16.09.2018 – Hydration on the high seas

Today the playful Atlantic spotted dophins (Stenella frontalis) were seen on all tours on both our traditional boat and on our zodiac. The guests and crew aboard our zodiac were also lucky enough to observe baleen whales feeding close to a tuna fishing vessel. This group may have consisted of both Tropical whales (Balaenoptera edeni) and Sei whales (Balaenoptera borealis). Both species enjoy a diet that includes small, schooling fish which may have congregated, together with some tunafish, near the fishing vessel. While toothed whales like dolphins actively hunt their prey, baleen whales filter their food out of the water column using their baleen plates. During feeding, both Tropical and Sei whales accelerate on their side near the waters surface with their mouths wide open in order to swallow as much water (and fish) as possible. The water is then expelled and their prey filtered through the baleen plates on their upper jaw.

We often have the pleasure of watching cetaceans hunt and feed during our tours but how do these animals drink? This is a question that has puzzled scientists for a while and is a topic that is frequently addressed by our guests. Like all mammals, cetaceans cannot directly drink seawater because of its high salt content which may cause dehydration, kidney failure or even death. The fact that marine mammals have little or no access to freshwater led to the popular assumption that they acquire their water indirectly through their food. Cetaceans feed on a variety of marine organisms including schooling fish, squid and crustaceans that already carry water in their bodies. Cetaceans separate the water containing their food into their food passage which is anatomically separated from their oxygen passage near their blowhole. This prevents the animal from accidentally ingesting water into their lungs while they consume their prey.

After ingesting and digesting their food, whales use an adapted filtration system in their kidneys to excrete excess salt in their urine. The filtration system is much more advanced in marine mammals than in land mammals and cetaceans do not lose as much water in the course of the day as we do; we lose water when we respire or sweat, cetaceans do not.

While these assumptions seem to be reliable explanations, scientists still do not fully understand osmoregulation in cetaceans as much as they’d like to. If these theories are correct we are not only watching cetaceans feed when they’re hunting, we are also watching them drink and hydrate. Marine birds belonging to the Tubenose family (Procelariiformes), a taxonomic group that also includes the often encountered Cory’s Shearwater (Calonectris borealis), drink a little differently. These animals are actually able to drink seawater! A small gland under their eyes that filters out the salt from the swallowed water makes this possible and this gland is only active in this family of birds.

By Paula Thake

Sightings of the day

Ribeira Brava

10:00 Atlantic spotted dolphins, Loggerhead turtle

Stenella

09:00 Atlantic spotted dolphins, Sei whales, Tropical whales, Loggerhead turtle

12:00 Atlantic spotted dolphins, Sei whales, Tropical whales, Loggerhead turtle