When in the water, Common Hippos lift their slit-like nostrils up on the surface to breathe, at intervals of up to 6 min. A reflex response ensures that the nostrils and ears are closed as soon as they come into contact with water. By lifting the head above the water level, the ears, eyes, and nostrils are allowing for visual, acoustic, and olfactory perception.
Because of the insulating properties of their thick skin, heat loss in the water is greatly reduced, the skin functioning as a ‘diving suit’. This, however, is an obvious disadvantage when moving on land as it reduces metabolic heat dissipation. Therefore, Common Hippos undertake long excursions to their grazing grounds during the cooler night hours.
Although Common Hippos spend much time in the water, they can neither float nor swim – at least there is no convincing evidence that they do in freshwater.
They can even sink to the bottom fully inhaled and exhaled underwater or when they come out for the next breath. This can easily be observed in the wild. Depending on the depth of the water, the animals lie in sternal recumbency on the bottom, completely under or with just the tops of their backs showing; in deeper water, they stand on all fours, and in even deeper water they stand on their hind legs.
To reach the surface, they either just lift the head or push themselves off the bottom. Shallow water for the adults is deep water for the babies able to stand on their hind legs, straighten up to breathe, or sit on their mother’s back apparently floating like a cork when the mother is submerged.
It is quite possible that Common Hippos are able to swim in seawater, and that would explain their former appearance on Madagascar, Cyprus, and Malta as well as on islands off the African coast. As the sea levels were lower during the Pleistocene than at present, the distances to cross would have been way less.
It is still a remarkable achievement for Common Hippos not only to have reached Madagascar but also for establishing viable populations there for tens of thousands of years. This achievement is even more impressive when one realizes that none of the many other ungulate species of Africa that are good to excellent swimmers ever managed to make it across to Madagascar.