Sea otters (Enhydra lutris) are unique among marine mammals because they consume saltwater. Living perpetually in the nearshore ocean environment requires specialized strategies to manage the constant intake of sodium chloride, a substance that would rapidly dehydrate most terrestrial mammals. Their survival hinges on two main factors: a diet that supplies a significant amount of fresh water and a highly specialized organ system for processing salt.
Hydration Through Prey Consumption
The primary source of hydration for the sea otter is the food it eats, which provides what is known as preformed water. Sea otters must consume a tremendous amount of food daily, often between 20 to 30 percent of their body weight, to sustain their high metabolic rate necessary for warmth in cold northern Pacific waters. This large food volume naturally contains a substantial amount of water that helps meet their daily needs.
Their diet consists mainly of marine invertebrates such as sea urchins, clams, crabs, and various mollusks. The soft tissues and body fluids of these prey items contain a lower concentration of salt than seawater, which helps offset the salt they inevitably ingest.
This reliance on dietary water is a form of behavioral adaptation that minimizes the need to actively drink seawater. However, because their prey lives in the ocean, the food still carries a significant salt load that must be managed. The protein-rich nature of their diet also produces a large amount of nitrogenous waste, or urea, which requires water to be excreted efficiently. Therefore, while diet is their main water source, it also creates the demand for a sophisticated salt-processing system.
Specialized Physiological Adaptations for Salt Processing
The ability of sea otters to thrive in a marine environment, despite their high-salt diet and occasional consumption of seawater, is due to their highly efficient kidneys. Sea otters possess kidneys that are relatively larger in size compared to those of most terrestrial mammals of similar body mass. These organs are multilobulated, a structure found in many marine mammals, which increases the surface area for filtration.
The specialized kidneys are adapted to concentrate urine significantly, a mechanism that allows them to excrete excess salt without losing a large volume of body water. Sea otters have been shown to produce urine with sodium and chloride concentrations similar to, or even slightly higher than, that of surrounding seawater. This physiological capability is what prevents dehydration.
This process is a stark contrast to human physiology, where consuming saltwater leads to severe dehydration as the body uses more fresh water to flush out the excess salt than was taken in. The otters’ powerful kidney function ensures that the salt they ingest, whether from their prey or from drinking, is effectively removed. This internal adaptation enables the sea otter to maintain proper water balance while living entirely in a saltwater habitat.