Which of the following are unique features of mammals?
The unique features of mammals are the presence of mammary glands and hairs on the skin.
Homeothermy is found in aves also.
Four-chambered heart is also found in crocodiles and aves.
Ureotelism is also found in amphibians and cartilaginous fishes apart from some invertebrates like earthworms.
Metanephric kidney is the functional adult kidney of all amniotes (reptiles, birds and mammals)
Viviparity is also seen in many other animals groups such as Chondrichthyes, Reptilia, etc.
Development is direct in all amniotes and many other animals.