Many people who observe hippos might notice a reddish, oily substance on their skin, leading to a common misconception that these large mammals are “sweating blood.” This belief has roots in ancient times, with early European discoverers in Africa even coining the term “blood sweat” due to its striking resemblance to blood, especially when exposed to sunlight. However, despite the visual similarity, hippos do not sweat blood. The reddish fluid is not blood, nor is it produced by typical sweat glands. Instead, this unique secretion serves several important biological purposes for the hippopotamus.
The Nature of Hippo Secretions
The reddish substance seen on hippos is a distinctive oily secretion produced by specialized subdermal glands located beneath their skin, not traditional sweat glands. When this fluid initially emerges from the hippo’s pores, it is clear and colorless. However, upon exposure to air and sunlight, it gradually changes color. This transformation occurs rapidly, turning red-orange within minutes and eventually polymerizing into a brownish hue as it dries.
The color change is attributed to two unique, highly acidic pigments identified within the secretion: a red pigment called hipposudoric acid and an orange pigment called norhipposudoric acid. These pigments are unstable on their own but maintain their color for several hours when mixed with mucus on the hippo’s skin before polymerizing further.
The Secretion’s Protective Functions
This unique secretion provides hippos with multiple layers of protection, important for an animal that spends significant time both submerged in water and exposed to the African sun. One of its primary roles is to act as a natural sunscreen. Both the red hipposudoric acid and orange norhipposudoric acid pigments absorb ultraviolet (UV) radiation across a broad spectrum, shielding the hippo’s sensitive skin from harmful sun damage. This built-in UV protection helps prevent sunburn on their largely hairless skin, which is susceptible to sun exposure.
The oily nature of the secretion also serves as a moisturizer, preventing the hippo’s skin from drying out and cracking when they are out of the water. Hippos have thick skin, but it is highly sensitive and requires constant moisture.
Beyond sun protection and moisturizing, the red pigment, hipposudoric acid, possesses strong antibacterial properties. This inhibits the growth of bacteria, aiding wound healing and preventing infections, which is particularly beneficial given that hippos frequently sustain injuries from territorial conflicts and spend time in muddy water environments.