The hippopotamus, a massive semi-aquatic mammal, spends a significant portion of its life in water. Despite their immense size, these animals move with surprising efficiency in their watery habitats. Their aquatic lifestyle involves physical traits and behaviors that facilitate survival.
Hippos in the Water: How Fast Do They Move?
Hippos do not truly swim; instead, their bodies are too dense to float, so they sink. They move through water by pushing off the bottom of riverbeds or lakes, effectively walking or galloping along the submerged terrain. An adult hippopotamus can achieve speeds of approximately 8 kilometers per hour (5 miles per hour) underwater. This speed is comparable to, or even faster than, the average human swimmer, who typically moves at about 1.5 to 2 meters per second (3.3 to 4.5 miles per hour). While their land speed can be much higher, reaching up to 30 km/h (19 mph) over short distances, their unique aquatic locomotion allows them to navigate their watery domain effectively despite their inability to float.
Physical Traits for Aquatic Prowess
The hippopotamus possesses several physical adaptations for aquatic movement. Their skeletal structure features exceptionally dense bones, known as pachyostosis, which allows them to sink. This density, combined with a low center of gravity, helps them maintain stability and walk along the bottom. Their short, powerful legs provide propulsion to push off the riverbed, enabling surprising underwater agility.
They have slightly webbed feet with four splayed toes that help distribute their weight and provide thrust during underwater movements. Their eyes, ears, and nostrils are positioned high on their heads, allowing them to remain almost entirely submerged while still seeing, hearing, and breathing. When fully submerged, their nostrils and ears can close to prevent water entry, and a clear membrane protects their eyes, enabling underwater vision. Adults can hold their breath for up to five minutes, supporting extended periods underwater.
Life in the Water: More Than Just Swimming
Spending most of their day submerged in water is fundamental to a hippopotamus’s life, extending beyond movement. This aquatic residency is primarily thermoregulatory, as water helps them manage their body temperature under the intense African sun. Their skin is sensitive and prone to drying out, cracking, and sunburn if exposed to the sun for prolonged periods.
To protect their sensitive skin, hippos secrete a unique reddish-orange fluid often called “blood sweat.” This fluid, not actual sweat or blood, contains two acidic pigments, hipposudoric acid (red) and norhipposudoric acid (orange), that act as natural sunscreen by absorbing ultraviolet light. It also functions as a moisturizer and possesses antibiotic properties, helping prevent skin infections. Beyond thermoregulation and skin protection, water provides refuge from predators and serves as a location for mating and giving birth. Hippos typically emerge from water at dusk to graze on land, returning to their watery sanctuary before the heat of the day intensifies.