Why can amphibians and reptiles stay underwater longer than birds and most mammals?

Mammals like humans are not very good at staying underwater but whales and porpoises are remarkable at staying under water. Equally some birds like cormorants and penguins are well adapted to being underwater and can stay there for a long time.

There are several reasons why amphibians and reptiles can stay underwater for a long time. One is because they are poikilotherms which means that they do not waste energy trying to keep warm. This will save a lot of oxygen. They may also be adapted to be able to have high concentrations of waste materials like carbonic acid and lactic acid in their blood without having any ill-effects. Most animals that go underwater also have strategies where metabolic function slows down. As this is temperature dependent and amphibians and reptiles can survive getting much colder they can slow right down and conserve energy. Some reptiles are also adapted to have wrinkly skin through which they can absorb oxygen from the water that can mean they don't need to go to the surface very much at all.