Question

Why are whales counted in mammals

Answer 1

because they have a nose and breathe air and give birth

Answer 2

Whales are mammals. There are four characteristics of mammals that make them different from fish and other animals.

Mammals are endothermic also known as warm-blooded. In order to survive, the need to produce heat through metabolism.
 
Fish lay eggs but Whales give birth. They also nurse their young ones.

Whales breathe oxygen from air through their nostrils, which are on the top of their heads called blowholes. 

Whales have hair follicles.