Pretty much all vertebrates yawn, says Rachel Metz, a curator at the Smithsonian’s National Zoo. Among marine mammals, pinnipeds, such as seals, do—but cetaceans, such as whales and dolphins, do not. They differ from other mammals in that they breathe through blowholes instead of noses or mouths, and they are conscious breathers, meaning they have to remember to breathe, even while sleeping. As for why mammals yawn, scientists still aren’t sure why—it could be sleepiness, boredom, a need to oxygenate the brain or some other cause.