Hippos are undeniably aquatic, spending the majority of their day submerged in water, but their method of movement is fundamentally different from a typical swimmer. Unlike many semi-aquatic animals that rely on buoyancy and stroking, the hippo’s physical structure dictates a unique form of underwater travel. The way they navigate their aquatic home challenges the conventional definition of swimming. This distinction highlights the specialized adaptations that allow them to thrive in the rivers and lakes of sub-Saharan Africa.
Aquatic Locomotion: The Underwater Walk
The key to understanding hippo movement lies in their bone density, which makes them non-buoyant. Their skeletal structure is graviportal, adapted to carrying an enormous weight, and their bones contain dense tissue, acting like a built-in ballast. This increased density allows an adult hippo, which can weigh up to 8,000 pounds, to sink rather than float freely at the surface.
The primary mode of travel for a hippo in water is a technique described as “bottom-walking.” They use their powerful, short legs to push off the riverbed or lake floor, effectively walking, trotting, or even bounding along the bottom. This allows them to move with surprising speed and agility despite their size, using the water’s buoyancy to reduce the burden of their weight.
When in deeper water where the bottom is unreachable, the hippo employs a series of porpoise-like leaps or high, prancing steps, often with periods of “flight” where no feet are touching the ground. This locomotion is facilitated by their dense body mass and the water’s lift. They can hold their breath for an average of five minutes, which facilitates these long stretches of underwater movement before they briefly surface for air.
Physical Adaptations for Submersion
The hippo’s body is specialized for its semi-aquatic existence, featuring sensory organs strategically placed for life in the water. Their eyes, ears, and nostrils are all located on the very top of their head. This anatomical arrangement allows the animal to remain almost entirely submerged, keeping its massive body cool and protected, while still being able to see, hear, and breathe.
The ears and nostrils are equipped with specialized valves that automatically close when the hippo submerges, creating a watertight seal. A clear membrane covers and protects their eyes while they are underwater, allowing them to maintain vision below the surface. The four toes on each foot are partially webbed, which assists their unique locomotion by providing better purchase for pushing off the ground or navigating slippery banks.
The dense bone structure and low center of gravity also serve as a stabilizer, ensuring that even when moving along the bottom, the hippo maintains balance and does not roll. This heavy, compact build, combined with the sensory adaptations, allows them to spend up to 16 hours a day in the water with minimal disruption to their vital functions.
The Role of Water in Hippo Survival
Water is not merely a habitat for the hippopotamus; it is necessary for regulating its body temperature and protecting its sensitive skin. Hippos lack sweat glands and quickly overheat when exposed to the intense African sun for prolonged periods. The water acts as a massive heat sink, drawing excess heat from their core and keeping their temperature stable.
Their skin is vulnerable to rapid dehydration and sunburn, making daytime submersion essential for survival. When they do leave the water, their skin secretes a thick, reddish-orange substance often referred to as “blood sweat,” though it is neither blood nor sweat. This secretion is a natural sunscreen and moisturizer that also possesses antiseptic properties, which helps prevent infection in their thin, hairless skin.
The hippo’s physiological vulnerability to heat and sun dictates their strict behavioral patterns. They emerge from the water only after sunset to graze on grasses for several hours, and then return to the water before the heat of the next day begins. This cycle ensures they remain cool, moist, and protected, demonstrating the water’s fundamental role in their existence.