The common hippopotamus is a large, semi-aquatic mammal recognized for its massive body and nearly hairless appearance. While they lack the dense, insulating coat seen on many other mammals, hippos do have hair, though it is extremely sparse and cannot be accurately described as fur. This minimal body covering is an adaptation to their lifestyle, which requires constant immersion in water to manage body temperature and maintain the integrity of their uniquely sensitive skin.
Hair Distribution and Function
Hippo hair is characterized as stiff, scattered bristles rather than a thick pelt. These strands are distributed across the body, with the highest concentration found on the muzzle, tail, and around the ears. The bristles on the muzzle function as vibrissae (whiskers), which are important sensory tools that help the hippo navigate and forage for grass. The stiff hairs on the tail assist the animal in sweeping or spreading its dung as a territorial marking behavior.
The rarity of hair across the hippo’s body surface is an evolutionary feature. Unlike terrestrial mammals that rely on fur for insulation, the hippo’s enormous size and semi-aquatic habits mean a thick coat would be counterproductive. Fur would soak up water, making the animal heavy and potentially leading to overheating or rapid chilling. Their near-naked skin allows for efficient heat exchange with the surrounding water, helping them regulate body temperature.
How Hippo Skin Manages Water
The hippo’s skin is incredibly thick, reaching up to 2 inches (6 centimeters) in places, yet it is highly vulnerable to dehydration. This thick dermal layer, which can account for up to 18% of the animal’s total body weight, provides protection but does not prevent water loss. The outer epidermal layer is thin and prone to cracking or bleeding if exposed to air and sun for too long.
Hippos do not possess the functional sweat glands that many other mammals use for evaporative cooling. This absence necessitates their semi-aquatic lifestyle, requiring them to spend most of the day submerged in water or mud. Remaining submerged prevents their sensitive skin from drying out and overheating under the intense sun. Prolonged exposure to dry air or intense sunlight can result in painful fissures and severe sunburn, making hydration a constant biological imperative for survival.
The Natural Sunscreen Secretion
To manage periods when they must be on land, hippos secrete a unique, reddish-orange fluid from subdermal glands, often mistakenly called “blood sweat.” This viscous fluid is neither blood nor true sweat, but a complex mixture of highly acidic compounds. Researchers have identified the two primary pigments as hipposudoric acid (red) and norhipposudoric acid (orange).
The secretion serves a dual function that is vital for the hippo’s exposed skin. Both pigments absorb ultraviolet light, providing an effective, broad-spectrum natural sunscreen to protect the skin from sun damage. The hipposudoric acid also exhibits strong antibiotic properties, which helps inhibit the growth of pathogenic bacteria. This antimicrobial action prevents infections from cuts and abrasions the hippo sustains in its aggressive social environment. The fluid also contains mucus, which helps it stick to the skin, providing a moisturizing layer that counteracts drying effects.