Book - Vertebrate Zoology (1928) 34
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXXIV Regulatory Mechanisms
All animals below the birds and mammals are what is usually called " cold-blooded," or poikilothermous. Actually, these animals are not so much cold as dependent on the environ- mental temperature, which may be hot. It is a mistake to regard " cold-blooded " animals as necessarily cold, lethargic and sluggish, for in a tropical climate their temperature is high and they may be very active. Nevertheless, since the processes of life can only go on within a certain limited range of tempera- ture, the fact that an animal is dependent on its environment for its temperature necessarily restricts the kinds of environ- ments in which it is capable of living. Further, within the suitable habitat, the degree of activity of the animal will depend on the temperature. This inconstancy of thermal conditions is a serious bar to the further evolutionary progress of the poikilothermous animals.
The advantage which the birds and mammals have in being " warm-blooded " (homothermous) is not only the fact that the temperature at which their biological processes go on is high, but still more the fact that this temperature is constantly maintained, regardless of the temperature of the environment.
The processes of metabolism, and especially muscular activity, entail the production of heat. Some warm-blooded animals shiver when they are cold, and their muscles are then thrown into series of contractions. There is therefore a source of heat within the organism which tends to make the tempera- ture rise. At the same time, heat is continually being lost by radiation from the surface of the animal. The maintenance of a constant temperature within the animal therefore depends on a regulation and balance of the amounts of heat produced and lost. Poikilothermous animals have a temperature only slightly higher than that of the environment. Some seem to be able to raise their temperature slightly for a period by muscular contractions, such as the python when it is coiled round its eggs. But these animals have no means of combating really cold external temperatures, during which they must either hibernate or die. Within limits, the hotter the tempera- ture, the better are the conditions for poikilothermous forms. Some lizards, however (Varanus, Uromastix), when exposed to great heat, increase their rate of breathing very considerably, and so resort to panting. Panting results in the lungs getting rid of large quantities of water vapour, and as heat is absorbed in the conversion of water into vapour, panting means loss of heat also. Uromastix, which inhabits deserts, is dark in colour up to a temperature of 41 ° C, but as the temperature rises above this point, it tends to become white. Since dark colours absorb heat and light colours reflect it, Uromastix has a peculiar mechanism which tends roughly to regulate its intake of heat from the environment. This method, however, is quite different from that of homothermous animals, birds, and mammals. In the first place, the homothermous animals have an external covering which is a bad conductor of heat ; this takes the form of feathers in birds, hairs in terrestrial mammals, and oil or blubber in birds and mammals which lead an aquatic existence. The effect of such a layer is to minimise the loss of heat by radiation. Next, they have more efficient respiratory and vascular systems, notably a four-chambered heart with complete separation of the arterial and venous circulations. In the Monotreme Echidna, the temperature is regulated by varying the amount of heat produced, but it has no method of varying the amount of heat which it loses. It has no sweat- glands, no increase in the amount of blood in the skin (vaso- dilatation), and it does not resort to panting. The heat- production of Echidna varies according to the difference between its temperature and that of the environment. How- ever, this regulation is not very efficient, for if the environ- mental temperature varies from 35 to 5 C, the temperature of the animal will vary by about io° C. Not only is the constancy of the temperature less than that of higher mammals, but the actual normal internal temperature is lower, being about 30 C. In cold weather, Echidna hibernates. Its protective covering of hair is poor, and, like a few other mammals (such as the marmot), it becomes almost poikilo- thermous. On the other hand, in hot weather when the temperature rises above 35 C, Echidna dies of apoplexy (unless it is activating, deep beneath the ground), for its only method of countering a rise in the environmental temperature is to reduce its own internal heat-production, and a point is reached below which it cannot reduce its metabolism and still live.
The other Monotreme, Ornithorhynchus, has a slightly higher normal temperature, 3 2° C, and it keeps it a little more constant. Not only can it vary its heat-production, but it can also vary its loss of heat by means of evaporation of water from its sweat-glands.
The higher mammals regulate their temperature almost entirely by controlling the heat-loss. This they do by three methods : by the evaporation of water from the sweat-glands, by the dilatation of the blood-vessels in the skin, and by the acceleration of respiration or ' ' panting. ' ' The heat-production in these animals is not increased unless the external temperature drops considerably. The Marsupials are intermediate between the Monotremes and the higher mammals in the efficiency of their temperature-regulations.
In birds, heat is lost by evaporation of water through the lungs and air-sacs.
The advantages accruing from the possession of a high and constant internal temperature are very great. Not only does it allow of a higher rate of living, since chemical reactions are accelerated at high temperatures, but it enables differentia- tions and specialisations to arise which would be wrecked if the speed of the metabolic processes (or in other words, the internal temperature) were not constant. Further, it enables the animals to inhabit climates in which poikilothermous forms either cannot live, or have to spend considerable time hibernating against the cold or aestivating against the heat. So it is found that the supreme and dominant animals in arctic regions are the birds and mammals, while in the tropics, reptiles can compete successfully with birds and mammals.
It is interesting to notice that during most of the period of incubation, the embryo chick is poikilothermous. It is only shortly before hatching that it acquires the capacity of maintaining a uniform temperature. The same is true of new-born mice, which become homothermous by the tenth day after birth.
