Hippopotamuses (Hippopotamus amphibius) are large, semi-aquatic mammals primarily found in sub-Saharan Africa’s rivers and lakes. Their strong association with water is a defining characteristic, revealing several adaptations crucial for their survival.
Keeping Cool and Hydrated
Hippos spend most of their day submerged in water for thermoregulation. Their immense body mass, up to 3,300 kg, generates substantial heat. Unlike many mammals, hippos lack true sweat glands, limiting evaporative cooling and making them highly susceptible to overheating in the intense African sun. Water effectively dissipates body heat, maintaining their internal temperature.
Their sensitive skin requires constant hydration and sun protection. It is thin and lacks sebaceous glands, prone to cracking and severe sunburn during prolonged exposure. To counteract this, hippos secrete a reddish, oily fluid, often called “blood sweat,” which acts as a natural sunscreen, moisturizer, and antiseptic. However, this fluid alone is insufficient for prolonged terrestrial exposure, making water immersion essential for defense against dehydration and sun damage.
Navigating with Ease and Seeking Safety
Water offers significant advantages for hippo movement and defense. Water’s buoyancy supports their bulk, making underwater movement more energy-efficient than on land. Hippos often “bottom-walk,” pushing off the riverbed with dense bones to propel themselves. They can hold their breath for up to five minutes, enabling extended underwater periods.
The aquatic environment serves as a crucial refuge from terrestrial predators. While adult hippos have few natural predators due to their aggressive nature and formidable size, water provides safety, especially for young calves. Lions and hyenas are less effective hunters in aquatic settings. Hippos are highly territorial in their aquatic domains, and their powerful jaws make them dangerous to any perceived threat.
Life on Land
Despite their strong reliance on water, hippos are terrestrial grazers, feeding almost entirely on grasses. They leave the water at dusk for nightly foraging excursions, returning before dawn. During these nocturnal periods, they consume large quantities of vegetation, often 40 to 50 kilograms of grass in a single night.
Their time on land is necessary for acquiring food, but it also exposes them to increased vulnerability. On land, hippos are less agile and more susceptible to threats, including human-wildlife conflict. They follow well-worn paths from the water to their grazing grounds, which can extend several kilometers. This nocturnal feeding strategy minimizes their exposure to daytime heat and sun, which their sensitive skin cannot tolerate.
Not All Hippos Are the Same
While the common hippopotamus is known for its aquatic lifestyle, the Pygmy Hippopotamus (Choeropsis liberiensis) exhibits different behaviors. Pygmy hippos are considerably smaller, weighing approximately one-tenth that of their common relatives. They are also less aquatic, spending more time on land and inhabiting forested areas and swamps rather than large open bodies of water.
Pygmy hippos are generally solitary and more reclusive, contrasting with the social groups of common hippos. Their physical characteristics also differ, with a rounder head, less webbed feet, and proportionally longer legs, adaptations suited to their more terrestrial existence. This smaller species still utilizes water for hydration and skin protection, but their reliance on it is not as pronounced as that of the common hippopotamus.