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Adaptation to the Desert
LEADING ARTICLES
1025
in an hour) yet have relatively little osmotic disturbance, because much of the loss is of tissue broken down by these animals’ high metabolic rate. In the body-weight/ evaporative-need curve, man and the donkey come just 9 MAY 1964 LONDON before the inflection, and so are able to rely on evaporative cooling when avoidance fails; whereas the dog comes just beyond the bend, when evaporative requirements are rising fast. A dog needs to evaporate, weight Adaptation to the Desert for weight, twice as much as a man, and so must depend IT used to be said that man shared with the burro the much more on avoiding heat. Although a dog can cool ability to withstand desert conditions lethal to all other itself by rapid shallow ventilation, with open mouth and animals.l Certainly anyone who has read McGEE’s lolling tongue, in extreme conditions it will probably account of the Mexican who got lost in the desert for be at a disadvantage compared with a man in whom eight days2 would agree that man’s capacity to survive is from the whole surface of the body. A in an utterly hostile desert environment is sometimes evaporation few months ago there were newspaper reports of a beyond all reasonable expectation. Man and the burro, with their dog dying of thirst in the Australian however, are by no means unique or even outstanding in family after they had run out of petrol. Whereas the men this tolerance. Since the war, workers in California,33 desert will have died from acute water loss, the death of the Israel,4 and elsewhere have recognised that the desert dog must have been partly due to hyperpyrexia. teems with animals, mostly small, which are adapted In limited quantities, water is required by the smaller either by their physiology or by their behaviour to make animals for getting rid of waste products; and there are it their habitat. What is known today of desert animals has now been certain inevitable leaks such as water loss in the stools and the evaporative loss from the skin and lungs. But set out by SCHMIDT-NIELSEN in a fascinating book.5 Here sections of modern physiology are interspersed the true desert animals have kidneys which, by reason of with pages of what can only be described as 19th-century their highly developed Henle’s loop, can concentrate the natural history, complete with the trials and tribulations glomerular filtrate to fantastic proportions: in small of a naturalist at work and anecdotes of what the camel desert mammals such as the jerboa and sand-rat the boy said to the physiologist: it can take its place beside urine/plasma osmotic ratios are more than 15-three or ADOLPH’s classic on the Physiology of Man in the even four times as high as man can achieve. Animals Desert.6 Like the recent volume of the American such as these can thrive on sea water and on salty Handbook on the physiology of adaptation,’ SCHMIDT- vegetation, and, by means of adaptations not yet defined, NIELSEN’S Desert Animals shows how far both behaviour they can consume vegetable oxalate with impunity. and physiology have evolved since the first vertebrate Their calcium metabolism is also interesting. SCHMIDTventured into an environment where heat can be a NIELSEN relates how the urine collected directly from the danger. So successfully have animals surmounted this urethral opening of some small mammals crystallised risk that the publication of a book on Heat Stress and out as it was collected. Carnivores rely to some extent on the water content of Heat Disorders8 seems almost a confession of adaptatheir tional backsliding on the part of modern man. Much victims, and on the water of oxidation of the flesh, of what Nature has to teach us seems to have been for- for excreting the quantity of urea they inevitably progotten, though it can be as necessary in the modern duce. Though they cannot concentrate urine as highly as the desert rodents, they can generally concentrate it workshop or mine 9 10 as it was to our ancestors. The two main threats of the desert are the high more than their usually ungulate victims: cats, for ambient and radiant temperatures during the day and example, can drink sea waterY This ability of the the continual lack of water. Applying the body-weight/ carnivore to live on the body water of the animals it eats surface-area rule, animals fall into two classes. There may be likened to the supposed ability of the fish-eating seal to keep in water and osmotic balance while living in a are the larger animals, which can keep cool by the small of but of amounts water, may (like hypertonic environment. But, as WOLF 12 has pointed evaporation man) be fatally dehydrated when their loss of water out, it all depends on the sort of fish the seal chooses to reaches about 15% 6; and there are the smaller and very eat; with heavy protein-rich fish, like the halibut, small animals, which may lose a surprising proportion of balance would not be possible; but on a diet of Pacific their weight (the kangaroo-rat may lose 13% of its 100 g. herring, with which the calculation was originally made, the seal could well maintain it. If the seal was like the 1. Dill, D. B. Life, Heat, and Altitude. Harvard University Press, 1938. 