Many people wonder if hippos are truly pink, a common misconception. While hippos may appear to have a reddish or pinkish hue, their inherent skin color is not pink. This perception stems from a unique biological phenomenon: a specialized skin secretion that changes color upon exposure to air and plays a significant role in their survival.
Actual Skin Color
A hippopotamus’s skin is typically grayish-brown, purplish-gray, or blue-black, often with lighter, brownish-pink areas on their underside and around their eyes and ears. Their thick skin, up to 6 centimeters (2.4 inches) thick, has very little hair. Environmental factors, such as being covered in mud or water, can temporarily alter their appearance. Despite their robust appearance, hippo skin is sensitive to drying and sunburn when exposed for extended periods.
The Reddish Secretion
The reddish appearance comes from a unique, oily fluid secreted from special mucous glands in their skin. This substance is neither blood nor true sweat, despite its common nickname, “blood sweat.” When initially secreted, the fluid is colorless, but it rapidly changes to a red-orange color upon contact with air. Over several hours, this secretion can gradually turn a darker brown as it polymerizes.
This fluid contains two primary pigments: red hipposudoric acid and orange norhipposudoric acid. These acidic compounds are unstable on their own but become more stable when mixed with mucus, remaining on the hippo’s skin for hours. Japanese researchers identified these compounds, noting their surprising acidity. The color change results from these molecules reacting in the air and linking into longer, colored polymer chains.
Protective Roles of the Secretion
The reddish secretion serves several important functions for the hippo, acting as a natural multi-purpose skin protectant. Both hipposudoric and norhipposudoric acids absorb ultraviolet (UV) radiation, providing effective sun protection. This natural sunscreen is important because hippos spend many hours exposed to direct sunlight, often up to 16 hours a day.
Beyond sun protection, the secretion also possesses antiseptic and antibiotic properties. Hipposudoric acid, even in low concentrations, can inhibit the growth of certain pathogenic bacteria, such as Pseudomonas aeruginosa and Klebsiella pneumonia. This antibacterial action helps prevent infections from cuts and scrapes that hippos frequently acquire. The fluid’s sticky, oily nature also helps keep their skin cool and moist when out of water, assisting in thermoregulation.