Seals are marine mammals found across diverse ocean environments. A common question arises regarding their survival in these vast saltwater habitats: how do these creatures stay hydrated without drinking the surrounding salty water? Their survival relies on biological strategies that differ from land animals. Seals do not consume seawater to meet their hydration needs.
The Marine Mammal Hydration Strategy
Seals primarily obtain the water they need through their diet. Fish, squid, and crustaceans, which constitute a large part of their prey, are rich in moisture and contain a much lower salt concentration than seawater. The moisture content of fish, for example, typically ranges from 60% to 80% of their body weight, sometimes even reaching over 81%. A seal’s digestive system is highly efficient at extracting this water from their food.
Beyond the water directly consumed from their prey, seals also generate water internally through a process known as metabolic water production. This occurs when their bodies break down fats and proteins from their food and blubber reserves. This process yields water as a byproduct of cellular respiration, supplementing their hydration, particularly during periods of fasting or when food is scarce. This dual approach of extracting water from their diet and producing it metabolically allows seals to maintain their fluid balance without needing to drink seawater, which would be harmful due to its high salt content.
Specialized Internal Adaptations
Seals possess highly specialized internal adaptations that enable them to efficiently manage salt and conserve water. Their kidneys are particularly effective, far surpassing human kidneys in their ability to concentrate urine. This allows seals to excrete excess salt ingested from their prey with minimal water loss, producing urine that can sometimes be saltier than the surrounding seawater. In humans, urine can be concentrated up to about four times the osmolality of blood plasma, but seals can achieve much greater concentrations to eliminate the salt load.
Other physiological mechanisms further contribute to their water conservation. Seals have reduced sweating, minimizing water loss through their skin compared to many terrestrial mammals. Their thick layer of blubber not only provides insulation in cold waters but also serves as a significant energy reserve. The metabolic breakdown of this blubber during fasting periods further contributes to their water supply, demonstrating a comprehensive adaptation to their marine environment.