2. McGee, W. J. Interstate med. J. 1906, 15, 279. 3. Chew, R. M. Biol. Rev. 1961, 36, 1. penguin and other sea birds, and could excrete salt from 4. Bodenheimer, F. S. in Desert Research, Research Council of Israel, its nasal glands,13 it would not have to be choosy about special publication no. 2. Jerusalem, 1953; p. 205. 5. Schmidt-Nielsen, K. Desert Animals: Physiological Problems of Salt its diet; but since the seal lacks this accomplishment, and Water. London: Oxford University Press 1964. Pp. 277. 45s. 6. Adolph, E. F., and associates. Physiology of Man in the Desert. New shellfish, for example, as with many of us, may disagree York and London, 1947. 7. Dill, D. B., Adolph, E. F., Wilber, C. G. (editors) Adaptation to the within.14 Astonishingly, camels and other ungulates can
THE LANCET
Environment. American handbook of physiology, section 4. American
8.
Physiological Society, 1964. Leithead, C. S., Lind, A. R. Heat Stress 1964.
9. 10.
Wolf, A. V., Prentiss, P. G., Douglas, L. G., Swett, R. J. Amer. J. Physiol. 1959, 196, 633. 12. Wolf, A. V. Nutr. Rev. 1956, 14, 161. 13. Schmidt-Nielsen, K., Slader, W. J. L. Nature, Lond. 1958, 181, 1218. 14. Parry, G. in Adaptation to the Environment (footnote 7).
11.
and Heat Disorders.
Caplan, A. Trans. Instn Min. Metall., Lond. 1944, 53, Ladell, W. S. S. Brit.J. industr. Med. 1955, 12, 111.
95.
London,
1026
cycle their urea when they are protein-deficient; the urea is diffused back into the rumen and is utilised there by bacteria for the synthesis of bacterial proteins which pass on down the gut and are digested and absorbed, and thus both nutritional and secretory problems are solved. Especially with big animals, the amount of water lost in the stools is largely a reflection of the water intake. The fxces of desert rodents which habitually take no free water, and live on dry food such as grain, contains less than solids, whereas in animals which live on a wet diet the reverse is true. Anyone with experience of human dehydration knows how the fxces grow more inspissated as the dehydration develops-a source, it is related, of great discomfort to shipwrecked mariners. Evaporation from the skin can be kept low by keeping the skin cool and protected from radiation-a purpose well served by an animal’s fur. Evaporation from the respiratory tract is a function of the temperature of the expired air: air can carry only as much water as will saturate it, and the lower the temperature of the air the smaller this amount will be. The kangaroo-rat has a counter-current cooling system in the nose; the incoming hot dry air passes through the wet nasal passages, evaporating the moisture and cooling the mucosa as it goes. On expiration, the air laden with moisture at body temperature passes back over the cool turbinates; and, as these are now well below the dewpoint of alveolar air, water is deposited as it passes over them and the air eventually expired contains much less. Because of this and of its other water savings in the kidneys and in the stools, the kangaroo-rat can live without access to water at any time. Ungulates, having a lot of water in their gut, can go without drinking for several days. But when they return to the water they drink their fill fast: SCHMIDT-NIELSEN has seen camels take in more than 20% of their body water in ten minutes. A suggestion has been made that the camel avoids possible hxmolytic effects of rapid water
absorption by having abnormally " strong " erythrocytes, a cow’s red cells being completely haemolysed at 0.36% sodium chloride, whereas a camel’s can withstand 0-11%. Some animals may lose more than a third of their body-weight, mostly from the gut, before they have to drink again. The camel, like man, will on occasion overdrink-possibly, as man may do, in a deliberate attempt to cool itself; but in general most animals monitor their drinking very carefully. From fistula experiments on the dog it seems that there may be two monitors for volume, one (in the oesophagus) concerned with flow, and the other (in the stomach) with distension.16 Other monitors on the electrolyte content of the fluid water
drunk report with these to the thirst centres.17 When an animal’s water is restored, there is a very sharp cut-off, whereas in man there may be temporary overhydration, soon corrected by diuresis.18 But contrary to SCHMIDTNIELSEN’S belief, man too can rehydrate fast: perhaps he has never heard of the " yard of ale contests " ? The secret of desert survival is to waste no water, 15. Perk, K. Nature, Lond. 1963, 200, 272. 16. Adolph, E. F., Barker, J. P., Hoy, P. A. Amer. J. Physiol. 1954, 17. Andersson, B., McCann, S. M. J. Physiol. 1962, 129, 33P. 18. Ladell, W. S. S. Brit. med. Bull. 1947, 5, 9.
