Manatees, often called sea cows, are large, gentle marine mammals found in warm, shallow coastal areas, estuaries, and rivers. These slow-moving herbivores spend much of their lives in environments that fluctuate between salty ocean water and freshwater input from rivers and springs. Given their prominent presence in marine habitats, many people assume their hydration needs are met by the surrounding saltwater. Unlike many true marine mammals, manatees cannot rely on the ocean for all their fluid intake, making their hydration needs counter-intuitive for an animal that spends so much time in the sea.
The Requirement for Fresh Water
Manatees must actively seek out and drink fresh water to survive. While they inhabit marine environments for extended periods, they cannot maintain proper water balance solely on seawater or brackish water. Studies have shown that captive manatees kept in saltwater environments without a freshwater source experienced significant increases in the salt concentration of their blood plasma within days. This demonstrates that their physiology is not adapted to process the high salt load of the ocean indefinitely.
Manatees do consume some water through the aquatic plants they eat, but this is often insufficient, especially when feeding primarily on salt-laden seagrasses. They need regular access to a source of pure or low-salinity water to counteract the salt they ingest from their food and habitat. Wild manatees typically return to freshwater sources every one to two weeks to drink and rehydrate. This periodic requirement makes access to freshwater a matter of survival.
Behavioral Adaptations for Hydration
The necessity of fresh water has led manatees to develop predictable and opportunistic behaviors to ensure they stay hydrated. They frequently migrate between salty coastal waters and the mouths of rivers, natural springs, and canals where fresh water flows into the sea. These freshwater plumes act as underwater oases, allowing the manatees to drink large quantities of water when they need it.
Manatees are also known to exploit man-made sources of fresh water, showing an ability to adapt their behavior to the human landscape. They will congregate near storm drain outfalls, boat docks, or even discarded water hoses to drink from the runoff. When at the surface in a marine environment, manatees have been observed tilting their heads up and siphoning the thin layer of rainwater that floats on top of the denser saltwater. This behavior, along with drinking rain falling directly from the sky, highlights their resourcefulness in finding potable water.
Seasonal changes significantly influence these behaviors, as drought conditions can reduce the flow from rivers and springs, forcing manatees to travel farther to find fresh water. This increased reliance on specific, localized sources can sometimes put them at higher risk of boat strikes or other human-related dangers. Their need for fresh water is so strong that they will bypass potential dangers to access a known source.
Physiological Mechanisms for Salt Management
The manatee’s reliance on fresh water stems from its specific internal processes for managing salt and water balance, known as osmoregulation. While manatees can tolerate fluctuations in salinity, their kidneys are relatively less efficient at concentrating urine compared to specialized marine mammals like dolphins or whales. Manatee kidneys can concentrate urine to an osmolality greater than that of seawater, which helps them conserve water when in a salty environment.
However, manatees lack the highly specialized reniculate kidneys found in cetaceans and pinnipeds that are adapted for continuous salt excretion. The manatee’s system is more similar to that of a terrestrial mammal that has secondarily adapted to a semi-aquatic life. The incidental salt load ingested from their seagrass diet and the surrounding water must be managed by the kidneys, which can excrete the excess salt but at the cost of expending more water.
Hormonal responses, such as the regulation of aldosterone and vasopressin, help manatees adjust to different salinities by conserving water or sodium as needed. For instance, when deprived of fresh water in a saltwater environment, manatees show endocrine changes that help them retain water and manage the electrolyte balance. This physiological flexibility allows them to inhabit both fresh and marine waters, but it does not eliminate the fundamental requirement for periodic, active consumption of fresh water to truly offset their salt intake and maintain long-term hydration.