Despite looking round and slow, hippos are excellent swimmers and can hold their breath underwater for up to five minutes. They can also close their nostrils like little valves, which is very handy when you spend most of your life in a river. Baby hippos are born underwater and can swim before they can walk properly on land.
Hippos have a natural sunscreen - they produce a reddish, oily liquid from their skin that acts like a moisturiser and sun-blocker combined. Scientists think this liquid also helps prevent infections from river bacteria. For a long time people thought hippos were sweating blood, which sounds dramatic but is not true at all!
In Gambia, hippos are found in the quieter, more upstream stretches of the river and in the River Gambia National Park - also known as Baboon Islands. Boat trips at dawn and dusk give the best chance of spotting them, since hippos are most active when it is cooler. You often hear them before you see them: their calls are a sort of booming laugh.
Hippos are very important for the river's health. Their dung fertilises the water, which feeds tiny creatures, which feed fish, which feed birds and people. Scientists call this the 'hippo nutrient pump'.