It is true that mammals are most adapted among vertebrates. Following are some of the adaptions shown by mammals:
(i) Mammals can be found in almost all types of habitat, e.g. in mountains, deserts, grasslands, plains and even in aquatic habitat.
(ii) Mammals have two pairs of limbs which help in running, jumping, walking, flying, climbing and burrowing.
(iii) Mammalian skin is equipped with sweat glands which help in osmoregulation and thermoregulation.
(iv) Hairs on skin provide insulation.
(v) External ears help in catching sound waves in efficient way.
(vi) Four-chambered heart facilitates double circulation which is more efficient than single circulation.
(vii) They are warm blooded animals which means they can control their body temperature.
(viii) Sexual dimorphism is well developed which is an important tool for survival. 
(ix) Most of the mammals are viviparous. This helps in longer life span of mammals.