Hippos, like all mammals, cannot breathe underwater and require air. Their remarkable capacity to remain submerged for extended periods, often leading to the misconception they can breathe submerged, is supported by unique adaptations that allow them to thrive in watery habitats, despite needing to periodically surface for air.
Remarkable Physical Adaptations
Hippos exhibit several distinct physical features that facilitate their semi-aquatic existence. Their eyes, ears, and nostrils are positioned high on their heads, allowing them to see, hear, and breathe while most of their massive bodies remain submerged. A clear membrane also covers their eyes, providing protection underwater while still allowing them to see.
Their skeletal structure includes dense bones, which contribute significantly to their weight and enable them to sink to the riverbed. This density allows them to walk or push off the bottom rather than expending energy to stay submerged. Hippos also possess unique skin that lacks true sweat glands and is highly sensitive to the sun. To combat this, their skin secretes a reddish, oily substance, often called “blood sweat,” which acts as a natural sunscreen and moisturizer, and is believed to have antiseptic properties.
Mastering Underwater Survival
Hippos employ both physiological and behavioral strategies to manage their time beneath the surface. Adult hippos can hold their breath for an impressive five minutes or longer. This breath-holding ability is aided by a diving reflex that significantly slows their heart rate from approximately 90 beats per minute to about 20 beats per minute when submerged. This reduction in heart rate conserves oxygen, allowing for extended dives.
Their nostrils and ears have muscular sphincters that automatically close upon submersion, preventing water from entering. Hippos typically move by walking or galloping along the bottom of rivers and lakes, using their powerful legs to push off the ground. They can even sleep underwater, relying on an automatic reflex that brings them to the surface to breathe without waking before sinking back down.
The Importance of Water for Hippos
Water is an indispensable element for the survival of hippos, dictating much of their daily routine. Spending most of their daylight hours submerged helps them regulate their body temperature, protecting them from the intense African sun and preventing overheating. Their sensitive skin, which can quickly dehydrate and crack if exposed to air for too long, necessitates constant moisture, which the water provides.
The buoyancy offered by water also supports their immense body weight, reducing stress on their joints and making movement less strenuous than on land. For young hippos, the aquatic environment offers a degree of protection from land-based predators, as mothers often give birth and nurse their calves in the water. This reliance on water underscores why hippos are so well-adapted to their semi-aquatic habitats.