The hippo's skin secretes a mucous-like substance that is red-orange in colour. Initially, this led people to believe that they sweat blood, but this secretion is neither sweat nor blood.
This substance starts out colourless, but within minutes it turns red-orange and will eventually turn brown. It can absorb light and this light absorption capability is maximised in sunlight's ultraviolet range. This then creates a sunscreen effect.
If the animal stays out of the water too long, this natural sunscreen will dry and their skin may crack. No wonder then that they'll spend up to 16 hours a day in water to stay cool.
Like hippos, we also need sun protection. Unlike hippos, we cannot secrete our own and we need a little help from a different source.
When choosing your sunscreen, make sure to select one that provides excellent broad-spectrum protection (UVA and UVB) and that it contains moisturising and anti-oxidant ingredients.