Salt glands are still fascinating: they turn on within minutes of excess salt being detected in the blood; although small in size they can secrete concentrated salt solutions at a very high rate, and to support this blood flow through them is amongst the highest recorded in the animal kingdom. Even now there remains much to be known about them, from the ecological level, the extent to which the salt glands are used by different birds in different habitats, for example, right down to secretory mechanism at the cellular and molecular levels.
I learned a great deal from reading the proofs for Krogh's osmoregulation book. One problem that especially intrigued me was how marine birds survive with no fresh water to drink. In search of an answer to this question, I developed methods I could use under primitive conditions, and at the end of the spring term in 1939, after securing permission from the Norwegian authorities to capture birds, I set off for the coast of northern Norway to try to solve the problem. This journey was the first of what would become a long series of field studies around the world, ranging from the Sahara Desert to the Amazon River, seeking answers to problems of how animals survive in hostile environments.
In mid-June 1939 I arrived at Röst, a small island in northern Norway, off the Lofoten chain and facing the Arctic Ocean. Millions of auks, puffins, and gulls nest on vertical cliffs that rise out of the ocean beyond Röst. The birds seek their food at sea, and except for rainfall there is no fresh water. Do they drink sea water? I wanted to find out.
It was already known how whales and seals can manage. If they drink sea water, the extra salt is excreted by the kidneys. Whale kidneys are powerful and can produce urine more concentrated than sea water. Although no one knows whether whales and seals actually drink sea water, they could readily eliminate the excess salt…
Humans have less powerful kidneys than seals and whales. A human castaway at sea who drinks sea water merely hastens the approach of death because the kidneys are unable to excrete the excess salts. Birds seemingly are worse off; their kidneys are even less efficient than humans' in eliminating salts. The problem was to find out if birds get sufficient water in their food, or if they drink sea water and somehow are able to excrete the salts…
During the summer I examined skuas, auks, puffins, and kittiwake gulls. The salt concentration in their guts was invariably low and showed no evidence that any of them drank sea water. I also examined five seals shot by a local fisherman; the results were similar. Neither the salt nor magnesium content in the seals' stomachs and intestines suggested that they had drunk sea water.
So far I had only negative evidence. The next step was to find out what happens when a bird actually swallows sea water. I captured a few kittiwake gulls and caged them in empty orange crates. They greedily devoured the fish I fed them. Fish doesn't have a high salt content, so I gave one of the birds an ample volume of sea water by stomach tube. If the kidneys excreted the salts, there should be a high salt concentration in the urine.
The bird produced copious volumes of urine, but to my amazement the urine had little salt in it. I repeated the experiment with other birds, and again the urine was nearly salt-free. Wondering if my analytical methods were wrong, I tested every step with solutions of known salt content; my methods were 100 percent correct.
No matter how much sea water I gave the birds, little salt appeared in the urine. Could it be that the birds retained the salts? If so, the salt concentration in the blood should increase. But my analysis of their blood showed no elevated salt levels. Where was the salt going? I knew it had entered the body, yet I couldn't find it in the urine or in the blood. It seemed that the salt had simply disappeared.
At the end of the summer I returned to Copenhagen, disappointed that I had found no solution to the original problem. I was anxious to talk to Dr. P. B. Rehberg, a prominent renal physiologist, who usually gave young scientists excellent advice. However, he said little, and I felt that he perhaps thought I hadn't done a very good job; he didn't even look at my meticulously kept data books. In desperation I suggested that if the salt doesn't come out the rear of the bird, it must somehow come out the front. Rehberg didn't comment…
I wanted to tackle this problem again, but the war intervened, and then other projects took all my time. Not until eighteen years later, in 1957 [actually 1956], did I return to the study of marine birds. As I had suggested to Rehberg, a salt load is indeed eliminated from the front end of the bird, as a salty fluid dripping from its beak. Then I understood why I hadn't noticed the phenomenon when I was on Röst. The primitive conditions where I worked, the orange crates and the rough wooden floor, made it difficult to see drops of fluid the birds shook from their beaks. That summer in Norway, I thought the few drops I noticed were no more than a little sea water regurgitated by the bird.
The research that led to the discovery of salt glands was done during the summer of 1956 at Mount Desert Island Biological Laboratory (MDIBL), Bar Harbor, Maine, then a gathering place for those studying the kidney and renal excretion during the long vacation, including Knut’s then wife, Bodil, August Krogh’s daughter.
