The hippopotamus is one of Africa’s most recognizable animals, often pictured submerged in water with only its eyes and nostrils visible. This semi-aquatic lifestyle leads to the common assumption that these massive animals are powerful swimmers. The surprising reality is that despite spending up to 16 hours a day in rivers and lakes, the hippo is not a conventional swimmer. Their unique physiology and density mean they use a grounded method of moving through their watery habitat.
The Truth About Hippo Locomotion
Hippos are unable to float naturally because their bones are extraordinarily dense, acting as a ballast that causes them to sink. The hippo’s skeleton, particularly the limbs, exhibits osteosclerosis, meaning porous bone has been replaced with compact bone tissue. This high bone density and the lack of insulating fat prevent them from floating freely on the water’s surface. Instead of swimming, the hippo utilizes the riverbed or lake bottom for propulsion.
The preferred method of movement is a unique form of “walking,” “bouncing,” or “galloping” along the ground underwater. They use their powerful, short legs to push off the substrate, achieving a gait compared to a reduced-gravity gallop. This movement allows them to glide through the water in extended intervals, almost like an astronaut taking large strides. This “hippo hop” can propel them at speeds of up to 8 kilometers per hour (5 mph), making them fast aquatic movers.
While a hippo can paddle for short distances in water too deep to touch the bottom, their primary and most efficient mode of travel relies on maintaining contact with the ground. This grounded locomotion plays a role in keeping slow-moving waterways open. Their movements help maintain channels and minimize the effects of siltation.
Specialized Aquatic Adaptations
The hippo’s body is specifically adapted to facilitate this semi-aquatic life. The sensory organs—eyes, ears, and nostrils—are positioned high on the skull. This arrangement allows the animal to remain almost entirely submerged while still being able to see, hear, and breathe on the surface. This adaptation is functionally similar to a periscope, enabling them to monitor their surroundings without exposing their massive bodies.
When fully submerged, the hippo’s nostrils automatically close, and a clear membrane covers their eyes, functioning like built-in goggles for underwater sight. The animal can hold its breath for extended periods, typically around five minutes, though some individuals stay underwater longer. They can even sleep submerged, using a reflex action to automatically rise, take a breath, and sink back down without waking.
Their feet also show adaptations suited for their environment. Hippos have four toes on each foot, connected by a slight webbing. This webbing does not function as a flipper to aid swimming. Instead, it helps distribute the hippo’s immense weight, providing better traction and preventing the animal from sinking too deeply into the soft, muddy river and lake bottoms.
The Purpose of Aquatic Life
The primary reason hippos spend so much time submerged is for thermoregulation and skin protection. They lack functional sweat glands and have sensitive, nearly hairless skin that is highly susceptible to drying out and cracking when exposed to the sun. Staying in the water keeps their skin moist and helps regulate their body temperature in the hot African climate.
When hippos leave the water, their skin secretes a thick, oily, reddish-orange fluid mistakenly called “blood sweat.” This substance is neither blood nor sweat but a mixture of two acidic pigments, hipposudoric acid and norhipposudoric acid, which serve multiple functions. These pigments absorb ultraviolet radiation, acting as a natural sunscreen.
The reddish pigment also possesses antibacterial properties, inhibiting bacteria growth and helping to heal wounds sustained from territorial fights. This natural, protective lotion only works when the skin is damp, requiring the hippo to return to the water multiple times a day. Being submerged also serves as a defensive measure, offering a refuge from land-based predators.