178, 538.
either in excretion or otherwise, and to avoid the heat. There is no trick which some animal or other does not adopt. As the Arab, with his thick woollen robes, has discovered, fur keeps the heat out as well as in. When animals stand close together, they present a smaller collective radiation area to the sun and to the terrain; so they keep close to keep cool. The kangaroo-rat and other small desert animals burrow deep into the sand where the temperature is constant and well below that of the surface -a circumstance now known to air-conditioning engineers who use the subsoil as a heat sink. The very threat itself can be turned to good purpose; but for the sun’s heat, the muscles of poikilothermic reptiles would not reach the temperatures at which they can be really active; and the camel and other desert animals whose body temperature is more labile than that of animals in temperate climates have a reduced radiation and convective gain when their body and skin temperatures rise. Living tissue depends on the constituent protein; above 40° C cytoplasmic protein begins to be denatured, and 40° C is also the threshold subcutaneous temperature for pain.19 Hence it is not surprising that the lethal limit for body temperature should be within a few degrees for all animals, whether reptiles or birds, placental mammals or marsupials; the lowest lethal limit cited by SCHMIDTNIELSEN is 445° for the kangaroo-rat, and birds and lizards can be heated to 46° or 47° before they succumbthe lizard showing a reversible loss of neuromuscular control before actual death. There is a similar but only slightly wider range in the resting temperature of homotherms generally. What more striking demonstration could there be of the common units of the stuff of life ?
Purification of a Gene THE capacity or " competence " of bacteria to absorb D.N.A. molecules from their environment, and hence to become genetically transformed, is a direct clue to the role of D.N.A. in the hereditary mechanism of cells. Transformation was first observed thirty-six years ago by GRIFFITH.2O He injected mice with a mixture of two strains of pneumococci-one live and non-virulent, the other virulent but heat-killed. Many mice died, and postmortem cultures showed the presence of a live virulent strain of the type contained in the heat-killed inoculum. This remarkable experiment was repeated and confirmed by other research-workers, and it was suggested that the heat-killed cells contained a substance capable of producing a predictable and type-specific change in other strains of pneumococci. The transformation of the non-encapsulated (non-virulent) into the encapsulated (virulent) pneumococci was found to take place in vitro 21; and in 1932 ALLOWAY 22 separated an active principle from the virulent, encapsulated pneumococci. Cell-free extracts of the " transforming principle were purified by AvERY et a1.23 in 1944. The purified agent could bring about transformation at very high dilution. It was identified as D.N.A. and was devoid of "
19. 20. 21. 22. 23.
Benjamin, F. B. J. appl. Physiol. 1951-52, 4, 907. Griffith, F. J. Hyg., Camb. 1928, 27, 113. Dawson, M. H., Sia, R. H.. P. J. exp. Med. 1931, 54, 681. Alloway, J. L. ibid. 1932, 55, 91. Avery, O. T., MacLeod, C. M., McCarty, M. ibid. 1944, 79, 137.
