Hippopotamuses are semi-aquatic mammals, known for their immense size and spending much of their day submerged in water. Native to sub-Saharan Africa, they are among the largest terrestrial mammals. Their unique adaptations allow them to thrive in both aquatic and terrestrial environments, raising questions about how they rest.
The Underwater Sleep Secret
Hippos can sleep underwater without drowning. This is possible due to an involuntary reflex that allows them to surface, breathe, and submerge again without fully waking. Their nostrils and ears are equipped with muscular valves that automatically close when they dive, preventing water from entering. Adult hippos can hold their breath for four to six minutes, or even longer. While sleeping, their bodies instinctively rise to the surface for air, take a breath, and gently sink back down, a subconscious behavior that ensures they continue to respire even in a deep sleep state.
Land vs. Water Sleep
Hippos exhibit distinct sleeping patterns depending on their environment. During the day, they primarily rest in water, which helps regulate their body temperature, protects sensitive skin from the intense African sun and dehydration, and its buoyancy supports their immense body weight, reducing leg stress. While in water, hippos may sleep in shallow areas with nostrils just above the surface, or they can fully submerge, relying on their automatic surfacing reflex. At night, hippos typically leave the water to graze on land for several hours. They can also sleep on land, often choosing spots near the water’s edge, lying down with their legs folded underneath them; this land sleep usually occurs during the day, as their nocturnal hours are dedicated to foraging.
Calf Sleep Behavior
Hippo calves display specific sleep behaviors adapted to their early life in water. Newborn calves hold their breath for about 40 seconds, much shorter than adults (several minutes) or older young hippos (two to three minutes). When born underwater, the mother immediately pushes the calf to the surface for its first breath. Young calves often rely on their mothers for support and protection while resting in the water. Mother hippos are highly protective of their offspring, staying with their newborns for several days without grazing; nursing can even occur underwater, demonstrating the calves’ early aquatic proficiency, and in deeper water, young hippos may be carried on their mothers’ backs for secure resting.