Why do whales tail up and down and sharks tail left and right?

Whales, dolphins and sharks were the earliest large marine animals. When we see a big guy swimming in the distance on the sea, how can we know whether it is a fierce great white shark or a docile dolphin? It's simple. The tail is a shark and the tail is a dolphin. When a photographer takes an aerial photograph, how can he distinguish sharks from dolphins at once? It's also very simple. Sharks wag their tails left and right, and dolphins wag their tails up and down. Why are sharks so different from cetaceans?

The shark's tail stands high.

Two different spinal structures

Although whales and dolphins now have streamlined bodies like fish, they are mammals. The vertebrate structure of mammals is completely different from that of fish. Their spines mainly move up and down, while fish spines swing from side to side. The bone in the tail is the coccyx, which is a part of the spine and is consistent with the movement direction of the whole spine. So mammals, including whales, mainly swing their tails up and down, while fish swing their tails left and right.

The tail of cetaceans is horizontal and divided into two leaves.

Why is there such a difference? Because mammals live on land, although whales are now in the sea, their ancestors are also on land. The first thing on land is to stand up, so have legs. But animals' legs can't move their bodies like table legs, so they need joints to connect their legs with their bodies, and then rotate in a certain direction to push their bodies forward.

Mammals should move forward and swing their limbs back and forth. If the spine swings from side to side, the spine movement and limb movement are not on the same plane, and the whole body will be stiff; But it is very convenient if it is designed to swing up and down. So mammals finally chose the spine that swings up and down, which is swinging back and forth for humans who walk upright. We should thank our ancestors for this great evolution, otherwise we will have to cross the aisle like crabs.

Bottlenose dolphin, grey dolphin, pike.

Fish have lived in water for 500 million years since their birth, but they have never lived on land. In water, fish can swim easily without legs, so the fish never changes its spine and keeps its original left-right swing.

We pay attention to observation and find that there are significant differences between fish and mammals. The fish's spine is stiff from the side, while the mammal's spine has a certain curvature and looks elastic.

Marin skeleton

In addition, the ribs used by mammals to support body cavities are connected to the left and right sides of the spine, while the spinous processes (that is, fishbones that we all hate) that support the body of fish grow up and down from the spine, and they all avoid the direction of the spine swinging. The tail of a fish is not only vertical, but the whole body is vertical and flat, while the body of a whale is more round.

Mammals all evolved from fish (fish → amphibians → reptiles → mammals), but about 50 million years ago, when their ancestors decided to return to the ocean, they had been out of water for 250 million years. Their body structure has been highly adapted to land life and cannot be changed back to the appearance of fish ancestors. Therefore, they have to adjust and transform the existing structure, the forelimbs are specialized into flippers, the useless hind limbs degenerate, and the tails become heavy and powerful.

Bones of bottlenose dolphins

Limited by the structure of the spine, whales can't swing from side to side like fish. They find it easier to swing up and down and swim in the waves, so whales choose this sport. Other marine mammals, including seals, sea lions and manatees, swim in this way, but whales are the most adaptable to water, and their tails are particularly developed and eye-catching.

Seals also have a horizontal tail, which swings up and down when swimming.

Advantages and disadvantages of two kinds of tails

Which is more efficient, the "horizontal tail" of whales and dolphins or the "vertical tail" of fish? Furthermore, which is better, the up-and-down movement of the spine or the left-and-right movement? This problem needs to be distinguished. On land, the advantages of spine moving up and down are obvious, because animals move faster and have stronger sports ability in this way.

Crocodiles, lizards and snakes in the existing terrestrial vertebrates all swing from side to side and can only crawl on their bellies. The spine of higher animals, birds and mammals mainly moves up and down. Quadrupeds originated from fish and initially swayed from side to side. However, the new forms of sports that appeared later quickly took the lead. For more than 200 million years since dinosaurs, animals with upper and lower spines have ruled the earth. Some people think that the revolutionary progress of dinosaurs in the way of movement is one of their advantages in gaining dominance.

Blue marlin rushed out of the water.

But in the water, the upper and lower tails and the left and right tails have no obvious advantages. The facts are in front of us, and no one has excluded anyone from these two tails. Fish has ruled the ocean for 500 million years, and it is still prosperous today. Although whales have only been in the water for tens of millions of years, their adaptation to the ocean is undoubtedly excellent. At present, they have occupied all the major ecological niches of the ocean, including the blue whale, one of the largest animals in history, and the killer whale, the dominant marine whale today.

Whales and fish are excellent at swimming and diving, which proves that both horizontal and vertical tails can adapt well to aquatic life. Whales breathe with their lungs and must breathe oxygen in the air, so their diving ability is not as good as that of fish. Aerobic exercise mainly focuses on lung breathing, so the explosive force and sprint speed of whales are not as good as those of fish, but the cruising speed is better. These differences have nothing to do with the shape and movement of the tail.

The blue whale is not only the largest creature, but also the fastest cruising speed.

Finally, it should be declared that this article is about the main direction of spine movement, not that the spines of these animals can only move in one direction. For example, the snake's spine mainly swings from side to side, but it also has the ability to move up and down. Our human spine can bend left and right to a certain extent, but it is obviously not as strong as the ability to move back and forth. Everyone sees cats and dogs wagging their tails from side to side every day. In fact, the range of their tails swinging from side to side is limited, but the upper and lower hem can reach nearly 180 degrees.