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For all mammals, hydration is of importance to live. No one can live without fresh water supply to the cells. Body sodium and minerals concentration is of importance as well and must be maintained within a narrow range for the correct functioning of the organism (it is the homeostasis). The balance therefore requires a delicate equilibrium to be maintained between the ingestion and excretion (urine) of sodium (Bernal et al., 2023).
But if we, as mammals, drink salted water from the ocean, it causes important damages to our body and cells. This is due to the osmose effect: when two liquid masses are separated by a porous membrane (for example: cell wall), the concentration in ions of the two water masses tend to an equilibrium. There is a water exchange from the water mass the less concentrated in ions towards the water mass the more concentrated in ions.
Figure 1. Schematic representation of the osmosis process. Before the solvent movement, the liquid on the left side on the membrane is less concentrated than the one on the right side of the membrane. In our example, the solvent is water (H2O) and the molecules are dissolved salt (NaCl). The semi-permeable membrane let the water go through but not the molecules (as the cell wall do). After the solvent movement, the liquids on the two sides of the membrane are equally concentrated.
Let’s take the cell and seawater example: each cell of our body is surrounded by a membrane. Our body cells are filled with a liquid (intracellular liquid) which is note pure water (H2O), it also contains a certain ions concentration (mostly salt and potassium) and the cells are surrounded by a saline intercellular liquid. If we drink too much of fresh water, the ions concentration outside the cells is well below the one inside the cells, and a lot of water goes through the membranes (the cell walls) making the cells to explode. This is why drinking huge quantities of water is dangerous: it is called hyponatremia and is an emergency (Adrogué et al., 2022; Bernal et al., 2023).
But on the other side, drinking seawater is way worst. It is highly concentrated in ions, mostly salt (sodium chloride, NaCl), with a concentration between 31 and 38 g/kg, depending on the location in the Ocean (mostly influenced by the fresh water supply by rivers, precipitations, melting glaciers and evaporation). This makes the intercellular fluid NaCl concentration to increase well above the ion concentration in the cells, far higher than what the body can tolerate and most beyond what the kidney can process. In these conditions, the water leaves the cells through the cell walls. The cells are not filled with water anymore and the body is full of liquid between cells, which can cause severe dehydration, a lot of damages to all organs and a life-threatening emergency. It is called hypernatremia (Bernal et al., 2023). The same thing happens when you eat food with too much salt in it and it is also why it is dangerous to drink seawater when you are at sea or in extreme survival context.
Ok so we, as mammals need to hydrate ourselves in a good quantity for our cells and kidneys to work properly, but not too much and absolutely not seawater, it needs to be fresh water. But in these conditions, how the ocean mammals (i.e. pinnipeds, manatees, sea-otters, polar bears and cetaceans) are doing to drink? How are they doing to provide for their needs in fresh water?
Let’s focus on the cetaceans. Cetaceans group embraces the extinct zeuglodonts (Archaeoceti), the whalebone whales (Mysticeti), including for example the blue whale, the right whale, and the humpback whale, and the toothed whales (Odontoceti), including, among others, killer whales, dolphins, and sperm whales. From the Early Eocene, around 50 million years ago (Ma) they evolved from terrestrial to aquatic until becoming incredibly adapted to their current environment (Ortiz, 2001; Thewissen and Williams, 2002). This implies morphological and physiological changes, especially adaptation to a highly saline environment for most of cetaceans. The vast majority of cetaceans are hypoosmotic against seawater: they live in an environment with a higher salt concentration than their inner body salt concentration (i.e, there is a tendency to lose water naturally by osmosis, leaving them in constant risk of dehydration). Since the composition of cetacean blood is similar to that of terrestrial mammals, life in a hyperosmotic (highly salted) environment requires specific evolutionary adaptations. This includes morphological and physiological changes in the kidneys and in all the body systems involved in the maintenance of homeostasis for both the cetaceans living in fresh and in sea water (Ortiz, 2001). The metabolic cost of drinking sea water exceeds the benefits. To deal with this problem, several strategies are employed by the different aquatic mammals to increase water retention in the body and achieve a balanced osmolarity (Ortiz, 2001; São Pedro et al., 2015). There is general consensus that for cetaceans life in seawater requires an active process to maintain internal fluid composition (Ortiz, 2001).
Mammals fully living in sea water rely mostly on food for their required water intake (Costa, 2002; Séon et al., 2023). Cetaceans conserve their body water by producing more salt concentrated urine than the terrestrial mammals, a unique osmoregulatory mechanisms by which they excrete strongly salted urine to maintain homeostasis in marine habitats (Ballarin et al., 2011; Birukawa et al., 2005; Ridgway and Venn-Watson, 2010). There are two likely processes for this: the production of vasopressin, an hormone secreted by the mammal to increase the NaCl concentration in urines (also present in small quantities among terrestrial mammals) (Ballarin et al., 2011) and the presence of aquaporins, that are membrane proteins present in all forms of life with the main function of osmoregulation (São Pedro et al., 2015). This combined with their anatomically adapted kidneys, allows them to thrive in a strictly marine environment (Ortiz, 2001).
In their evolution, Cetaceans benefited from a strong genetic selective pressure in the aquaporin AQP2 gene, exclusively involving the cetacean lineage. The AQP2 is the only aquaporin found exclusively in the kidneys and involved in the reabsorption of water, being clearly required for production of concentrated urine, a crucial osmoregulation mechanism in cetaceans (São Pedro et al., 2015).
Regarding the vasopressin, Ballarin et al. (2011) found that urinary vasopressin levels increase after the meal and correspond to a rise in NaCl levels, a sign indicating that the effect of vasopressin is aimed at curtailing the dispersion of food-derived water. Cetaceans secrete vasopressin in response to food intake to exploit their principal source of water. There is a difference in NaCl supply between the macro-organisms (e.g. fish, squid) and plankton source of food, making marine mammals face different osmoregulatory problems depending on the type of prey that they consume. This makes slightly different adaptation between baleen whales and toothed whales. Birukawa et al. (2005) found indeed higher concentrations of NaCl in the baleen whales urea than in the toothed whales urea. Statistical analysis indicates that vasopressin levels in the urine (and possibly in the blood) are correlated to water intake through food (Ballarin et al., 2011).
Figure 2. Schematic representation of the NaCl ingestion and excretion of Cetaceans, and important processes in the maintain of their homeostasis.
As a conclusion, most of the marine mammals, and especially cetaceans are not drinking sea water. Their hydration is supplied from their food and they are highly adapted to the marine life. Their ability to increase the NaCl concentration in their urea make possible the released of salt from their body to only keep the fresh water they need. This prevents them from severe dehydration.
Drinking is not a common behaviour in marine mammals, only few exceptions are known to day: sea otters, who commonly drink sea water and manatees, who are exclusive herbivorous and living in the full range of water salinity so they can frequently drink fresh water (Ortiz, 2001).
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December 2025
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