The large, gray, semi-aquatic bodies of manatees and hippopotamuses invite a common question about their ancestry. Both mammals spend significant time submerged in water, possess thick, nearly hairless skin, and rely on aquatic environments for survival and thermoregulation. This physical resemblance, however, is a classic example of nature’s tendency to repeat successful designs, rather than evidence of a close familial tie. The definitive answer lies not in their appearance, but deep within their genetic code and evolutionary history. This analysis will trace their distinct evolutionary paths, revealing their true relatives and explaining why they look so much alike despite belonging to entirely different branches of the mammalian family tree.
The Phylogenetic Reality
Manatees and hippos are not closely related, despite their shared semi-aquatic lifestyle and similar body shape. Scientific classification places them in entirely separate superorders of mammals, indicating their common ancestor existed millions of years ago. Modern molecular and fossil evidence confirms that the evolutionary split between their lineages occurred long before either animal adapted to its current watery existence. Their superficial similarities are misleading and should not be used to infer a close biological connection.
Taxonomically, manatees are members of the Order Sirenia, while hippos belong to the Order Artiodactyla (even-toed ungulates). This immense evolutionary distance demonstrates that their shared traits developed independently, a consequence of adapting to similar environmental pressures.
The Manatee’s True Family Tree
The manatee, along with the dugong, belongs to the Order Sirenia, which is placed within the Superorder Afrotheria. This superorder represents a group of mammals that originated and diversified on the African continent. The manatee’s closest living relatives are the elephants (Proboscidea) and, more distantly, the rock hyraxes and aardvarks.
This unexpected kinship is supported by shared anatomical and genetic features tracing back to a common African ancestor. For instance, both manatees and elephants share a unique pattern of molar replacement, where new teeth emerge from the rear of the jaw and move forward to replace worn teeth. Manatees also possess internal, undescended testes, a trait shared with elephants and hyraxes.
Fossil evidence suggests the ancestors of manatees were four-footed land mammals that began transitioning to an aquatic life over 60 million years ago. The Sirenia lineage is part of the Paenungulata group, which also includes elephants and hyraxes, emphasizing their shared origin.
The Hippo’s True Family Tree
The hippopotamus belongs to the Order Artiodactyla, traditionally known as the even-toed ungulates (including deer, cattle, and pigs). Modern genetic analysis places the hippo within the Superorder Cetartiodactyla, reflecting a discovery about its closest relatives. The hippo’s closest living relatives are the entirely aquatic cetaceans—whales, dolphins, and porpoises.
The common ancestor of hippos and whales diverged from other even-toed ungulates approximately 60 million years ago. Genetic evidence overwhelmingly supports that the hippo family, Hippopotamidae, is the sister group to the entire Cetacea group.
This relationship is also supported by shared traits, including the lack of a thick coat of hair and the absence of sebaceous glands, which are adaptations beneficial for an aquatic lifestyle. The fossil record provides transitional forms, such as extinct terrestrial artiodactyls called anthracotheres, which are thought to be the ancestors of hippos and shared a common origin with early whales. Hippos and whales also share specialized aquatic traits, such as underwater vocalizations.
Why They Look Alike: Convergent Evolution
The striking physical resemblance between the manatee and the hippopotamus is a textbook example of convergent evolution. This is the process where two unrelated species independently evolve similar traits because they are adapting to similar environmental pressures. Despite their vast difference in ancestry, both animals faced the challenge of thriving in a semi-aquatic environment.
The manatee and the hippo both evolved large, bulky bodies to aid in thermoregulation, as high body mass minimizes heat loss in water. Their thick, sparse-haired skin also developed as an adaptation to reduce drag and streamline movement. These similar solutions to the shared problems of buoyancy and heat retention drove their bodies to look alike.
This process demonstrates that the environment acts as the selective force, shaping organisms toward similar functional outcomes regardless of their genetic starting point. The manatee’s evolution stemmed from its elephant-like ancestor, while the hippo’s aquatic adaptations evolved from its whale-like ancestor. The end result is a similar external body plan achieved through entirely separate evolutionary journeys.