The physical resemblance between the rhinoceros and the hippopotamus is striking, leading many people to assume they are closely related. Both are massive, gray-skinned herbivores that dominate their respective African and Asian landscapes. However, modern genetic and fossil evidence reveals that this visual similarity is misleading, as these animals belong to two distinct evolutionary branches that diverged tens of millions of years ago. The surprising truth is that the closest relatives of the rhinoceros are not the hippo, but rather horses and tapirs, while the hippo’s most unexpected kin are the whales and dolphins.
Why the Confusion Exists: Convergent Evolution
The common misconception stems from the superficial likenesses developed through a process known as convergent evolution. This biological phenomenon occurs when unrelated species independently evolve similar traits or adaptations in response to comparable environmental pressures or ecological niches. In the case of rhinos and hippos, both lineages independently developed a large, barrel-shaped body, short, robust legs, and thick, nearly hairless skin to manage their large size and partially aquatic or humid lifestyles.
The dense skin of both species serves as protective armor, though the hippo’s skin secretes a unique reddish fluid that acts as a natural sunscreen and antiseptic. The massive size of both animals (rhinos up to 5,000 pounds, hippos nearly 10,000 pounds) is an adaptation allowing them to process large amounts of fibrous vegetation. The hippo’s need for buoyancy and the rhino’s need for terrestrial bulk resulted in similar overall body plans, even though their internal anatomy is quite different. The resemblance is a prime example of form following function, where a similar solution arose twice for the challenge of being a large herbivore in a hot climate.
The Evolutionary Path of Rhinos (Perissodactyla)
The rhinoceros belongs to the mammalian order Perissodactyla, commonly known as the odd-toed ungulates. This order is defined by a unique anatomical structure where the body weight is primarily supported by the central, or third, toe on each foot, giving them an odd number of toes—one or three—on their hind feet. All five living rhino species have three toes on each foot, a defining feature separating them from their presumed relatives.
The closest living relatives of the rhinoceros are the horses, asses, and zebras (Equidae), and the tapirs (Tapiridae). The earliest members of the Perissodactyla order appeared in the Paleocene epoch, about 56 million years ago, and they dominated the large herbivore niche during the Eocene. The fossil record of the Rhinocerotidae family is extensive, showing a wide diversity of forms, including the extinct, semi-aquatic Amynodontidae, which superficially resembled hippos. These ancient relatives were hindgut fermenters, a digestive strategy shared by all Perissodactyls, where plant matter is broken down in the colon and cecum, distinguishing them physiologically from many other large herbivores.
The Surprising Relatives of Hippos (Cetartiodactyla)
The hippopotamus is classified within the order Cetartiodactyla, or the even-toed ungulates and whales, a group that includes animals such as deer, pigs, cows, and camels. This order is characterized by a foot structure where the weight is borne equally on the third and fourth toes, resulting in an even number of functional toes. Within this group, the hippo belongs to a suborder known as Whippomorpha, which it shares exclusively with the cetaceans—whales, dolphins, and porpoises.
Molecular genetic studies show that hippos are the closest living terrestrial relatives of whales, a connection not initially apparent from physical appearance. Analysis of genetic markers, such as short interspersed elements (SINEs) and long interspersed elements (LINEs), has shown that hippos and whales share unique DNA sequences that are absent in other even-toed ungulates. This molecular data suggests a common ancestor for the hippo and cetacean lineages that lived approximately 50 to 60 million years ago.
The fossil record supports this surprising genetic link, with the discovery of extinct species like Indohyus, a small, deer-like mammal that lived about 48 million years ago. Indohyus is considered a close relative of the ancestor of both hippos and whales. This discovery, along with other fossils, links the semi-aquatic hippo to the fully aquatic cetaceans, suggesting they both arose from an ancestral group of semi-aquatic mammals. Shared evolutionary history is further reflected in physiological traits, such as both hippos and whales giving birth and nursing their young underwater, and a lack of hair and sebaceous glands, adaptations for an aquatic existence.
The Ancient Divergence of Ungulate Orders
The definitive answer to the relationship between rhinos and hippos lies in the ancient split between their respective orders, Perissodactyla and Cetartiodactyla. The last common ancestor of all modern ungulates (both odd-toed and even-toed hoofed mammals) lived near the Paleocene-Eocene boundary, approximately 55 to 60 million years ago. This timeframe marks the deep evolutionary divide that separates the rhino’s lineage from the hippo’s.
Rhinos and horses belong to the super-order Laurasiatheria; the Perissodactyla separated from the ancestor of the Cetartiodactyla long before the hippo’s ancestors branched off from other even-toed species. Consequently, the rhino is phylogenetically distant from the hippo, despite their outward appearance. The vast span of geological time confirms they are merely distant mammalian cousins, not the closely related species their massive bodies might suggest.