The question of whether hippos are purple stems from the animal’s unique and often misunderstood skin coloration. The hippopotamus is not purple, but the confusion arises from a remarkable biological process. Their skin secretes a reddish-orange fluid that often gives them a temporary, vibrant hue mistaken for purple or pink.
Actual Hippo Coloration
The true skin color of the Hippopotamus amphibius is a shade of grayish-brown or dark slate. This coloration provides excellent camouflage when the animal is submerged in the muddy waters of its habitat in sub-Saharan Africa. The skin is nearly hairless, a trait that helps them move easily through the water without becoming waterlogged.
The underlying skin is surprisingly sensitive despite its incredible thickness, which can reach up to two inches. The underside of the hippo is a lighter, brownish-pink color, particularly around the belly, eyes, and ears. Since hippos spend most hot daylight hours submerged, their skin lacks traditional sweat glands and is highly susceptible to drying out and sunburn when they are on land.
The Myth of Purple and “Blood Sweat”
The visual phenomenon leading to the purple or pink myth is an oily, thick secretion covering the hippo’s body. When the hippo emerges from the water, this initially colorless liquid quickly changes color upon contact with the air.
This secretion is neither sweat nor blood, but its striking, reddish-orange appearance earned it the incorrect nickname of “blood sweat.” The ancient Greeks and early naturalists believed this discharge was blood. The color can look pink, bright red, or purplish when mixed with water and mud, explaining the persistent misconceptions about the animal’s true coloring.
The Science of Hippo Sunscreen
The red-orange fluid is a complex natural substance secreted by specialized subdermal glands beneath the hippo’s skin. It is composed primarily of two distinct, highly acidic, pigmented compounds: red hipposudoric acid and orange norhipposudoric acid. Both compounds are synthesized by the animal’s body, likely from precursors like the amino acid tyrosine.
These pigments serve multiple functions for the hippo’s sensitive skin. Both the red and orange pigments are effective at absorbing ultraviolet light, providing a natural sunscreen that protects against the harsh sun. Their light-absorption profile peaks directly in the ultraviolet range, which is important for an animal vulnerable to sun damage and cracking when dry.
The secretion also functions as a powerful moisturizer, helping to keep the hippo’s skin supple and preventing it from cracking during prolonged periods out of the water. Additionally, the red pigment, hipposudoric acid, possesses strong antibiotic and antimicrobial properties. This protective feature helps to inhibit the growth of disease-causing bacteria, which is particularly important for an animal that frequently sustains wounds from fighting and spends its life in muddy, bacteria-rich environments.