Animals and salt
All living things need salt, but they can only tolerate it in small quantities. We will demonstrate how nature solves this problem effectively, using a number of vertebrates as examples. Our selection is not entirely random, because the creatures' desalination strategies are best illustrated by their contrasting living conditions.
Die Evolution der Wirbeltiere hat verschiedene Organe zur Osmoregulation befähigt. So etwa die Haut und die Kiemen. Sie stehen als äussere Grenzflächen im direkten Kontakt mit dem umgebenden Medium, Meerwasser, Süsswasser oder Luft. Drüsen, die aktiv Salz ausscheiden, sind im Tierreich weit verbreitet. Bei Reptilien und Vögeln sind sie im Augen-, Nasen- und Mundbereich zu finden. Ebenso effizient sind die Nieren an der Regelung des Wasser- und Salzhaushaltes beteiligt.
Atlantic salmon (Salmo salar)
The salmon is far more than an excellent fish to eat. Its migrations are legendary. The fry leave their native waters, migrate downstream and disappear into the mythical Sargasso Sea. On attaining sexual maturity, they embark on the return journey to fresh waters, guided by their phenomenal sense of smell and an overpowering compulsion to spawn.
The salmon is simultaneously a freshwater and seawater fish, which makes it even more interesting.
In freshwater, its body has a higher concentration of salt than the surrounding water, which therefore enters continuously through the skin and gills. The salmon is at risk of swelling. The kidney must excrete water and retain salt. For this purpose, it produces large quantities of heavily diluted urine. In seawater, on the other hand, the salmon’s body has a lower salt concentration than the surrounding water. The saltwater constantly removes bodily fluid from the fish through its skin and gills. Paradoxically, the salmon is at risk of dying of thirst due to dehydration. The fish therefore keeps drinking large quantities of saltwater; the kidney retains the freshwater and excretes the salt. It produces small amounts of heavily concentrated urine. The gills of sea fish can also excrete salt.
Roadrunner (Geococcyx californianus, G. velox, American roadrunner)
The roadrunner or earth cuckoo is originally a desert bird that is excellently able to thrive in its habitat. Two American species are known. They occur in the western USA, e.g. in the Sonora and Mojave deserts, and in Central America, and they are known for their spectacular turns of speed. They can reach almost 30 km/h. Roadrunners are omnivores, but they mainly chase reptiles and insects. They do not disdain colibris or rattlesnakes. The roadrunner’s exuberance has also made it popular as a comic character (The Road Runner by Chuck Jones (Warner Brothers)).
Roadrunners are very sparing in their consumption of water. Water retention in the rectum is extreme, so the faeces are bone dry. In the searing midday heat, the roadrunner reduces its activity by one half. Excess salt is excreted through a salt gland in its nose, but the kidney plays no part in salt regulation.
Sea snakes (Hydrophiidae)
Sea snakes inhabit tropical waters from the Persian Gulf to the south-west Pacific Ocean. All of them are poisonous. They usually grow to lengths of about 1.5 metres. In most cases, young sea snakes already hatch inside the female’s body, so they are born live. Most species never leave the water. They are excellent swimmers but they need to obtain air as a basic requirement. They can also remain underwater for several hours by swallowing water, absorbing the oxygen it contains and expelling the water again. Sea snakes feed on fish. Unless they feel threatened, sea snakes do not usually attack humans. Sea snakes excrete excess salt through glands located under their tongue.
Camel (Camelidae)
The single-humped camel (Camelus dromedarius) and the two-humped camel (Camelus ferus) live in the desert regions of Asia and North Africa. Both species have been domesticated since time immemorial, and are highly valued as mainstays of livelihood or as racing camels. Camels are able to withstand long periods of drought and great heat without consuming water. They display an extreme version of control over the water and salt balance. The fatty tissue in their humps is both an energy reserve and a water store. As the fat is reduced, water is released into their metabolism. The quantity of urine is small but with high concentrations of urea and salt.
