Eli5 How did mammals end up in the ocean?

Question

I’m watching dolphins pop out of the water to breathe, and it just seems absurd that animals that live in water need to breathe air.

In: 9

Answer 1

The closest land relative of dolphins is the hippopotamus. It’s a giant cow-like animal that eats plants (mostly grass) at night and sleeps in shallow water during the day.

Sleeping in water seems to be mainly an adaptation for living in extremely hot environments: it’s a way of keeping cool despite the hippo’s bulk. (A side benefit is that it helps protect the hippo from parasites and predators while it sleeps.)