Dolphins, like all mammals, require fresh water to survive. Despite living in an ocean, they do not primarily drink saltwater. Instead, they possess physiological adaptations to thrive in their salty habitat without direct consumption of seawater.
Water Sources for Dolphins
Dolphins primarily obtain water through their diet. Their prey, including fish, squid, and crustaceans, contains significant water. These marine animals’ body fluids are considerably less salty than seawater, typically about one-third the salinity. By consuming large quantities of prey, up to 13 kilograms of fish per day for an adult dolphin, they ingest substantial fresh water.
Another important source is metabolic water, generated internally as a byproduct when their bodies metabolize fats and proteins from food. For instance, metabolizing 100 grams of fat can yield over 100 grams of water. This process significantly contributes to the water balance of many marine mammals and animals in arid environments. Dolphins also conserve water by not losing it through sweating, a common mechanism of water loss in land mammals.
How Dolphins Handle Salt
Dolphins inevitably ingest some salt from their diet, but their bodies are highly specialized to manage this load and maintain internal balance. Their kidneys are uniquely adapted to process consumed salt. These organs are reniculate or lobulated, composed of numerous small, distinct kidney-like structures called renicules. This unique architecture significantly increases the surface area for filtration.
Dolphin kidneys excrete highly concentrated urine, allowing them to eliminate excess salt while minimizing fresh water loss. Their ability to produce urine saltier than seawater is an important adaptation for osmoregulation in their marine environment. Dolphins can also adjust urine concentration and solute excretion in response to varying water or salt levels.
Why Drinking Seawater is Problematic for Mammals
For most mammals, drinking seawater leads to dehydration. Seawater contains a much higher salt concentration than the fluids inside a mammal’s body. When water with a higher salt concentration is ingested, osmosis causes water to be drawn out of the body’s cells to dilute the excess salt. This results in dehydration.
Furthermore, most mammal kidneys are not equipped to handle seawater’s high salt content. Human kidneys, for example, can only produce urine less salty than blood. Given that seawater is over three times saltier than human blood, a mammal would need to excrete more water than consumed to eliminate ingested salt. This creates a negative water balance, burdening kidneys and potentially leading to kidney dysfunction and severe dehydration.