Why do marine mammals sway their tails vertically, while fish sway them horizontally?

Question

Why do marine mammals sway their tails vertically, while fish sway them horizontally?

In: Biology

Answer 1

Marine mammals sway their tails vertically because their tail fins are horizontal. Fish sway their tails horizontally because their tail fins are vertical. They get far more thrust by swaying their tails perpendicular to their tail fins than they would if they swayed them parallel to their tail fins.