The question of whether a hippopotamus has gills is common, stemming from the fact that these massive creatures spend up to 18 hours a day submerged in water. Despite their highly aquatic lifestyle, the answer is definitively no; hippos do not possess gills. As members of the class Mammalia, they are air-breathing vertebrates that must surface to take in atmospheric oxygen. This semi-aquatic existence, where they retreat to water bodies like rivers and lakes during the hot African day, creates the illusion of a creature adapted for underwater breathing.
Why Hippos Need Lungs
Hippopotamuses are classified as mammals, placing them in a biological group that uses lungs to process oxygen from the air. Like all mammals, hippos rely on a diaphragm and chest cavity movements to draw oxygen-rich air into their lungs. This reliance on atmospheric air means they cannot extract dissolved oxygen directly from the water, which is the function of gills in fish. Gills are intricate organs that require water to flow over them constantly for gas exchange, a mechanism completely absent in the hippo’s anatomy. Pumping water through a mammal’s lung structure would be an impossible and energy-inefficient task, underscoring why hippos must return to the surface for every breath.
Physical Adaptations for Submersion
While lacking gills, the hippo has developed remarkable physical features that maximize the time they can spend submerged for thermoregulation and skin hydration. Their eyes, ears, and nostrils are positioned high on the dome of their head, an arrangement known as a nuchal crest. This allows a hippo to remain almost entirely underwater while still maintaining awareness of its surroundings, seeing, hearing, and breathing with minimal exposure. When fully submerging, the hippo can actively seal its nostrils and ears shut using muscular valves to prevent water from entering these passages.
Adult hippos possess a large lung capacity and can typically hold their breath for about five minutes, though some individuals may extend this to seven or eight minutes. Their massive, dense bone structure also plays a role in their aquatic behavior, as it makes them negatively buoyant. They do not float or swim in the traditional sense but rather walk or push off the bottom of the riverbed, which conserves energy during submerged movements. This ability to sink and walk helps them stay anchored and hidden beneath the surface during the day.
The Involuntary Breathing Mechanism
Perhaps the most fascinating adaptation related to their breathing is the involuntary reflex that allows hippos to rest and even sleep completely submerged. When a hippo is resting or sleeping underwater, its body automatically manages the need for air without waking the animal. This is accomplished through a subconscious mechanism that monitors blood oxygen levels.
As the hippo’s oxygen reserves deplete, a reflex is triggered that causes the immense animal to rise to the surface. It will take a breath, often a loud exhalation followed by a quick inhalation, and then immediately sink back down into the water. This entire process occurs without conscious thought, allowing the hippo to maintain a continuous state of rest, only momentarily breaking the surface every few minutes to replenish its oxygen supply.