The hippopotamus, with its thick, gray hide and preference for basking near water, often causes confusion about its biological identity. Despite its somewhat reptilian appearance and habit of spending hours submerged, the answer is clear: the hippopotamus is not a reptile. It is firmly categorized within the class Mammalia, alongside humans, elephants, and whales. Its massive size and water-dwelling behavior simply mask the fundamental traits that place it squarely within the mammal lineage.
The Definitive Classification: Hippos as Mammals
The common hippopotamus is scientifically classified as a member of the class Mammalia, specifically the order Artiodactyla, or the even-toed ungulates. This classification is based on several defining physiological traits shared by all mammals. Hippos are endothermic, meaning they generate their own internal body heat to maintain a stable temperature regardless of the external environment.
They possess mammary glands and feed their young with milk. Hippos give birth to live young after a gestation period of about eight months, with calves often born underwater. Though their skin appears nearly hairless, a sparse covering of thin hair is present on their bodies, particularly on their heads and tails. Their closest living relatives are cetaceans, the group that includes whales and dolphins, from which they diverged about 55 million years ago.
Separating Mammals from Reptiles
The distinction between a mammal and a reptile is based on fundamental biological differences. The most significant contrast lies in thermoregulation. Reptiles are ectotherms, relying on external heat sources like the sun to regulate their body temperature. Mammals, including the hippo, are endotherms that use internal metabolism to achieve a constant body temperature.
Skin structure is another separating feature. Reptiles are covered in scales or scutes made of keratin, while mammals have hair or fur, even if scarce, as seen in the hippo. The reproductive method also differs sharply: reptiles lay shelled, amniotic eggs, while the vast majority of mammals give birth to live young. These core criteria confirm the hippopotamus is not a reptile.
Specialized Adaptations that Cause Confusion
The confusion surrounding the hippo’s classification stems from its specialized adaptations to a semi-aquatic lifestyle. Hippos spend up to 16 hours a day submerged in rivers and lakes, a behavior often associated with reptiles like crocodiles. Their skin is thick, ranging from blue-gray to brown, and appears smooth and leathery, lacking the obvious fur associated with most land mammals.
The placement of their eyes, ears, and nostrils high on the skull allows them to remain almost entirely underwater while still breathing and observing their surroundings. They secrete a reddish, oily fluid from mucous glands in their skin, which was once mistakenly called “blood sweat.” This secretion acts as a natural sunblock and moisturizer, protecting their sensitive skin and potentially providing antiseptic properties.