longer used)
    • Order Eulipotyphyla
    • Order Chiroptera
    • Order Carnivora
    • Order Perissodactyla
    • Order Artiodactyla
    • Order Cetacea
  • Superorder Euarchontoglires
    • Order Rodentia
    • Order Lagomorpha
    • Order Primates

Images

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  Polar bear

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  Bat

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  Camel

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  Manatee

Related pages

• Mammalia (taxonomy)