In fact, although a whale looks like a fish, it is not a fish, but a mammal, similar to the situation that a bat looks like a bird but not a bird, and it is also a real mammal. Unlike fish, whales are viviparous. Generally speaking, they have a baby every two years. The cubs are fed by their mothers' milk.

Fish are oviparous and can lay thousands of eggs at a time. Once hatched, young fish can live independently without breastfeeding. The whale's body temperature is constant, with an average of 35.5℃, which is maintained in cold water and tropical waters. Fish is a temperature-changing animal, and its body temperature changes with the change of environmental temperature.

Whales breathe with their lungs. They often need to surface to breathe air. On the other hand, fish can always stay underwater by absorbing oxygen dissolved in water through gills. Whales and fish are similar in appearance because they have lived in the same living environment for a long time, which is a "convergence phenomenon".

Characteristics of whales:

Cetaceans are characterized by constant body temperature, about 35.4℃. Bare skin, no body hair, only a few bristles on the tail, no sweat glands and sebaceous glands. The fat under the skin is very thick, which can keep the body temperature and reduce the proportion of the body in water. The skull is developed, but the skull is smaller, the face is larger, the frontal bone and maxilla are obviously prolonged, forming a long snout.

The neck is not obvious, the cervical vertebra is healed, and the head is directly connected with the trunk. The forelimbs are fin-shaped, the toes are not separated, there are no claws, and the elbow and wrist joints cannot move flexibly, which is suitable for swimming in the water. The hind limbs are degenerated, but there are still pelvic and femoral residues, and residual bone fragments can be seen.

The tail degenerates into fins, and the skin at the end expands horizontally from left to right, forming a pair of large tail leaves without bone support. The spine gradually tapers in the long and narrow caudal trunk and finally disappears before entering the caudal fin. Unlike fish, the caudal fin can swing up and down and is the main organ for swimming. Some species also have dorsal fins to balance their bodies.

Their bones have spongy tissue, and there is more fat in the body cavity, which can increase the volume of the body, reduce the proportion of the body and increase buoyancy.