Curiosity about particular phenomena—investigations were carried out to determine how desert animals prevent water loss in their wastes.
Water, water, every where,And all the boards did shrink;Water, water, every where,Nor any drop to drink.The Rime of the Ancient Mariner
As a zoology major at university, I spent many hours pouring over Knut Schmidt-Nielsen’s classic text, Animal Physiology. He was widely regarded as the father of comparative physiology and made groundbreaking contributions to the discipline of Ecophysiology, the study of how animal physiology is adapted to the environment. A group of animals that came in for particular notice in many of his studies were the desert rats (Dipodomys sp.).
These rats eat only dry food, principally seeds, yet almost never drink water, even if it is available. Their habitat, the deserts of SW North America, see little rainfall and baking temperatures. How is this possible?
Firstly, a primer on water balance (adapted from Schmidt-Nielsen, 1962). Animals gain water through either drinking free water, water in their food and the water produced from the oxidation of glucose in respiration (metabolic water). Water is lost from the body by evaporation from the skin and lungs, in urine and in feces. In order to survive without drinking water and on a dry-food diet, these animals must therefore minimise their water loss. As this topic is linked to 11.3 (Osmoregulation) we will focus on their kidney adaptations.
We learn when studying topic 11.3 that the Loop of Henle is crucial in allowing the development of a urine that is hyperosmotic to the blood plasma. Desert rats have a proportionally very long Loop of Henle and a thicker medulla, which allows for the establishment of even more concentrated urine. Additionally, as a small animal, their high metabolic rates mean they have more densely packed mitochondria and transport pumps throughout the nephron, allowing the maintenance of a concentration gradient.
The result is that various species of desert rats can excrete a urine that is up to 16 times the concentration of their blood plasma and over four times more concentrated than seawater!
With a greater amount of solute in the urine, proportionally less water is needed to excrete it. These rodents also take it a step further by producing very dry, pellet-like feces. They also ingest their feces to reabsorb what little water is in them. They are therefore able to survive only by using metabolic water and the water in their food – a useful adaptation for the desert.
But the curiosity doesn’t end there! Schmidt-Nielsen wanted to know if they could survive on drinking seawater. Seawater has a concentration of between 1000-1200 mOsm/L. If a human drank 1 litre of seawater, they would need 2 litres of urine to flush out the excess sodium chloride, thus leading to dehydration very quickly. But given the ability of the desert rat kidney’s to concentrate the urine, he reasoned that they should be able to do it.
Firstly, to induce thirst, he fed the rats on a diet of soybeans. These are high in protein and produce a lot of urea that needs to be removed. Normally, the rats don’t drink water so they need to be induced to do this; the high protein diet accomplishes this. He then provided different groups of rats seawater, freshwater and no water. The results are displayed below:
Amazingly, the rats provided with sea water showed no change in body weight after 2 weeks, almost exactly the same as those provided with fresh water. Proof of the tremendous concentrating power of their kidneys, and more specifically, their loops of Henle.
And remember, should you ever be stuck on a becalmed ship or wrecked on a desert island, don’t drink the water!
Coleridge, Samuel Taylor. “The Rime of the Ancient Mariner .” Poetry Foundation, Poetry Foundation, http://www.poetryfoundation.org/poems-and-poets/poems/detail/43997.
Ingles, Dr. Lloyd Glenn. “Merriam’s Kangaroo Rat (Dipodomys merriami)” University of Texas, El Paso. museum2.utep.edu/chih/theland/animals/mammals/dipomerr.htm, 1999. Web. 9 Feb. 2017.
Schmidt-Nielsen, Knut. “Comparative Physiology of Desert Mammals.” Agricultural Experimental Station, vol. 21, Dec. 1962, animalsciences.missouri.edu/research/bec/Brody%20Memorial%20Lectures%201/Lecture%202%20Knut%20Schmidt-Nielsen.pdf.
Willmer, Pat, et al. Environmental Physiology of Animals. 2nd ed., Malden, Mass., Blackwell Publ., 2011.