Some salient features of class Mammalia

Natural history:

  • Mammals are primarily terrestrial vertebrates but few like whales and dolphins are aquatic.
  • They are the most dominant animals on earth and have the capacity to learn because of their better developed brain.
  • Mammals have evolved from Therapsid (mammals-like) reptiles some 220 million years ago in the Triassic period.


  • There are two pairs of pentadactyl limbs (each foot bears 5 or fewer toes provided with horny claws, nails or hooves).
  • They are variously adapted for walking, running, jumping, climbing, burrowing, swimming, flying etc.


  • The skin is glandular (sweat glands, sebum glands etc.) and mostly covered by a horny epidermal exoskeleton of hair and fur.
  • The hair on the skin and adipose tissue beneath the skin conserve body heat.
  • Mammary glands are well developed in females that produce milk to suckle the young ones.

Digestive system:

  • The mouth is relatively small and has movable lips.
  • Buccal cavity has true salivary glands.
  • Teeth occur in both the jaws and they are thecodont (developed in sockets) and heterodont (different types of teeth growing in two sets).
  • Tongue is mobile.
  • Distinct liver and pancreas are present.
  • Alimentary canal is complete and opens by anus.

Respiratory system:

  • Respiration occurs only by lungs which are elastic and spongy.
  • They are enclosed in the pleural cavities.
  • Rib muscles and diaphragm (a muscular partition separating thoracic cavity from abdominal cavity) play a role in breathing.
  • Glottis is guarded by epiglottis.

Circulatory system:

  • The circulatory system is closed.
  • The heart is four chambered with two auricles and two ventricles.
  • Heart pumps oxygenated blood to different parts of the body.
  • Hepatic portal system is present but renal portal system is
  • RBCS are circular, biconcave and non-nucleated most species.
  • They are homeothermic (warm-blooded), which gives them high rate of metabolism and makes them active.

Nervous system:

  • The brain has large cerebrum and cerebellum.
  • There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.
  • Optic lobes are divided into four bodies; corpora quadrigemina.
  • Corpus callosum connects the two cerebral hemispheres.

Sense organs:

  • The olfactory sacs open by the internal nares far back into the pharynx.
  • Eyes have movable lids.
  • Internal ear has an organ of corti, middle ear has 3 bony ear osciles (malleus, incus and stapes) and external ear generally has a large, fleshy pinna.

Excretory system:

  • Two bean shaped kidneys are present which areĀ metanephric.
  • They are ureotelic (excrete urea).
  • Ureters open into the urinary bladder and urine is fluid.

Reproductive system:

  • Sexes are often distinguishable externally (sexual dimorphism).
  • The testes usually descend into scrotal sacs in the adult.
  • Male has a copulatory organ called penis.
  • Gonoducts lead directly to the exterior and fertilization is internal.
  • Mammals are mostly viviparous (directly give birth to the young ones).
  • Development occurs in the uterus of the female. Embryo has amnion, allantois and chorion.
  • A placenta fixes the fetus to the uterine wall for nourishment, respiration and excretion.
  • Young ones are nourished with milk for some time after birth and are brought up with great love and care.