1025
in an hour) yet have relatively little osmotic disturbance, because much of the loss is of tissue broken down by these animals’ high metabolic rate. In the body-weight/ evaporative-need curve, man and the donkey come just 9 MAY 1964 LONDON before the inflection, and so are able to rely on evaporative cooling when avoidance fails; whereas the dog comes just beyond the bend, when evaporative requirements are rising fast. A dog needs to evaporate, weight Adaptation to the Desert for weight, twice as much as a man, and so must depend IT used to be said that man shared with the burro the much more on avoiding heat. Although a dog can cool ability to withstand desert conditions lethal to all other itself by rapid shallow ventilation, with open mouth and animals.l Certainly anyone who has read McGEE’s lolling tongue, in extreme conditions it will probably account of the Mexican who got lost in the desert for be at a disadvantage compared with a man in whom eight days2 would agree that man’s capacity to survive is from the whole surface of the body. A in an utterly hostile desert environment is sometimes evaporation few months ago there were newspaper reports of a beyond all reasonable expectation. Man and the burro, with their dog dying of thirst in the Australian however, are by no means unique or even outstanding in family after they had run out of petrol. Whereas the men this tolerance. Since the war, workers in California,33 desert will have died from acute water loss, the death of the Israel,4 and elsewhere have recognised that the desert dog must have been partly due to hyperpyrexia. teems with animals, mostly small, which are adapted In limited quantities, water is required by the smaller either by their physiology or by their behaviour to make animals for getting rid of waste products; and there are it their habitat. What is known today of desert animals has now been certain inevitable leaks such as water loss in the stools and the evaporative loss from the skin and lungs. But set out by SCHMIDT-NIELSEN in a fascinating book.5 Here sections of modern physiology are interspersed the true desert animals have kidneys which, by reason of with pages of what can only be described as 19th-century their highly developed Henle’s loop, can concentrate the natural history, complete with the trials and tribulations glomerular filtrate to fantastic proportions: in small of a naturalist at work and anecdotes of what the camel desert mammals such as the jerboa and sand-rat the boy said to the physiologist: it can take its place beside urine/plasma osmotic ratios are more than 15-three or ADOLPH’s classic on the Physiology of Man in the even four times as high as man can achieve. Animals Desert.6 Like the recent volume of the American such as these can thrive on sea water and on salty Handbook on the physiology of adaptation,’ SCHMIDT- vegetation, and, by means of adaptations not yet defined, NIELSEN’S Desert Animals shows how far both behaviour they can consume vegetable oxalate with impunity. and physiology have evolved since the first vertebrate Their calcium metabolism is also interesting. SCHMIDTventured into an environment where heat can be a NIELSEN relates how the urine collected directly from the danger. So successfully have animals surmounted this urethral opening of some small mammals crystallised risk that the publication of a book on Heat Stress and out as it was collected. Carnivores rely to some extent on the water content of Heat Disorders8 seems almost a confession of adaptatheir tional backsliding on the part of modern man. Much victims, and on the water of oxidation of the flesh, of what Nature has to teach us seems to have been for- for excreting the quantity of urea they inevitably progotten, though it can be as necessary in the modern duce. Though they cannot concentrate urine as highly as the desert rodents, they can generally concentrate it workshop or mine 9 10 as it was to our ancestors. The two main threats of the desert are the high more than their usually ungulate victims: cats, for ambient and radiant temperatures during the day and example, can drink sea waterY This ability of the the continual lack of water. Applying the body-weight/ carnivore to live on the body water of the animals it eats surface-area rule, animals fall into two classes. There may be likened to the supposed ability of the fish-eating seal to keep in water and osmotic balance while living in a are the larger animals, which can keep cool by the small of but of amounts water, may (like hypertonic environment. But, as WOLF 12 has pointed evaporation man) be fatally dehydrated when their loss of water out, it all depends on the sort of fish the seal chooses to reaches about 15% 6; and there are the smaller and very eat; with heavy protein-rich fish, like the halibut, small animals, which may lose a surprising proportion of balance would not be possible; but on a diet of Pacific their weight (the kangaroo-rat may lose 13% of its 100 g. herring, with which the calculation was originally made, the seal could well maintain it. If the seal was like the 1. Dill, D. B. Life, Heat, and Altitude. Harvard University Press, 1938. 