Seals are marine mammals that spend their lives in the ocean, an environment dominated by saltwater. A common question is how they stay hydrated without freshwater. Seals have developed physiological and behavioral adaptations to thrive in such an environment. They have evolved specific mechanisms to obtain water and manage the salt content in their bodies, allowing them to survive without directly consuming seawater.
How Seals Stay Hydrated
Seals primarily obtain water from their diet of fish and invertebrates. These prey items contain significant amounts of freshwater within their tissues, which the seals efficiently extract during digestion. Their digestive systems are specifically designed to maximize water absorption from these moist food sources.
Beyond dietary water, seals also produce metabolic water. This water is generated within their bodies through the chemical breakdown of fats and proteins from their blubber reserves. During periods of fasting, such as breeding or molting, or when food is scarce, seals can catabolize their blubber, releasing water that helps sustain them. Some Arctic seal species can even supplement their water intake by consuming snow or ice.
The Role of Kidneys in Salt Regulation
Seals have efficient kidneys that manage the salt load they acquire from their marine diet. Their kidneys are adapted to concentrate urine, allowing them to excrete excess salt without losing much water. This capability means their urine can be up to 2.5 times saltier than seawater and seven to eight times saltier than their own blood.
This specialized renal function enables seals to maintain their internal fluid balance despite consuming a diet that inherently contains salt. The ability to produce such concentrated urine is a key physiological adaptation, ensuring that the salt ingested from their prey is effectively removed from their system. This efficient salt excretion is crucial for their survival in a saltwater environment, preventing the accumulation of harmful salt levels in their tissues.
Why Seals Avoid Drinking Seawater
Seals generally avoid drinking seawater because its high salt concentration, typically around 3.5%, is significantly greater than the salt concentration in their bodily fluids, such as blood, which is approximately 0.9%. Ingesting water with a higher salt content than their internal fluids would create an osmotic imbalance.
To process and excrete the excess salt from seawater, their kidneys would need to use more water than the seal would gain from drinking it. Drinking seawater would effectively make the seal thirstier and could lead to hypernatremia, where salt levels in the blood become too high. Consequently, seals rely on their specialized hydration strategies and kidney adaptations to avoid the detrimental effects of consuming saltwater directly.