Eli5 the difference in shark and whale tails

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So why do sharks have the tail that goes up and down but whales have the tail that goes side to side. Is there an actual reason for it or is something that just kinda happened?

In: Biology
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It’s the other way round, sharks and other fish have the tail that goes side to side, whales have the tail that goes up and down. Because whales are mammals and they have evolved from surface animals which find it easier to swim by waving their rear end up and down (humans can swim the same way).

Whales are mammals with a backbone, sharks are made from cartilage their genetic backgrounds are completely different, with whales being land animals which returned to the sea.

Evolution.

When animals evolved to live on land their skeletons evolved for this when they evolved legs. The spine needed to move up and down for walking on land rather than the side to side motion fish have as up and down is way better suited for walking on land than side to side motion.

Whales are animals that returned to the sea. They have kept the spine movement of up and down and so that’s how their tail works.

Evolution doesn’t plan and moving up and down works well enough so it seems there hasn’t been any pressure to change it back to how fish use it.

Edit:

As to why fish evolved side to side in the first place? Probably just chance. Either would have been fine. There doesn’t seem to be much of an advantage of one over the other.

Whales are mammals, and our mammal bodies have a spine that articulates (moves) up and down. Try wiggling side-to-side like a fish in water. You’ll drown. They had to compensate with what evolution already gave them, so a tail to flattens out horizontally will provide the most propulsion while accommodating for the spine to move up and down like any other mammal.

Sharks don’t have this problem, and side to side is actually more efficient in water. Because they never moved on land, they move side to side when swimming too be able to use their entire body to move.

Interestingly whales sleep unihemispherically – meaning that only half their brain (either the left or right) goes to sleep while the other half is still fully conscious and controlling half the body’s muscles.

Possibly the difference in the tail and basic swimming motion means that sharks are unable to use this form of sleep.