2. McGee, W. J. Interstate med. J. 1906, 15, 279. 3. Chew, R. M. Biol. Rev. 1961, 36, 1. penguin and other sea birds, and could excrete salt from 4. Bodenheimer, F. S. in Desert Research, Research Council of Israel, its nasal glands,13 it would not have to be choosy about special publication no. 2. Jerusalem, 1953; p. 205. 5. Schmidt-Nielsen, K. Desert Animals: Physiological Problems of Salt its diet; but since the seal lacks this accomplishment, and Water. London: Oxford University Press 1964. Pp. 277. 45s. 6. Adolph, E. F., and associates. Physiology of Man in the Desert. New shellfish, for example, as with many of us, may disagree York and London, 1947. 7. Dill, D. B., Adolph, E. F., Wilber, C. G. (editors) Adaptation to the within.14 Astonishingly, camels and other ungulates can
THE LANCET
Environment. American handbook of physiology, section 4. American
8.
Physiological Society, 1964. Leithead, C. S., Lind, A. R. Heat Stress 1964.
9. 10.
Wolf, A. V., Prentiss, P. G., Douglas, L. G., Swett, R. J. Amer. J. Physiol. 1959, 196, 633. 12. Wolf, A. V. Nutr. Rev. 1956, 14, 161. 13. Schmidt-Nielsen, K., Slader, W. J. L. Nature, Lond. 1958, 181, 1218. 14. Parry, G. in Adaptation to the Environment (footnote 7).
11.
and Heat Disorders.
Caplan, A. Trans. Instn Min. Metall., Lond. 1944, 53, Ladell, W. S. S. Brit.J. industr. Med. 1955, 12, 111.
95.
London,
1026
cycle their urea when they are protein-deficient; the urea is diffused back into the rumen and is utilised there by bacteria for the synthesis of bacterial proteins which pass on down the gut and are digested and absorbed, and thus both nutritional and secretory problems are solved. Especially with big animals, the amount of water lost in the stools is largely a reflection of the water intake. The fxces of desert rodents which habitually take no free water, and live on dry food such as grain, contains less than solids, whereas in animals which live on a wet diet the reverse is true. Anyone with experience of human dehydration knows how the fxces grow more inspissated as the dehydration develops-a source, it is related, of great discomfort to shipwrecked mariners. Evaporation from the skin can be kept low by keeping the skin cool and protected from radiation-a purpose well served by an animal’s fur. Evaporation from the respiratory tract is a function of the temperature of the expired air: air can carry only as much water as will saturate it, and the lower the temperature of the air the smaller this amount will be. The kangaroo-rat has a counter-current cooling system in the nose; the incoming hot dry air passes through the wet nasal passages, evaporating the moisture and cooling the mucosa as it goes. On expiration, the air laden with moisture at body temperature passes back over the cool turbinates; and, as these are now well below the dewpoint of alveolar air, water is deposited as it passes over them and the air eventually expired contains much less. Because of this and of its other water savings in the kidneys and in the stools, the kangaroo-rat can live without access to water at any time. Ungulates, having a lot of water in their gut, can go without drinking for several days. But when they return to the water they drink their fill fast: SCHMIDT-NIELSEN has seen camels take in more than 20% of their body water in ten minutes. A suggestion has been made that the camel avoids possible hxmolytic effects of rapid water
absorption by having abnormally " strong " erythrocytes, a cow’s red cells being completely haemolysed at 0.36% sodium chloride, whereas a camel’s can withstand 0-11%. Some animals may lose more than a third of their body-weight, mostly from the gut, before they have to drink again. The camel, like man, will on occasion overdrink-possibly, as man may do, in a deliberate attempt to cool itself; but in general most animals monitor their drinking very carefully. From fistula experiments on the dog it seems that there may be two monitors for volume, one (in the oesophagus) concerned with flow, and the other (in the stomach) with distension.16 Other monitors on the electrolyte content of the fluid water
drunk report with these to the thirst centres.17 When an animal’s water is restored, there is a very sharp cut-off, whereas in man there may be temporary overhydration, soon corrected by diuresis.18 But contrary to SCHMIDTNIELSEN’S belief, man too can rehydrate fast: perhaps he has never heard of the " yard of ale contests " ? The secret of desert survival is to waste no water, 15. Perk, K. Nature, Lond. 1963, 200, 272. 16. Adolph, E. F., Barker, J. P., Hoy, P. A. Amer. J. Physiol. 1954, 17. Andersson, B., McCann, S. M. J. Physiol. 1962, 129, 33P. 18. Ladell, W. S. S. Brit. med. Bull. 1947, 5, 9.
