The Distinctive Secretion and Its Hue
Hippos are large semi-aquatic mammals, often observed partially submerged in water. This behavior helps them manage body temperature under the African sun. Their unique physiology includes a fascinating skin secretion that has long intrigued observers. This substance is not ordinary sweat, but a specialized fluid that plays a significant role in their survival. Many have wondered about the unusual reddish appearance of this secretion, which has historically led to misconceptions.
The fluid released from a hippo’s skin is not conventional sweat, as hippos lack typical sweat glands. Instead, this unique substance originates from subdermal glands. When initially secreted, this fluid is clear and colorless, with a viscous, somewhat oily consistency.
Upon exposure to air and sunlight, a rapid chemical transformation occurs. The clear fluid changes color within minutes, developing a reddish-orange hue. This can also appear brownish or pinkish depending on concentration and environmental factors. This visible color change results from pigments within the secretion reacting with oxygen. Over time, as these pigments oxidize and polymerize, the coating on the hippo’s skin can gradually darken, eventually turning dark brown.
Biological Role and Pigment Chemistry
The distinctive color of the hippo’s secretion comes from two primary pigments: hipposudoric acid (red) and norhipposudoric acid (orange). These non-benzenoid aromatic compounds are highly acidic and synthesized by the hippo’s body, likely from precursors like tyrosine. Researchers found these pigments unstable when isolated, but they maintain their color on the hippo’s skin for several hours due to other components in the mucus.
This remarkable secretion serves multiple functions. Both hipposudoric acid and norhipposudoric acid absorb ultraviolet (UV) radiation, acting as a natural sunscreen. This protects the hippo’s sensitive skin from intense sun. Hippos have relatively thin skin prone to sunburn and drying out when exposed to air for extended periods.
Beyond sun protection, the secretion also possesses antiseptic and antibiotic properties. The red pigment, hipposudoric acid, inhibits the growth of certain bacteria, helping prevent infections in cuts and wounds acquired in aquatic environments. This antimicrobial action helps maintain skin health. The oily nature of the secretion also moisturizes the skin, preventing cracking and drying when the animal is out of water.