Why do sea mammals move their tailfin up & down while fish move theirs side-to-side?

Question

I'm starting to wonder whether these mammals returned to the sea from the land. And whether man's affinity with water and these creatures could undermine Darwinian theories of evolution with respect to humankind.

Answer 1

Their just showing off.

Answer 2

I think Darwin undermined his own theories when he mentioned we descended from apes. This question will cause him to re-think.

Answer 3

Maybe evolution is a crutch used by scientists to allow them to reject God.

I personally believe God created these animals just the way they are.  To date, I haven't seen any professional swimmers children start to develop webbed arms/legs, and I don't think I ever will.

Answer 4

their bodies are just constructed that way

Answer 5

because fish can only swim 1 way and mammals can swin different ways

Answer 6

the answers obvious---because they regularly need to go to the surface to breathe air---in other words --they swim up and down more than left and right----if they just swam at one depth the vertical (fish) tail would be more useful---and that would have 'evolved'

Answer 7

Is it because their spines are near the top whereas fish have spines in the middle of their bodies.

Answer 8

It's to do with their spinal columns.

Sea mammals are descended from land animals and when a four footed animal walks or runs the spine flexes up and down. Flexing side to side would be pointless (No porpoise to it ;-) )

Try this your self. Next time you go swimming try to move like a dolphin and then like a fish. Your spine is not designed to move like a fish.

So any land mammal returning to the water is more likely to evolve a tail that works with the spines natural movement.

Answer 9

If you look at how the pelvic bones of mammals are shaped, you'll see it is much more natural for their fins to lay horizontal.   If a mammals tailfin was like a fish's (veritcal) it would be like twisting your hip at a whole uncomfortably 90 degrees in order to swim.  Just imagine if you were to develop fins.  Your feet would become the fin, and it would be much more natural to swim moving them up and down than side to side.  The bone structure inside of the tailfin, you will see there are phalanges (similar to our fingers) supporting this evidence.

I believe that they left the land for the sea.

Answer 10

Yes, all marine mammals are descended from land-dwelling ancestors. It doesn't undermine darwinian evolution at all. It makes perfect sense that, since most of the planet is covered in water, animals would evolve to take advantage of that habitat!

Answer 11

The difference between mammals and other types of animals is sometimes made out to be more simple than it actually is. Perhaps the vertical tailfin movement is one of the few attributes that determines a mammal sea creature. On the other hand, the common opinion of a mammal is live birth, but there are a handful of sharks that give birth to live babies while they are fish.

Answer 12

It's because of the orientation of the tail fins.

Most fish fins are vertical (i.e. up & down) so they need to move it back and forth to generate movement via resistance.

The dolphin and other mammals tail fin is horizontal (i.e. side to side like the spoiler on a car).  They need to move it up and down to generate movement via resistance.

Take a piece of paper and hold it flat (parallel to the ground)...wave it back and forth...not much resistance.  Wave it up and down...all of a sudden you've got resistance (and you're moving air).  Same principle.

Answer 13

because they have lungs fish breath in the water

Answer 14

because mammals are cool like that. =]

Answer 15

It's something they learned in their SCHOOLING.