Many wonder if rhinoceroses and hippopotamuses are related due to their large size and similar appearances. Despite superficial resemblances, these two mammals are not closely related. Their evolutionary paths have diverged significantly over millions of years, placing them in distinct mammalian orders.
Evolutionary Lineage
Rhinoceroses belong to the order Perissodactyla, known as the odd-toed ungulates. This group is characterized by having an odd number of toes on each foot, with the middle toe bearing the most weight. Other members of this order include horses, zebras, and tapirs. The rhino lineage split from other Perissodactyls millions of years ago, with African and Eurasian rhino lineages diverging around 16 million years ago.
Hippopotamuses are part of the order Artiodactyla, which encompasses the even-toed ungulates. This order is defined by having an even number of toes, typically two or four, with weight borne equally on the third and fourth digits. Pigs, deer, giraffes, and cattle are also members of this diverse group. Surprisingly, the closest living relatives to hippos are cetaceans, including whales, dolphins, and porpoises, sharing a common semiaquatic ancestor approximately 60 million years ago. This means hippos are more closely related to whales than they are to rhinos.
The apparent similarities between rhinos and hippos are a result of convergent evolution. This occurs when unrelated species develop similar traits independently, often due to similar environments. Both rhinos and hippos are large, herbivorous mammals that spend time near water, leading to some shared physical characteristics despite their different evolutionary backgrounds.
Distinguishing Characteristics
Physical Traits
Rhinoceroses have prominent horns made of keratin, the same protein found in human hair and fingernails. Most rhino species have thick, wrinkled gray or brown skin, sometimes with folds that give an armored appearance. Their feet have three toes, each ending in a broad, blunt nail.
Hippopotamuses do not have horns but possess large tusks, which are elongated canine and incisor teeth used for defense and display. Their bodies are largely hairless with very thick protective skin, and they have four toes on each foot.
Diet and Feeding Behavior
Rhinos are exclusively herbivorous; their diet varies by species and habitat. White rhinos are grazers, using wide, square lips to feed on grasses, while black rhinos are browsers, consuming leaves, branches, and shrubs with prehensile, pointed upper lips. They spend a significant portion of their day foraging.
Hippos are also primarily herbivores, feeding almost entirely on short grasses, often traveling kilometers from water sources to graze at night. An adult hippo can consume 40 to 50 kilograms (88 to 110 pounds) of grass per night, using strong lips to crop vegetation.
Habitat and Social Structure
Rhinoceroses inhabit various environments, including grasslands, savannas, and forests across Africa and Asia. Rhino social structures differ by species; black rhinos are generally solitary, while white rhinos can form groups, often consisting of females and calves.
Hippos are semi-aquatic, found in rivers, lakes, and swamps across sub-Saharan Africa, requiring permanent water sources to regulate body temperature. They are highly social, living in groups called pods or bloats, which can range from a few to over one hundred individuals, congregating in water during the day.
Water Dependency
Both animals spend time in water for different reasons and to different extents. Rhinos use mud wallows and water to cool down and protect their sensitive skin from sunburn and insect bites. While needing access to water, they are not continuously submerged.
Hippos are far more dependent on water, spending up to 16 hours a day submerged to keep their skin cool and moist, as they lack sweat glands and are susceptible to sunburn. Despite their aquatic nature, hippos do not swim or float; they walk along the bottom of water bodies.