Why do marine mammals sway their tails vertically, while fish sway them horizontally?
In: Biology
Mammals have to go up and down regularly to breath air at the surface and then get back to wherever they like to spend time.
At the risk of overgeneralizing, most fish through most of their evolutionary history have spent their lives at more or less the same depth so that movement in the plane was at least as important. A lot of non-mammals do cycle to and from the surface too, though, including vast quantities of phytoplankton and all the fish that eat them (directly or one or two steps away on the food web).
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