Nowadays, the two-humped camel is only found in the wild in south-western Mongolia and north-western China. Less than 1000 of these animals remain. At the start of 2001, the discovery of an allegedly unknown species of camel that could drink salt water hit the headlines. 600 specimens were still said to be living in the remote Kum-Tagh dunes in the Chinese province of Xinjiang, on the edge of the Tibetan mountains. There were no photographs and so far, there has been no confirmation of this report.
Penguin (Spheniscidae)
This comical bird has won the hearts of adults and children alike. Their “morning dress” and upright waddling gait may seem ungainly, but in water penguins display superb swimming skills. They are also very resilient: after all, most of the 18 species of penguin live in the Antarctic or on sub-Antarctic islands. Other species are native to the coasts of Australia, South Africa and South America, as well as the Galápagos and Falkland Islands. If penguins are denied access to fresh water, their salt glands begin to function in order to regulate the salt concentration in their bodies (as happens with other birds). Penguins’ salt glands are located behind their eyes and they open into the nasal cavities. The concentrated salt solution exits through their nostrils and drips off their beaks.
Flamingos (Phoenicopteridae)
An impressive spectacle can be seen when thousands of shrieking flamingos rise into the air from the lagoons, making the sky shimmer in hues of pink and white. But these birds with their long legs, long necks and curved bills are also fascinating to watch in a zoo as they majestically scout for food, as if walking on stilts. Flamingos breed in various regions of the earth, mostly in still bodies of water or on low-lying islands in flat ponds, salt lakes and lagoons. The perfect way they adapt to habitats that are usually very salty is not so well known. They are not even deterred by the all-engulfing soda lakes of East Africa. Flamingos excrete excess salt through salt glands in their noses. Some species of flamingo drink fresh water from geysers and hot springs.
Marine iguana (Amblyrhynchus christatus)
This ancient reptile belongs to the unique fauna of the Galápagos archipelago. Charles Darwin developed his fundamental ideas on evolution and the development of species here because he witnessed live evidence before his very eyes. The marine iguanas live on the surf-swept cliffs, and their dull coloration and gnarled appearance give the impression that they have emerged directly from the lava. However, they are harmless herbivores that feed on algae and various types of seaweed. They use salt glands to regulate the concentration of salt in their bodies. These glands are located in front of their eyes, and they open into the nasal cavity. The concentrated salt solution exits from their nostrils in the form of fine droplets.
Hawksbill sea turtle (Eretmochelys imbricata)
This sea turtle inhabits all warm seas and oceans. Its shell can reach lengths of up to 90 centimetres. The shape of its upper jaw resembles the beak of a bird of prey. Hawksbill sea turtles feed on fish, molluscs, crustaceans and various marine plants. They are hunted and decimated to obtain their shells. To detach the horny plates (“scutes”) from the turtle’s carapace, the creatures are often exposed to a source of heat while they are still alive. The turtles can survive this procedure, after which they form new scutes which are no longer suitable for decorative purposes because they are of lesser quality. Like other marine creatures, the hawksbill sea turtle must constantly excrete excess salt in strong concentrations. Special salt glands perform this function. Their opening is located in front of the turtle’s eye, and the trickling fluid becomes encrusted here when exposed to air. This is why turtles and other creatures with salt glands appear to weep.
Whitefish (Coregonus spec.)
This famous game fish (edible fish) is one of the species that most resembles salmon. It is primarily native to the cold, deep lakes of the northern hemisphere. It is a typical freshwater fish which feeds on plankton and invertebrates.
Its body tissue has a higher concentration of salt than freshwater, so it continuously takes water in through its skin and gills. The whitefish does not drink water and it must retain salt. Its kidney actively filters the salt out. Consequently, it produces large quantities of heavily diluted urine.
Seals (Pinnipedia)
These mammals populate all marine regions in the cold and temperate climate zones. Monk seals are the only species found in tropical regions. Seals consist of three families: eared seals (sea lions, fur seals), walruses and earless seals (elephant seals). The ancestors of the seals used to live on land and they retreated into the water in the course of the evolutionary development of their species. They are almost perfectly adapted to life in the water, although they return to the coast or drift ice in order to mate and raise their young. As predators, they feed on fish, crabs, molluscs and other marine creatures. They regulate their salt balance via the renal function. Their urine is heavily concentrated.
