The hippopotamus (Hippopotamus amphibius) is a large semi-aquatic mammal, spending up to 16 hours a day submerged in rivers and lakes to protect its sensitive skin from the African sun. Despite their aquatic lifestyle, hippos are air-breathing mammals who must surface to survive. While healthy adults possess extraordinary adaptations for underwater existence, they can still drown if they are incapacitated, trapped, or prevented from reaching the surface.
Hippo Locomotion: Why They Don’t Need to Swim
Adult hippos possess negative buoyancy, meaning their body density is greater than that of water. This is due to osteosclerosis, a skeletal condition where bone tissue is denser than in most other mammals. These heavy bones act as ballast, allowing the animal to sink and remain stable on the riverbed without expending significant energy. Thus, a hippo does not swim using propulsive strokes.
Instead, the hippo moves through the water by walking, trotting, or “prancing” along the bottom of the lake or river. They use their powerful, short legs to push off the substrate in a slow-motion gallop. They control their descent and ascent by regulating respiration, exhaling air from their lungs to increase their density and sink more quickly. This specialized locomotion allows them to navigate shallow and deep waters with ease.
Specialized Physiological Adaptations for Submergence
The hippo’s anatomy is finely tuned for prolonged submergence while maintaining awareness of its surroundings. Their eyes, ears, and nostrils are positioned high on the top of their massive head, often referred to as a nictitating plane. This arrangement allows the hippo to remain almost completely submerged, gaining the benefit of the water’s cooling effect while still being able to breathe, see, and hear above the waterline.
For full submergence, the hippo has evolved muscular sphincters that automatically seal off its nostrils and ears. A healthy adult hippo can hold its breath for an average of three to five minutes, sufficient for routine movements along the riverbed. This controlled apnea is a voluntary action when the animal is awake, but an involuntary reflex takes over when the hippo is resting.
This involuntary surfacing mechanism is why adult hippos rarely drown, even while sleeping. The animal can sleep completely submerged; when oxygen levels drop, a reflex causes it to automatically ascend to the surface, take a breath, and sink back down without waking up. This automatic respiratory control ensures the animal’s need for air is met without interrupting its rest cycle.
The Specific Risk of Drowning for Hippo Calves
While the adult hippo is well-protected by its reflexes and physical structure, drowning is a significant cause of mortality for the young. Hippo calves are born weighing between 50 and 100 pounds, and they are not initially proficient swimmers, despite their natural affinity for the water. Newborns are relatively buoyant compared to adults, and they lack the muscular control and lung capacity necessary for prolonged, independent submergence.
Newborn calves can only hold their breath for approximately 40 seconds at a time, making them highly dependent on their mothers for survival in the water. If a calf is born fully submerged, the mother must immediately guide or push it to the surface for its first breath. The young calf often nurses underwater, requiring the mother to assist in bringing it up to breathe frequently throughout the day.
This vulnerability means that young hippos are highly susceptible to drowning when separated from the protective care of their mother. Strong river currents can easily overwhelm a calf, and they may lack the strength to fight the flow or climb back to the surface. Furthermore, territorial disputes or acts of infanticide by aggressive adult males can result in the calf being bitten, crushed, or held underwater, leading to a forced drowning that is a consequence of social conflict.