Another matter for which a regulatory mechanism has been evolved in the vertebrates is the osmotic pressure of the blood. Of aquatic invertebrates it may in general be said that their body-fluids have roughly the same osmotic pressure and the same percentage of salts as the water in which they live, and that these vary as the water varies. It is interesting to find that in the Selachii, the osmotic pressure of the blood is not constant either, but varies with the water. The salts in the blood are only about half as concentrated as in sea water, but the blood of Selachians contains urea, which makes up the difference. In the Teleosts, the osmotic pressure of the blood is about one-third that of sea water, but it is kept more or less constant. This regulation is more efficient in some forms than in others ; the osmotic pressure varies with that of the sur- rounding water slightly in the cod, varies more in the plaice, and varies still more in the eel, which alternates between fresh and sea-water. As a rule the osmotic pressure of the blood of fresh- water Teleosts is lower than that of the marine forms.
In the land- vertebrates, the osmotic pressure of the blood is kept constant, and regulated by the kidneys, in spite of variations in food and drink.
In an animal like a Selachian, living in the sea, it is not of much importance if water and salts be lost from the body, as they can be replenished from the medium in which it lives. In a land-vertebrate the case is different, and the loss of water and salts is regulated. The importance of maintaining a constant osmotic pressure of the blood lies in the fact that it entails constancy in the concentration of salts, or in other words, a stable " internal environment " ; and stability of conditions is essential for highly specialised and co-ordinated processes of life.
The relation between the quantities of oxygen and C0 2 in the blood is regulated by the respiratory system, controlled by a centre in the brain. If the blood is rich in C0 2 the respiratory movements are accelerated, and conversely they are retarded if the quantity of C0 2 is low. In this connexion it must be remembered that the respiratory movements of the fish and amphibia are effected by the muscles of the visceral arches. These are visceral muscles, innervated by visceral efferent fibres in the dorsal cranial nerve-roots, and the centre which controls them is in the visceral sensory lobe of the medulla oblongata. In the Selachian (Raia) it is perhaps better to speak of several centres, one corresponding to each of the 7th, 9th, and 10th cranial nerves. Each of these segmental centres in Raia has a degree of autonomy of its own, for if separated from the others by cutting across the medulla, it continues to regulate the muscular movements in the visceral arch or arches to which it is connected.
In the amniotes, however, the respiratory movements are effected by the intercostal muscles (moving the ribs) and the muscles of the diaphragm. These are somatic (myotomic) muscles innervated by somatic efferent fibres through ventral nerve-roots in the region of the neck and trunk. Nevertheless, the " respiratory centre " is still in the medulla oblongata, in the primitive position which it occupied in the fish and amphibia, but it no longer shows the simple segmental arrangement.
Lastly, attention may be paid to two features which the higher vertebrates possess, and which though not strictly regulatory (compensating) mechanisms, nevertheless serve to ensure maximum constancy of conditions. The first of these is concerned with the fact that the ovary and testis in birds and mammals serve not only for the production of reproductive cells, but they also furnish a chemical secretion which evokes and maintains the development of the secondary sexual characters.
The other feature refers to the method of ossification of certain cartilage-bones by means of a diaphysis and two epiphyses, which is characteristic of the mammals. This method enables the bones in question to function as supports and hinges, and at the same time to grow and enlarge so long as the diaphysis and the epiphyses remain separated by cartilage. But once the diaphysis becomes firmly united by bone with the epiphysis at each end of it, the growth of the bone as a wrfole ceases. The maximum size of such bones is therefore limited, as is that of the animal. In several respects, therefore, the higher vertebrates differ from the lower. With the temperature, the osmotic pressure and the acid-base relations of the blood regulated and constant, the higher vertebrates are largely independent of the environment. Indeed, they have a constant internal climate and " environment " of their own, in which they live sheltered from external agencies, with, in mammals, a constant final adult size.
The possession of this " internal environment " is not only one of the chief means of survival of the higher vertebrates, but it has also enabled them to become as specialised and perfected as they are.
Literature
Dakin, W. J. The Osmotic Concentration of the Blood of Fishes taken from Sea Water of naturally varying Concentration, and Variations in the Osmotic Concentration of the Blood and Coelomic Fluids of Aquatic Animals, caused by Changes in the External Medium. Biochemical Journal, vol. 3, 1908.
Haldane, J. S. Respiration. Yale University Press, 1922.
Hide, I. H. Localisation of the Respiratory Centre in the Skate. American Journal of Physiology, vol. 10, 1904.
Krehl, L., und Soetbeer, F. Untersuchungen iiber die Warmeokonomie der Poikilothermen Wirbeltiere. ^Pfliiger's Archiv. f. d. Gesammte Physiologie, vol. 77, 1899.
Martin, C. J. Thermal Adjustments and Regulatory Exchange in Monotremes and Marsupials. Philosophical Transactions of the Royal Society, Ser. B, vol. 195, 1903.
Scott, G. G. A Physiological Study of the Changes in Mustelus Canis produced by Modifications in the Molecular Concentration of the External Medium. Annals of the New York Academy of Science, vol. 23, 1913.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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