The Hippopotamus amphibius, the world’s third-largest land mammal, spends up to sixteen hours a day submerged in African rivers and lakes. Like all mammals, the hippo relies entirely on lungs to extract oxygen from the air and cannot breathe underwater. The misconception that hippos can breathe beneath the surface is common, but the truth lies in their specialized ability to hold their breath and utilize physical and behavioral tools that allow them to remain submerged for extended periods.
The Truth About Hippo Respiration
Hippos must regularly surface to inhale air since they cannot process oxygen from water. An adult hippo typically holds its breath for three to five minutes while submerged, which is sufficient for patrolling territory or moving along the riverbed. This breath-holding capacity is a highly developed physiological skill, but this time limit necessitates a return to the surface for oxygen.
The mechanism that allows hippos to spend time in the water, even while asleep, is a powerful involuntary reflex. When resting or sleeping underwater, the hippo’s body automatically triggers a reaction to rise to the surface for air without the animal consciously waking up. The hippo bobs upward, takes a breath, and then descends back into the water while remaining asleep. This reliable reflex ensures the animal never drowns and acts as a built-in survival mechanism.
Specialized Physical Adaptations for Submersion
The hippo’s anatomy is specifically structured for constant submersion. Its eyes, ears, and nostrils are positioned high on the dorsal surface of the head. This “periscope” arrangement allows the hippo to remain almost entirely underwater, keeping its massive body concealed while still being able to see, hear, and breathe.
The nostrils and ears are equipped with specialized muscular valves, or sphincters, that automatically seal shut when the animal submerges its head. This reflexive closing prevents water from flooding the respiratory passages and ear canals during a dive.
The hippo’s body composition further aids its underwater existence. Hippos possess an extremely dense bone structure and lack significant body fat, resulting in negative buoyancy. This density allows the hippo to sink and remain on the bottom of the river or lake. This trait is a major factor in their underwater travel, as they do not need to fight their natural tendency to sink.
Hippo Behavior in the Water
The hippo’s body density dictates its primary mode of underwater movement. Instead of true swimming, hippos typically walk, trot, or “gallop” along the riverbed. Their powerful legs push off the bottom, allowing them to move quickly and efficiently through the water.
Hippos spend the majority of their daylight hours submerged for thermoregulation and skin protection. The water prevents the large animal from overheating under the African sun. Their skin is highly susceptible to drying out and cracking when exposed to the air for too long.
To combat dehydration, hippos secrete a reddish, oily fluid often referred to as “blood sweat,” which acts as a natural moisturizer and sunblock. This fluid offers protection when they are out of the water to graze at night.