The Order Sirenia, commonly known as sea cows, contains four living species of large, slow-moving, herbivorous marine mammals. These aquatic giants are grouped into two distinct families: the Trichechidae (three species of manatees) and the Dugongidae (the single dugong species). Although they share a common ancestor and similar general appearance, manatees and dugongs have evolved unique anatomical and behavioral differences tailored to their specific environments.
Defining Physical Characteristics
The most immediate physical distinction between the two animals is the shape of their tails. Manatees possess a horizontal, rounded, paddle-like tail used for propulsion. Conversely, the dugong has a deeply notched, fluked tail that is split into two points, similar in appearance to that of a dolphin or whale. This fluked tail provides a more powerful thrust.
Differences in feeding methods are reflected in the structure of their heads and snouts. The manatee has a shorter, more rounded snout with a highly flexible, divided upper lip. This prehensile lip acts almost like pincers, allowing the manatee to grasp and manipulate aquatic vegetation. The dugong, conversely, has a broader, more rigid snout that is sharply downturned.
This downward-facing snout is an adaptation for specialized bottom-feeding. The dental structure also varies significantly between the two sirenians. Manatees have a unique system of “marching molars,” where new cheek teeth continuously grow at the back of the jaw and move forward to replace older, worn teeth.
Dugongs lack this continuous molar replacement system. Mature males typically possess a pair of tusks, which are overgrown upper incisors. These tusks are thought to be used by males in intraspecies competition, particularly during mating season, and their presence highlights a clear divergence in evolutionary strategy. While both are large, manatees generally appear bulkier and can reach lengths of up to 4 meters, often outweighing the dugong.
Geographic Distribution and Habitat
The distribution of these two families is widely separated, with the three manatee species occupying regions in the Americas and West Africa. The West Indian manatee ranges along the coasts of the southeastern United States, the Caribbean, and down to Brazil, while the West African manatee inhabits coastal areas and rivers along the western coast of Africa. The third species, the Amazonian manatee, is strictly confined to the freshwater systems of the Amazon River basin.
This geographical spread indicates a greater tolerance for varied water conditions in manatees, who often migrate between freshwater, brackish estuaries, and marine environments. In contrast, the dugong is the only strictly marine herbivorous mammal, rarely venturing into freshwater. Its range is expansive but scattered across the warm, shallow coastal waters of the Indo-Pacific region, stretching from East Africa to the northern coasts of Australia.
The dugong prefers sheltered coastal bays and mangrove channels that support extensive seagrass meadows. This reliance on a saltwater, tropical environment makes the dugong more susceptible to localized habitat degradation. Manatees, particularly the West Indian species, move into warmer freshwater springs or power plant outflows during colder months to maintain their body temperature.
Feeding Ecology and Behavior
The morphological differences in the snout and teeth are directly linked to the animals’ distinct feeding habits. Manatees are generalist herbivores, consuming a wide variety of aquatic plants, including floating vegetation, submerged grasses, and algae. Their flexible upper lip is ideally suited for plucking and grasping plants growing at various depths.
Dugongs are highly specialized grazers that feed almost exclusively on bottom-dwelling seagrasses. Their rigid, downward-angled snout is used to root through the sand and sediment, often pulling up the entire plant, including the nutrient-rich rhizomes. This method of feeding creates visible trails, or “feeding scars,” along the seafloor.
This rooting behavior acts as a form of cultivation, preventing the overgrowth of certain seagrass species and promoting biodiversity. Manatees, while also consuming seagrass, tend to crop the leaves rather than excavating the entire plant, demonstrating a less disruptive feeding pattern. Both animals spend a significant portion of their day grazing due to their slower metabolic rate.
Differences also exist in their social structures. Both are generally considered solitary or found in small groups. Manatees are often seen alone or in loose aggregations when gathering at warm water sources, but these groupings are not considered a true social herd. Dugongs, while often encountered individually or in mother-calf pairs, occasionally form much larger, temporary herds of dozens to hundreds of individuals, particularly in the rich seagrass beds off the coast of Australia.