Hippos and rhinos, both large, grey, and herbivorous mammals, share visual similarities that often lead to questions about their relationship. Despite their robust appearance, these resemblances do not indicate a close evolutionary kinship.
Are Hippos and Rhinos Related?
Despite sharing superficial characteristics, hippos and rhinos are not closely related. Their large size, grey coloration, and herbivorous diets are examples of convergent evolution, where unrelated species independently develop similar traits due to adapting to comparable environmental pressures.
Tracing Their Evolutionary Journeys
Hippos and rhinos occupy distinct branches of the mammalian family tree. Hippos belong to the order Artiodactyla, which includes even-toed ungulates like deer, pigs, and cattle. Surprisingly, their closest living relatives are cetaceans, the group encompassing whales, dolphins, and porpoises. This connection stems from a common semi-aquatic ancestor that lived approximately 60 million years ago, with the hippo and cetacean lineages diverging around 55 million years ago. Early hippo ancestors, such as anthracotheres, were semi-aquatic, pig-like animals that thrived from about 55 to 23 million years ago, eventually leading to modern hippos.
Rhinos, on the other hand, are part of the order Perissodactyla, known as odd-toed ungulates. This group also includes horses and tapirs. The evolutionary path of perissodactyls diverged much earlier from the artiodactyl line, with their origins tracing back over 60 million years. Fossil evidence indicates that early rhino ancestors appeared in North America or Eurasia around 55 million years ago, showcasing a lineage independently adapted for terrestrial life.
Key Differences in Biology and Habitat
Beyond their evolutionary divergence, hippos and rhinos exhibit significant biological and habitat differences. Hippos are semi-aquatic, spending most of their day submerged in rivers, lakes, and swamps to keep cool and protect their sensitive skin from the sun. They emerge at night to graze on short grasses, often traveling several kilometers from water sources.
Their skin is nearly hairless and secretes a reddish, oily substance that acts as a natural sunscreen and moisturizer. Hippos possess a barrel-shaped body, short legs, and large canine tusks, which are enlarged teeth used for defense and display rather than true horns. They are highly social, living in groups called “pods” that can number up to 100 individuals, though social bonds among adults can be minimal.
Rhinos are primarily terrestrial, inhabiting savannas, grasslands, and forests, though they require access to water for drinking and bathing. Their diet varies by species, with some being grazers that consume grasses and others being browsers that feed on leaves, twigs, and shrubs. Rhino skin is thick and folded, often appearing armor-like, and they have sparsely distributed hair, mainly on their ears and tail. A defining characteristic of rhinos is their horn or horns, made of keratin, the same protein found in human fingernails. Unlike hippos, rhinos are generally solitary creatures, with the exception of white rhinos, which may form small, loose social groups.