Manatees are large, slow-moving aquatic mammals that inhabit shallow, marshy coastal areas, rivers, and estuaries. These herbivores are remarkable for their ability to navigate and feed across a wide range of aquatic environments, from freshwater springs to the high salinity of the ocean. Their need for fresh water is a fundamental biological reality concerning their survival in saltwater ecosystems.
The Simple Answer: Yes, They Need Fresh Water
Manatees, particularly the West Indian species, are euryhaline, meaning they can tolerate varied salinity levels and move freely between marine, brackish, and freshwater systems. This trait allows them to forage in both ocean seagrass beds and inland rivers. Despite spending extensive periods in the ocean, manatees cannot sustain themselves indefinitely on a purely marine diet without access to fresh water.
Experimental studies show that manatees kept in salt water and fed only seagrass experienced significant increases in plasma osmolality and electrolyte levels within nine days. This physiological stress suggests that prolonged exposure to high salinity without fresh water is unsustainable. Access to this resource directly influences their distribution and migration patterns, making fresh water a requirement for their long-term health.
The Biological Requirement for Freshwater
The underlying reason manatees must seek fresh water is to maintain internal balance, a process called osmoregulation. Unlike true marine mammals, such as dolphins or whales, manatee kidneys are less specialized for excreting massive salt loads. When manatees consume marine vegetation, they absorb salt, and the continuous diffusion of salt across their skin further contributes to their internal salinity.
To manage the salt load, manatees make physiological adjustments, including endocrine responses involving hormones like vasopressin and aldosterone. Vasopressin acts to conserve water, while aldosterone helps manage sodium levels. However, these mechanisms are considered a backup plan, as a purely marine existence without fresh water leads to the buildup of sodium and chloride in the blood.
If deprived of fresh water while in a marine environment, the manatee’s plasma osmolality increases, signaling dehydration and a state of salt stress. This condition can eventually cause manatees to stop eating their primary food source, the seagrass, further compromising their survival. Therefore, the intake of fresh water acts as a necessary diluent, helping their kidneys flush out the excess salts and maintain a stable internal environment.
How Manatees Find and Consume Fresh Water
Manatees employ specific behaviors to acquire the fresh water needed to counteract the effects of a saltwater environment. One of the most common methods is to travel up rivers and streams, migrating from the coastal ocean into inland waterways. They frequently gather at natural freshwater springs, which offer both warm water refuge in winter and a consistent source of drinking water.
In marine and estuarine areas, manatees also rely on less obvious sources of hydration. After heavy rainfall, fresh water is less dense than salt water and tends to collect on the surface in a temporary layer, which manatees will surface to “sip.” They have also been observed seeking out man-made sources of fresh water, such as drainage pipes or hoses, which can unfortunately draw them closer to boat traffic.
Furthermore, their diet of aquatic plants contributes to their overall water intake. While foraging, manatees consume both marine and freshwater vegetation, and the water content of these plants can provide a substantial amount of hydration. Studies indicate that manatees are intuitive feeders, often selecting water-concentrated vegetation when they are dehydrated.