Strange Facts About Not So Weird AnimalsThe third largest land mammal in the world, the hippopotamus (smaller only to the elephant and rhinoceros), spends a good deal of time in the water and adults are able to remain in it without breathing for about 25 to 30 minutes! Also peculiar is that these large beasts can actually sleep underwater and need not be conscious to keep breathing or to come up for air. The breathing process is automatic so that an underwater sleeping hippopotamus will rise to the surface to intake air and go back down under without waking up. They are also one of the few mammals to birth under water. Famous for sticking parts of their heads above water while the rest is under, hippos are able to launch sounds that travel simultaneously in water and in air to communicate with other hippopotamus.