The hippopotamus (Hippopotamus amphibius) is widely recognized as one of the most aquatic non-marine mammals, often spending up to sixteen hours a day submerged in water. Despite this deep connection to water, the definitive answer to whether this African giant can survive in the ocean is a clear no. Hippos are strictly freshwater animals, having evolved specific physiological needs and adaptations that make the marine environment an insurmountable barrier. While they are powerful and graceful in their native habitat, they lack the necessary biological equipment to thrive in the vast, salty expanse of the sea.
How Hippos Move in Water
The common perception of a hippo swimming is largely inaccurate. These animals are not buoyant like marine mammals because their bones are exceptionally dense, a condition known as pachyostosis. This density acts like built-in ballast, causing them to sink rather than float.
Hippos primarily locomote by walking, trotting, or performing a slow-motion gallop along the river or lake bottom. In deeper channels, they use powerful pushes off the ground to propel themselves forward, essentially bounding through the water. This movement style requires constant contact with the substratum, which is a fundamental limitation in the deep, bottomless sections of the open ocean. Their short legs are designed for walking on submerged land, not for the sustained propulsion needed to navigate strong ocean currents.
The Freshwater Imperative
The need for freshwater is non-negotiable for the hippo’s survival, primarily due to the unique requirements of their skin. Hippo skin is thin and highly sensitive, lacking the protective layer of hair or sebaceous glands found in many other land mammals. This delicate skin dehydrates rapidly when exposed to dry air or intense sun.
To combat this, hippos secrete a viscous fluid often called “blood sweat,” which functions as a natural sunscreen and antiseptic. This secretion contains the red pigment hipposudoric acid and the orange pigment norhipposudoric acid. This complex biological sunblock works optimally only when the animal is regularly submerged in freshwater. Prolonged exposure to air or the highly saline water of the ocean would disrupt the protective layer, leading to severe dehydration, cracking, and painful skin lesions.
Physiological and Ecological Barriers to Saltwater
Beyond the threat to their skin, the ocean presents several physiological challenges that hippos cannot overcome. Ingesting saltwater creates a severe osmotic imbalance, or osmotic stress, which their kidneys are not equipped to handle. A freshwater mammal drinking seawater must expend excessive water to excrete the high concentration of salt, leading rapidly to fatal dehydration.
Furthermore, their diet is completely incompatible with a marine environment, as hippos are strict terrestrial grazers. They spend their nights on land, traveling to consume land-based grasses. The ocean offers none of the specific plants that constitute their diet, making long-term survival impossible. Moving into the open ocean would also expose them to an increased risk of predation from large, unfamiliar marine predators, such as sharks, against which their bottom-walking movement offers little defense.