system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do sea mammals move their tails up and down while fish move it side to side?

Is there an evolutionary advantage to either one of them?
gpt
There's no "advantage" it's just different.

They're different because in a sea mammal's evolutionary history they were quadrupedal land animals. If you watch a dog or other land mammal run, you'll see how their spine arches up and down. A whale's tail swinging up and down is just an evolutionary tweak on the same movement they used before they adapted to the water.