Hippopotamuses, or hippos, are large semi-aquatic mammals found in African rivers and lakes. Despite their frequent presence in water, their aquatic movements differ significantly from conventional swimming.
How Hippos Move in Water
Hippos do not truly “swim” like other aquatic mammals. Instead, they move through water primarily by interacting with the riverbed. Their dense bodies and heavy bones prevent them from floating naturally, causing them to sink.
In both shallow and deep water, hippos primarily walk or gallop along the bottom, navigating submerged terrains with agility. When their feet cannot reach the bottom, they push off the submerged surface with their powerful legs, performing porpoise-like leaps to propel themselves forward. This thrust creates the illusion of swimming as they briefly glide before sinking to push off again. The water supports their immense weight, making their underwater locomotion resemble movement in a microgravity environment.
Hippos control buoyancy by adjusting their breath, allowing them to sink, stand, or rise for air. An adult can hold its breath for three to five minutes, or longer. Even while sleeping underwater, a reflex allows them to automatically surface, breathe, and sink back down without waking.
Unique Physical Adaptations
Several physical characteristics enable hippos to thrive in aquatic habitats and facilitate their unique movement. Their eyes, ears, and nostrils are positioned high on their heads, allowing them to see, hear, and breathe while most of their body remains submerged. This placement helps them stay hidden while aware of their surroundings.
When submerged, hippos can close their nostrils and fold their ears shut to prevent water entry. A clear membrane covers their eyes, functioning like goggles to protect them while allowing underwater vision. Their exceptionally dense bones contribute to negative buoyancy, making it easy to sink and walk on the riverbed. This dense structure acts as a ballast, aiding stabilization when submerged.
A hippo’s skin is thick, sensitive, and nearly hairless. To protect it, they secrete a reddish, oily substance often called “blood sweat,” though it contains neither blood nor actual sweat. This secretion, composed of hipposudoric and norhipposudoric acids, acts as a natural sunscreen, moisturizer, and may possess antibiotic properties, protecting their skin from sunburn, drying, and infections.
Why Hippos Love the Water
Hippos spend most of their day, typically 16 hours, submerged in water, which is essential for their survival. Their large bodies are prone to overheating, and water provides an effective cooling mechanism during hot African days. This thermoregulation is a primary reason for their aquatic lifestyle.
Water also protects their sensitive skin from the sun’s harsh rays. Without regular submersion, their skin can quickly dry and crack. Water acts as a natural barrier, complementing their skin’s protective secretions.
Beyond physical comfort, water offers refuge from land-based predators. While adult hippos have few natural enemies, water provides a safe haven, especially for young calves. Much of their social behavior, including mating and birthing, occurs in the water. Females give birth in water or on land; if born underwater, the mother assists the calf to the surface to breathe. Hippos are social animals, often forming groups called “pods” or “bloats” in the water, ranging from a few individuals to over a hundred.