Manatees are large aquatic mammals belonging to the order Sirenia. They navigate the water without the hinged joints common to land animals, meaning they lack external hind limbs and knees. However, their internal structure and evolutionary history reveal distinct clues about a past spent walking on land. The transition to a fully aquatic life required a complete redesign of the body plan, eliminating the need for complex limb joints.
The Loss of Hind Limbs
Manatees share a deep evolutionary connection with four-legged land mammals, belonging to the clade Tethytheria, which also includes modern elephants. Millions of years ago, manatee ancestors began a gradual move from land to shallow, aquatic environments. This lifestyle change meant that the paired hind limbs, once necessary for terrestrial locomotion, became inefficient for movement through water.
Natural selection favored individuals with more streamlined bodies and smaller, non-functional legs. This resulted in the complete external loss of the hind limbs and corresponding joints, including the knee structure. The powerful, muscular tail, known as the fluke, became the primary engine for propulsion.
Vestigial Anatomy
Despite the external absence of hind limbs, manatees still carry evidence of their terrestrial heritage in their skeletal structure. Deep within the muscle tissue near the tail, they possess small, disconnected bones representing the highly reduced pelvic girdle. These tiny, non-functional remnants are not attached to the vertebral column, unlike the pelvis in land mammals. Analysis of these bones sometimes reveals minute pieces of the femur, which would have formed the upper part of the knee joint.
These internal skeletal structures are evolutionary holdovers that no longer serve their original purpose for locomotion. The main function of these remnants today is to provide attachment points for some of the muscles used for reproductive and urinary functions.
Manatee Movement Without Knees
Without the hinged power of a knee joint, the manatee’s propulsion comes entirely from the caudal fluke. The animal moves by oscillating its large, paddle-shaped tail and the posterior portion of its body in a powerful, up-and-down (dorso-ventral) motion. This swimming style is highly efficient for their slow, grazing lifestyle as herbivores.
The front limbs evolved into flippers, which are primarily used for steering, balancing, and manipulating food toward the mouth. Manatees can also use their flippers to “walk” or crawl along the bottom of the riverbed or seafloor while foraging. They navigate their shallow water habitats effectively without relying on the structures needed for walking on land.