Hippos mostly try to avoid direct sunlight by lying in water during the day and feeding at night. The unique skin of the hippo, consists of a thin epidermis and a thick dermis with dense subcutaneous and fat tissue. It is very sensitive to both drying and sunburn.
No sebaceous or sweat glands exist, but large, subdermal mucous glands secrete a thick, oily red fluid—the so called “blood sweat” or "red sweat.” This viscous alkaline liquid is secreted over
their face and back. The colorless sweat (secretion) turns red within a few minutes, then gradually becomes brown polymers. This acts as a natural sunscreen and also protects the skin from becoming waterlogged when a hippo is in the water.
The pigments responsible for the color have been described as hipposudoric acid (the red pigment) and norhipposudoric acid (the orange pigment). The red pigment has been shown to have significant bacteriocidal activity by inhibiting the growth of pathogenic bacteria such as Pseudomonas aeruginosa and Klebsiella pneumonia.
Fowler’s Zoo and Wild Animal Medicine, Volume 8
Chapter 59 - Hippopotamidae (Hippopotamus)
Chris Walzer Gabrielle Stalder