Unlike most other aquatic animals, dolphins do not have gills because they are not fishes but cetaceans. Gills in fishes are adapted to help them breath while underwater, the gills function by absorbing dissolved oxygen in water, and replacing this with carbon dioxide – an excretory waste.
Since dolphins are not fishes but mammals, and have lungs; how then do they manage to take in oxygen, more so when they are asleep? According to an article by teenkidsnews.com, the explanation goes thus:
As mammals, they have to get to the surface to breathe air periodically. For example, the bottlenosed dolphin can only hold their breath for seven minutes or so before they have to come up for air.
So while sleeping, dolphins let one hemisphere of their brains nod off while the other half keeps an eye out for trouble. Literally, if the left brain is sleeping, the right eye stays open and vice versa.
This way, they always know when it’s time to surface and breathe and when to duck into safety from predators. They seem to switch off every two hours or so until they get a full eight hours a day of sleep.
