Most animals have some level of salinity in their blood. For terrestrial animals, maintaining that salt concentration is fairly straightforward - eat and drink water and your kidneys will do the rest. In aquatic environments, however, there is an added challenge. In a marine environment, the salt concentration of the water surrounding an animal is much higher than that of the animal itself, so drinking salt water actually dehydrates marine mammals (think how dehydrating salt water is for you when you accidentally drink some while swimming in the ocean!). So the question often arises - how on earth do marine mammals hydrate if they can't even drink the water around them?
The majority of marine mammals' freshwater needs are actually met solely through their diet. Fat and carbohydrates (the main ingredients in most prey items eaten by marine mammals) produce water as a bi-product when metabolized, leading to fairly efficient hydration from just day-to-day eating activities.
Focusing on the proper food types is also key to staying hydrated in the ocean. Eat animals instead of plants! Animals all have very similar internal salt concentrations, making them a great meal for larger mammalian predators, whereas plants have the same salinity as the ocean water surrounding them and would actually be very dehydrating to eat. This is one reason why almost all marine mammals have evolved to be carnivorous.
Inevitably, some salt water is going to be ingested if you're living your life surrounded by it. So a few adaptations have evolved in the kidneys of marine mammals to help retain water while excreting excess salt and maintaining the right internal salt concentrations. Blood is filtered much more meticulously in marine mammals than terrestrial mammals, only allowing water and salt to pass through the remainder of the kidney. This solution is then run through a very long, thin-walled tube called the loop of Henle that allows for water reabsorption, but not salt. The longer the tube, the more water can be reabsorbed. Most marine mammals have a much longer loop of Henle than terrestrial animals. This results in super salty, concentrated urine, allowing for extra salt to be excreted without very much water loss.
Kidney and diet adaptations are found all over the animal kingdom for a variety of different environments - from deserts to freshwater streams. Check back in the future for more blogs highlighting awesome animal adaptations across the world!
Naturalist Sarah C.