The mammals are the most successful and dominant animals today. They thrive very well in most environment of the world - polar ice caps, deserts, oceans, mountains, forests, grasslands and dark-caves. Man (a mammal) dominates the world. Unique mammalian characteristics are :
(a) Body covered with hair.
(b) Presence of sebaceous and sweat glands in skin.
(c) Presence of mammary glands in females.
(d) Presence of a pair of external ears (pinnae) and three ear ossicles except prototheria.
(e) Teeth are the thecodont, diphyodont and heterodont.
(f) The heart is four chambered.
(g) Presence of only left aortic arch.
(h) The red blood corpuscles (erythrocytes) are biconcave and enucleated.
(i) Thyroid cartilage in larynx present.
(j) Optic lobes form corpora quadrigemina.
(k) Corpus callosum unites two cerebral hemispheres.
(l) Testes extra-abdominal in scrotal sacs.
(m) Presence of penis for copulation in males.
(n) Mostly viviparous and embryo attached to the uterine wall by placenta. Prototheria are oviparous only.