Why do marine mammals sway their tails vertically, while fish sway them horizontally?
In: Biology
It’s a somewhat remnant of their ancient ancestors. Fish evolved from animals that slithered on the sea floor so an undulating movement helped them move, the fishes side to side moving tail is suited to it.
Marine mammals evolved from land mammals whos legs were underneath them and had an unstable gait to allow rapid direction changes. To extend their stride they flexed their spines up and down, so when they evolved to be fully aquatic the horizontail tail shape came with it.
“It reflects the evolutionary history of locomotion. Our wormy ancestors slithered on the sea floor, so undulated side-to-side. Fish inherited that movement, for which a vertical tail is best.
Their distant land mammal descendants evolved to run with limbs underneath: an unstable gait allowing rapid direction changes. To extend the stride, their spine flexes up-and-down. Marine mammals kept this movement, for which a horizontal tail is best.”
Source:
https://www.sciencefocus.com/nature/why-do-fish-have-vertical-tail-fins-and-whales-have-horizontal-ones/
Fish have tails oriented 90 degrees from marine mammal tails. Therefore the sway is 90degrees
I’m not one of those people who talk about missing links but in this case, I’d really like to see the transitionary species between the land version and the sea version of the aquatic mammals
Convergent evolution is the evolution of similar features in different species over different times (epochs). Eyesight, powered flight, etc.
A vertical tail or fin is more suitable for horizontal propulsion, and vice versa. Mammals need to climb up and down for air so horizontal tail is more optimal. Marine animals’ locomotion is adapted to their needs and habitat e.g. any direction movement by squid, octopus, jellyfish using jet propulsion.
More on animal movement in water [here.](https://en.wikipedia.org/wiki/Aquatic_locomotion)
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).
horizontal spine flexing is the original way all animals with spines moved. that’s causes a wriggle that allows fish to swim through water. then some fish went to land and developed limbs and an upright stance. Lizards, newts and salamanders still do the wriggle, but mammals run differently because their legs are directly unter the body. Their bodies don’t drag on the surface, their gait and limbs can be much larger and they can support more body weight that way. And the spine developed to flex vertically because that assists running as as extra joint that can contrac and extend, increasing opower and gait length. and when they went back into the water they kept it that way because it worked just fine.
Thinking about adaptation rather than evolution, a vertically moving tail would allow you to cover more depth like in an ocean, while in a shallower river you would need to maneuver side to side.
Mammals that live in the sea evolved from land mammals, which means that they are similar to land mammals. You are a land mammal. If you tied your legs together and laid on your stomach, would it be easier for you to move them side to side or up and down? Up and down, right? Since water mammals have similar bones to you, it’s also easier for them to move up and down.
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