Little is known about the ability of West Indian manatees to osmoregulate and maintain water balance, but previous studies have suggested that theyshould be able to actively drink salt water based on urine data, their renal anatomy, and their occurrence in marine habitats for extended periods. Certainly manatees are frequently seen “drinking” (or are they playing?) with water from hoses. Other sirenian species are strictly fresh water inhabitants (Amazonian manatees – Trichechus inunguis) or marine (dugongs – Dugong dugon). Because West Indian manatees inhabit both fresh water and marine environments, they provide an ideal species to address questions of water balance and osmoregulation in sirenians.
We worked with captive West Indian manatees at facilities in Florida and Brazil, as well as getting blood samples from free-ranging West Indian manatees in Puerto Rico and Colombia. We wanted to see how manatees could cope with a variety of experimental conditions: animals living in fresh water eating lettuce (something with a very high water content); animals living in salt water eating lettuce; and animals living in salt water eating seagrasses (high in salt content and low in water content) with and without access to fresh water to drink. In each case we measured how much fresh water they drank and how much they obtained directly from the food they were eating (measured using labeled water dilution) and the ability of the animal to maintain electrolyte and water balance.
Captive manatees held in salt water without access to fresh water and fed a diet of sea grass showed significant increases in plasma osmolality and plasma concentrations of sodium and chloride within 9 days. These manatees eventually refused to eat sea grasses. These data suggest that wild manatees may require regular access to fresh, or perhaps brackish, water to meet water balance needs. In captive situations, this need is met by drinking fresh water or by eating food that is high in free water (e.g., lettuce, which is approximately 94% water). Manatees living in fresh water and consuming lettuce showed the highest rate of daily water intake (145 ± 12 ml kg-1 per day) compared to manatees in salt water on a diet of lettuce (45 ± 3 ml kg-1 per day) or manatees living in salt water on a diet of sea grass (21 ± 3 ml kg-1 per day). These data suggest that manatees living in fresh water not only get a great deal of water from the food they eat but that they are also drinking large quantities of water. We were ultimately able to demonstrate that manatees do not actively drink salt water, and that incidental ingestion of salt water during feeding elicited typical mammalian endocrine responses resulting in excretion of the excess salt load.