I still can't fucking get over how cetaceans adapted to spend their entire lives in the water and just never bothered to redevelop water-breathing. there's motherfuckers who spend an hour or more diving and the evolutionary solution is just "breathe a lot on the surface and then lower your heart rate to a near-hibernation level while actively cruising the seafloor for stuff to eat". totally insane solution to one of the oldest solved problems in biology
Well, there are some good reasons for that.
First, foremost, and Steve Alten be damned, one cannot simply "re-evolve" gills. All the bits and bobs that used to be gills in our water-breathing ancestors are jaw and ear bones now. If some kinda mammal wanted to reacquire the ability to breathe water, it'd have to be from scratch. Evolution is a cheap-ass, so as long as there's a perfectly good set of lungs on that fella, it's not gonna bother to make an entirely new lung-alternative.
Secondly - and psych, this is actually more important - compared to air, water fukken sucks as an oxygen carrier! At its highest, water's potential for holding dissolved oxygen maxes out at 14.6mg/L...for fresh water...at 0°C. Saltwater carries about 7-8mg/L of dissolved oxygen. Good ol' air, meanwhile, holds a whopping 21mg/L of oxygen! Compared to water, that's pure jet fuel! It's just more efficient to go up to the surface for a breath of air and hold it.
TL;DR, mammals don't have the tools to evolve the ability to breathe water and no incentive to do so in the first place.
its weird to me that dolphins and great white sharks have like, not totally dissimilar sizes and speeds when they're getting the oxygen for those muscles in a completely different way.
Drag, I guess.
Yeah: Sharks are just so smooth that, even with the more inefficient breathing method, they can still maintain similar speeds and sizes u_u u_u