Bats and whales are classified as mammal because they possess the essential mammalian characteristics like presence of milk gland or mammae, presence of pinnae, viviparous nature, hairs on body, presence of diaphragm etc. Though bats and whales have four chambered heart, it cannot be considered as the main characteristic of being mammals because it is also present in some reptiles (e.g. crocodile) and all birds.