Hippopotamuses are large, semi-aquatic mammals known for spending much of their day submerged in water. Their skin secretes a distinctive fluid that plays a fundamental role in their survival and health.
The Unique Secretion’s Appearance
A hippo’s skin secretes a substance that undergoes a remarkable color transformation. Initially clear and viscous, it rapidly changes to a reddish-orange hue upon exposure to air, appearing as if the hippo is “sweating blood.” This striking visual phenomenon has historically led to misconceptions. Over several hours, the reddish-orange color deepens, eventually turning brownish as the compounds within the secretion polymerize.
Unpacking the “Sweat” Misconception
The hippo’s secretion is not true sweat, nor is it blood. Unlike humans, hippos do not possess eccrine sweat glands that produce watery perspiration for cooling. Instead, this oily, viscous fluid is produced by specialized subdermal glands located beneath the hippo’s skin. These glands release the fluid through visible skin pores, creating a protective coating.
Essential Roles of the Secretion
This unique secretion provides multiple benefits for the hippo’s well-being and survival. It acts as a natural sunscreen, absorbing harmful ultraviolet (UV) radiation across a broad spectrum, from 200 to 600 nanometers. This protection is important as hippos spend extensive periods under intense sunlight. The fluid also functions as an antiseptic, preventing bacterial infections. This helps wounds heal, especially since hippos often sustain injuries from territorial fights in unclean aquatic environments.
The secretion serves as a moisturizer, keeping the hippo’s thick, sensitive skin hydrated and preventing it from drying out and cracking when the animal is out of water. While not primarily for thermoregulation, it contributes to overall skin health by providing a protective, moisturizing layer. Researchers also theorize it might act as an insect repellent, as flies tend to avoid landing on the fluid.
The Chemistry of Hippo Secretion
The distinctive color and functional properties of the hippo’s secretion are attributed to specific organic pigments. Scientists have identified two primary compounds: hipposudoric acid (responsible for the red coloration) and norhipposudoric acid (imparting an orange hue). These pigments are non-benzenoid aromatic compounds that are notably acidic.
Both hipposudoric acid and norhipposudoric acid are highly effective at absorbing UV light. The red pigment, hipposudoric acid, also possesses significant antibiotic activity, capable of inhibiting the growth of pathogenic bacteria such as Pseudomonas aeruginosa and Klebsiella pneumoniae, even at low concentrations. These compounds are initially unstable, but when they dry on the hippo’s skin in the presence of mucus, they polymerize into more stable, colored solids, which helps them adhere and provide lasting protection. The precise mechanism by which hippos synthesize these pigments is still under investigation, though it is hypothesized they are produced from precursors like the amino acid tyrosine.