178, 538.
either in excretion or otherwise, and to avoid the heat. There is no trick which some animal or other does not adopt. As the Arab, with his thick woollen robes, has discovered, fur keeps the heat out as well as in. When animals stand close together, they present a smaller collective radiation area to the sun and to the terrain; so they keep close to keep cool. The kangaroo-rat and other small desert animals burrow deep into the sand where the temperature is constant and well below that of the surface -a circumstance now known to air-conditioning engineers who use the subsoil as a heat sink. The very threat itself can be turned to good purpose; but for the sun’s heat, the muscles of poikilothermic reptiles would not reach the temperatures at which they can be really active; and the camel and other desert animals whose body temperature is more labile than that of animals in temperate climates have a reduced radiation and convective gain when their body and skin temperatures rise. Living tissue depends on the constituent protein; above 40° C cytoplasmic protein begins to be denatured, and 40° C is also the threshold subcutaneous temperature for pain.19 Hence it is not surprising that the lethal limit for body temperature should be within a few degrees for all animals, whether reptiles or birds, placental mammals or marsupials; the lowest lethal limit cited by SCHMIDTNIELSEN is 445° for the kangaroo-rat, and birds and lizards can be heated to 46° or 47° before they succumbthe lizard showing a reversible loss of neuromuscular control before actual death. There is a similar but only slightly wider range in the resting temperature of homotherms generally. What more striking demonstration could there be of the common units of the stuff of life ?
Purification of a Gene THE capacity or " competence " of bacteria to absorb D.N.A. molecules from their environment, and hence to become genetically transformed, is a direct clue to the role of D.N.A. in the hereditary mechanism of cells. Transformation was first observed thirty-six years ago by GRIFFITH.2O He injected mice with a mixture of two strains of pneumococci-one live and non-virulent, the other virulent but heat-killed. Many mice died, and postmortem cultures showed the presence of a live virulent strain of the type contained in the heat-killed inoculum. This remarkable experiment was repeated and confirmed by other research-workers, and it was suggested that the heat-killed cells contained a substance capable of producing a predictable and type-specific change in other strains of pneumococci. The transformation of the non-encapsulated (non-virulent) into the encapsulated (virulent) pneumococci was found to take place in vitro 21; and in 1932 ALLOWAY 22 separated an active principle from the virulent, encapsulated pneumococci. Cell-free extracts of the " transforming principle were purified by AvERY et a1.23 in 1944. The purified agent could bring about transformation at very high dilution. It was identified as D.N.A. and was devoid of "
19. 20. 21. 22. 23.
Benjamin, F. B. J. appl. Physiol. 1951-52, 4, 907. Griffith, F. J. Hyg., Camb. 1928, 27, 113. Dawson, M. H., Sia, R. H.. P. J. exp. Med. 1931, 54, 681. Alloway, J. L. ibid. 1932, 55, 91. Avery, O. T., MacLeod, C. M., McCarty, M. ibid. 1944, 79, 137.