This Baby Hippo Is Adorable, Even Though it Looks Naked
Hippos lack hair for much the same reason that other aquatic and semi-aquatic mammals do; fat is a more useful insulator for large animals that spend most of their time in the water, and by implication, the hair might not be necessary for their survival. Therefore, this lack of hair does not make hippos vulnerable to the sun.
Instead, they secrete a light-absorbing substance that acts as a natural sunscreen. They need this because they do not have the glands that produce an oily substance known as sebum that other mammals rely on for skin lubrication.