• Fertilisation is internal.
• Except egg-laying monotremes, mammals are mostly
viviparous giving birth to young ones.
• Vivipary enables the mammalian mother to remain mobile
while incubating embryos, an advantage over birds.
• Development takes place in the uterus of the female. Embryo
has amnion, allantois and chorion. A placenta fixes the
foetus to the uterine wall for nourishment, respiration and
excretion.
• Development is direct.
• Females have mammary glands with teats
producing milk for nourishing the young
ones. They show high degree of
parental care.
• The body temperature
is regulated (homeothermy). This
gives the mammals a high rate
of metabolism and makes them
active animals like birds.
• Mammals, like birds, are
endothermic. Endothermy enables these
animals to live in extreme temperatures that
other land vertebrates cannot bear.
reasons for mammals being most
successful and dominant
• Presence of an insulating and protective hairy
exoskeleton.
• These are homeothermals and endothermic animals.
• Presence of well-developed brain.
• These are viviparous animals and show high degree of
parental care.
• These have more developed hearing efficiency.
• Ability of speech and grasping power due to thumb
opposability to fingers.CLASSIfICAtIon
• Mammals have been thoroughly described and adequately
classified. They include approximately 5,000 living species
(15,000 subspecies) and numerous fossil forms.
• The main characters forming the basis of their classification
into orders include : (i) mode of caring for their young
ones, (ii) nature of dentition, (iii) foot posture, (iv) nails,
claws and hoofs, (v) complexity of nervous system, and (vi)
systematics.
• G. G. Simpson provided a complete review of the group in a
publication entitled “The Principles of Classification and a
Classification of Mammals”.• A muscular partition, called diaphragm, separates the
anterior thoracic cavity from the posterior abdominal cavity.
• The mouth is relatively small having movable lips.
• A secondary palate separate the respiratory passage from
the food passage.
• Buccal cavity has true salivary glands.
• Teeth occur in both the jaws. They develop in sockets
(thecodont); grow in two sets: temporary or milk teeth
and permanent teeth (diphyodont); and are of four types:
incisors, canines, premolars and molars (heterodont).
• Alimentary canal terminates by anus,
there being no cloaca.
• The coelom is divided into four
cavities; a pericardial cavity
lodging the heart, two pleural
cavities each containing the
lung and an abdominal cavity
having the rest of viscera.
• Respiration occurs only by lungs.
The lungs are spongy and elastic. Rib
muscles and diaphragm play a role in
breathing. Glottis is guarded by epiglottis.
• The heart is 4-chambered, having two auricles and two
ventricles. It supplies oxygenated blood to all parts of the
body. There is neither sinus venosus nor truncus arteriosus.
Renal portal system is lacking.
• Red blood corpuscles are circular, biconcave and
denucleated except camel, llamas where RBCs are oval,
flat and nucleated.
• Only left systemic arch persist.
• The kidneys are metanephric. The cloaca is absent except in
monotremes (egg laying mammals). Ureters open in urinary
bladder.
• Excretion is ureotelic.
• The brain has large cerebrum and cerebellum. Optic lobes
are divided into four lobes called corpora quadrigemina.
Corpus callosum connects the two cerebral hemispheres
internally. Grey matter is more developed.
• 12 pairs of cranial nerves are present.
• Eyes have movable lids and are well developed.
• Ear has three regions: internal, middle and external. Internal
ear has an organ of Corti. Middle ear has 3 bony ear ossicles
(malleus, incus and stapes). External ear generally has a
large, fleshy pinna.
• Sexes are often distinguishable externally. Sexual
dimorphism is well marked.
• The testes usually descend into scrotal sacs in the adult.
Male has a copulatory organ, the penis.