Fifty-five million years ago, a common ancestor split from other even-toed ungulates. This lineage eventually gave rise to both whales and hippos. Naturalists once grouped hippos with pigs based on molar patterns until 1909. Blood proteins and molecular systematics later revealed the true relationship. The cetacean and hippo lineages diverged soon after branching off from Ruminantia. One branch evolved into fully aquatic whales like Pakicetus. The other became anthracotheres, four-legged beasts resembling skinny hippos. These groups lived across Eurasia and Africa during the Eocene and Oligocene epochs. A rough evolutionary lineage traces from Anthracotherium to Miocene species like Merycopotamus. Hippo ancestors entered Africa from Asia around five million years ago. The oldest unambiguous records of the modern Hippopotamus amphibius date to the Middle Pleistocene. Three species of Malagasy hippopotamus went extinct within the past thousand years. Evidence suggests humans hunted these island dwellers to extinction.
Anatomy And Physiological Adaptations
Adult male hippos average two tons while females weigh closer to one ton. Exceptionally large males have been recorded reaching three tons. Their barrel-shaped bodies support massive weight through graviportal skeletal structures. Dense bones allow them to sink and move along river bottoms. Eyes, ears, and nostrils sit high on the skull roof for underwater visibility. Nictitating membranes cover their eyes when submerged. The skin secretes a red-colored substance known as blood sweat. This secretion turns brown over minutes and acts as natural sunscreen. Two acidic pigments inhibit bacteria growth in the fluid. Hippos lack hair except for sparse bristles on their heads. Their jaws open 100 to 110 degrees using huge masseter muscles. Canines grow continuously up to fifteen inches long. These teeth serve primarily as weapons rather than tools for feeding. A hippo's lifespan typically spans forty to fifty years. Donna the Hippo lived at Mesker Park Zoo until her death in 2012 at age sixty-one. Bertha held the record for oldest hippo ever recorded at sixty-five years.