Although whales live in water, they are a class of mammals. Compared with terrestrial mammals, their living environment is different. However, as a group of mammals, whales share some key characteristics with terrestrial mammals. For example, from the way they breathe, they breathe with their lungs, they can not sink deep in the water to breathe, but can only periodically breathe with their heads above the water. In terms of body temperature, their body temperature is constant. From the point of view of reproductive methods, they use viviparous reproduction, but also lactation. These characteristics are the outstanding characteristics of whales as mammals.
Whales and fish live in the same environment, and the body shape of whales is very similar to that of fish, so many people will think that whales are also a kind of fish. However, they are not actually fish, as mentioned above, they are mammals. Although their body structure is similar to that of fish, this is only a convergence phenomenon in the long-term evolutionary process. When whales lived on land before, they did not have similar structures. When environmental changes brought them into the water, they gradually evolved similar characteristics.
The reason why whales are not fish is that whales are very different from fish in some key characteristics. For example, fish use gills to breathe and can breathe without leaving the water, but whales, like most mammals, use lungs to breathe. In addition, fish mostly use oviparous way of reproduction, unlike whales using viviparous way of reproduction.
