Hippopotamuses secrete a reddish oily fluid often described as \"blood sweat\", but the secretion is neither blood nor typical sweat; it contains two pigments, hipposudoric acid and nor‑hipposudoric acid, that protect the skin from sun damage, help prevent infection and aid hydration.
Overview
Hippos produce a sticky, oily secretion from specialised skin glands that initially appears colourless and then oxidises to orange‑red and brown tones. Historically misunderstood as bleeding, this secretion plays several physiological roles that are important for animals that spend long periods in sunlit, aquatic and muddy environments.
Chemical components
The secretion contains two notable pigments: hipposudoric acid, which gives a red pigment, and nor‑hipposudoric acid, which tends toward orange or brown. These organic compounds act both as pigments and as biologically active molecules on the skin surface.
Functions
The secreted acids serve multiple functions: they absorb and block some ultraviolet radiation acting like a natural sunscreen; they have antimicrobial properties that reduce the risk of skin infections in moist environments; and their oily nature helps retain moisture and limit excessive water loss through the thick hippo hide.
Mechanism of action
Hipposudoric and nor‑hipposudoric acids interact with the skin surface to form a protective film. Their chemical structure allows them to absorb certain wavelengths of light and to disrupt or inhibit growth of some microbes, while the lipid component improves skin barrier function and reduces desiccation.
Not blood
Although the secretion looks like blood, it originates from skin glands rather than blood vessels and serves adaptive protective roles rather than signalling injury; experiments and chemical analyses confirm its distinct composition and biological functions.
Quick related facts
- Appearance: initially clear, then red‑orange to brown on exposure to air
- Key chemicals: hipposudoric acid; nor‑hipposudoric acid
- Functions: natural sunscreen; antimicrobial action; skin hydration
- Source: specialised skin glands, not sweat glands or blood vessels
- Adaptive context: protects skin during long periods in sun, water and mud