The aquatic mammal that is physically incapable of swimming

Nature · Published 7 October 2026 Swipe more like this →

In short: Despite spending their lives in the water, hippos are biologically unable to swim. These heavy giants must walk along the riverbeds, relying on dense bone structures to stay submerged. In the shifting Okavango Delta, this physical limitation becomes a life-or-death struggle when floodwaters vanish in just months.

The hippopotamus presents a biological paradox: an aquatic mammal that cannot swim. This is due to the specific density of their bone structure combined with their immense body weight. Rather than paddling through the water, hippos must walk along the bottom of rivers and deltas, holding their breath until they can push off the floor to rise for air. To catch a breath, they simply push off the riverbed to reach the surface. This method allows them to navigate the depths while managing their oxygen levels.

In Botswana's Okavango Delta, this lifestyle becomes precarious. The region is prone to extreme seasonal shifts where deep floodwaters can dry to dust in a matter of months. When food becomes scarce due to these droughts, hippos exhibit surprising survival behaviors, such as regurgitating grass from their stomachs to chew it again, a process suggesting they may possess a ruminating digestive system similar to cows. During these dry periods, survival depends on these efficient nutritional strategies. Such adaptations are vital for enduring the harsh environmental changes of the delta.

Socially, hippos are far more complex than their aggressive reputations suggest. They are highly intelligent, sensitive, and even compassionate animals. For instance, mothers may experience jealousy or aggression toward older calves that bother a new baby, and they show protective instincts toward their families. However, their nervous nature, paired with poor eyesight and sensitive hearing, often contributes to their skittish and defensive behavior in the wild. Mating typically occurs within shallow waters where their massive weight is better supported. These social dynamics can be quite intense, with males often engaging in aggressive gaping displays to defend their territory.

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