Knut took up the story again in The Camel’s Nose:
For the study of marine birds I asked two postdoctoral collaborators to join me in Maine: Humio Osaki from Japan and a former classmate of mine from Denmark, Carl Barker Jörgensen. We caught some young cormorants, and to find out what effect sea water has on salt excretion, I gave one of them a liberal amount by stomach tube and placed the bird in a carefully cleaned plastic container. Within a minute or two I made the fastest scientific discovery I ever made. I noticed that the bird, with a quick movement of the head, shook off droplets of fluid that appeared at the tip of its beak. I sampled the clear liquid with a micropipette; it gave a massive precipitate with silver nitrate, revealing a high concentration of chloride. We were astounded, but the result confirmed what I had suggested decades before—that if salts do not come out one end of the bird, they must come out the other.
The very salty secretion is produced by glands in the bird's head and drips from the tip of the beak. Thus, if the birds drink sea water, the excess salt is eliminated, leaving a net gain of free water. Whether marine birds in the wild actually drink sea water is a question that is difficult to answer. Nevertheless, much of their food has a salt content high enough to necessitate the elimination of excess salt by the glands we had discovered. For simplicity we decided to call them salt glands. Our discovery received a great deal of attention from physiologists as well as the popular press, for no such gland was known in any animal, and it solved a long-standing problem.
I continued these studies over the next two years, both at Duke and in Maine. All marine birds we examined—gulls, pelicans, petrels, eider ducks, and so on—use the same mechanism to excrete excess salt. I had a marvelous collaborator in a Swedish friend, the animal physiologist Ragnar Fänge, who described the detailed anatomy of the salt gland and refined our understanding of its function.
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| Knut Schmidt-Nielsen working on sea birds at MDIBL (from here) |
Schmidt-Nielsen spent the following summers until 1959 at MDIBL, adding to the previous work. The history also showed that he worked with the MDIBL stalwarts, William L Doyle and Thomas H Maren but they published their salt-gland work without Knut as a co-author. Doyle published the first electron micrographs of the gland. Others there also published on salt glands later, Hubert and Mabel Frings, for example.
The discovery of salt glands in birds and then in reptiles was just one part of Knut Schmidt-Nielsen’s series of seminal contributions to How Animals Work (the title of one of his books). Obituaries by the late Steven Vogel (1940-2015) and Ewald Weibel can be found here and here.
As Steven Vogel (1940-2015) wrote in Knut’s obituary, in discussing the discovery of salt glands, ‘…the work has taken its place as common knowledge with only rare reference to the seminal reports’. I can only add that, sadly, not only are the references to the seminal reports rare but that the information given on salt glands, particularly in blogs and websites, is so often completely, utterly and completely wrong that it can only be classified as drivel.
At a symposium in Sandbjerg, Denmark to celebrate Knut Schmidt-Nielsen’s 65th birthday in July 1980 there were seven co-authors (out of a possible ten) of his papers on salt glands that were all published between 1957 and 1964 (Ragnar Fänge (1920-1999), Carl Barker Jörgensen (1915-2007) , Maryanne Robinson (Maryanne Robinson Hughes), Arieh Borut, Eugene C. Crawford, Stephen Thesleff, Francis G Carey (1931-1994)). In addition, two of us there (Dennis Bellamy and me) had worked on salt glands later. All the participants and contributors to the proceedings, entitled A Companion to Animal Physiology, received a commemorative medal which show Schmidt-Nielsen’s famous books and the animals with which he was most associated: kangaroo rats, gulls, camels, frogs and snails.
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| 5th International Symposium on Comparative Physiology, Sandbjerg, Denmark, July 1980 To commemorate Knut Schmidt-Nielsen's 65th birthday |
And there were Golden Orioles in the trees.
Taylor CR, Johansen K, Bolis L (editors). 1982. A Companion to Animal Physiology: Perspectives. Cambridge: Cambridge University Press
Evans DH. 2015. Marine Physiology Down East: The Story of My Desert Island Biological Laboratory. New York: Springer
*Schmidt-Nielsen K. 1957. Extrarenal excretion of salt in birds. J Elisha Mitchell Scientific Society 73, 235
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