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Computer Graphics	Repose or absence of muscular activity Exactly as there are great variations in the anatomical structure and exterior make-up of warm-blooded animals there are corresponding variations in their needs for activity during daily life, and with the superimposition of their activities variations in their body temperatures are found. Animals are not constructed simply and solely to remain quiescent. They must be able to live and reproduce their species under conditions obtaining on the earth and in the air. It thus follows that these animals must have a structure enabling them to increase greatly their activity and their heat production and at times to an extremely great degree. Hence it is conceivable that one should judge the anatomy and comparative physiology of various animals upon the maximum heat that they can produce. In other words, the machine must be constructed to stand the greatest strains possible and not simply to run idly. In their combat throughout life, which means the survival of the fittest, animals must escape enemies. This requires both ability for flight and to fight, that is, strength to out-distance the pursuing enemy or strength to overcome the enemy in individual combat. The effort of flight or chase may be continued for hours. This of itself is no mean energy demand. That of combat may be continued for an even greater time but usually is of much shorter duration. In the same species, quite aside from efforts to secure food, there are combats for supremacy between males. These may be strenuous and long-continued, although for the most part they are seasonal, but in the struggle for existence animals must be so designed as to conquer in such combats. Hence during a large part of its life the muscular activity of an animal may be at a level far above that required for mere existence, and the design of the organism is imperfect unless provision is made for the capacity for these intense muscular efforts, not only momentary but often sustained. Because of the tremendous variability in the nature and demands of these combats it is conceivable that no comparison between animals can be drawn upon the basis of their potential maximum effort. In listing these demands emphasis has been placed upon the maximum demand possible, what one might call the "breaking load" for animals. The effort, when prey must be chased, may be a daily occurrence. On the other hand, when animals are to escape the enemy, the effort may be much less, as but a small number of a group will be selected for attack on any one day. In general, however, in the life of every animal a certain amount of effort will be spent in searching for food and, if not escaping from an enemy, in securing points of advantage where the enemy can not easily surprise and capture it. This effort may be nothing more than the alertness, for example, of the bird, but it necessitates muscular movements that are essential to the life of the organism. Few, if any, measurements have been made with regard to the maximum possible effort that can be expended by various animal species. Because of his intelligent cooperation man has been studied more than any other animal, and it has been found that man is capable of sustained effort resulting in an energy expenditure of ten or more times his basal needs for an hour or longer. From studies of the respiratory exchange during explosive efforts, such as sprinting, and based upon the calculation of the oxygen debt (which is measured only with difficulty), it has been shown that man is capable of muscular effort for 10 seconds which would be comparable to an energy expenditure of 3.5 calories per second. This gives a fairly good idea of the highest strain to which the human organism can be put. The body may be and, indeed, must be designed for maximum rather than basal effort, but the maximum effort is seldom expended and with most individuals may never be called for. Practically no measurements have been made on other animals from this standpoint, and hence it is out of the question to attempt to compare animals on the basis of their maximum capacity. It is conceivable, however, that animals may be compared with reference to their heat production during the entire course of a day's life. If the effort required for the usual activities of life were the same proportionately with all animals, then measurement of the day's energy needs would be an excellent basis for comparison, but because of the variations in milieu from the severe cold of the Siberian wastes to the intense heat of the tropics and because of the scarcity of food in the northern climate and the abundance of food at the equator, great variations in the daily efforts that are necessarily put forth to secure food are to be expected. It is inconceivable that animals live for any appreciable part of the day at or near a minimum level of energy expenditure. They must get food, and most animals are the exact antithesis to domestic animals, such as the hog, whose food is brought to it by man. Hence one of the most important needs in the daily life of an animal is the activity necessary to obtain food. The body must be prepared for an enormous increase in activity, at times of explosive nature, although this latter is not the rule but rather the exception. This situation applies more specifically to the carnivorous animals and the birds of prey. The herbivorous animals are much more fortunate in this respect. They may be chased by animals of prey, but they have no long chase to secure their food. However, they may have seasonally long treks to secure new pasture or grazing grounds, new water holes and salt licks. The carnivorous animals must chase their prey and attack them in order to secure sustenance. Perhaps no animal secures its prey with such a minimum of exertion as does the snake. The snake lies in wait near some water hole or some path to a water hole over which its prey must pass. The time element plays no particular role. The snake may lie there not only for days and weeks but for months. When the prey approaches, there is a short attack, a short struggle, the prey is then engulfed, and the snake's transient period of intense activity (barely 20 minutes) is followed by a long period of repose. These activities or essential life processes vary enormously in degree. As implied above, probably the lowest heat production incidental to the securing of food by the vertebrates is that of the snake. Probably the greatest energy expended in the effort to secure food is by birds of prey. Among the insectivorous birds, the energy expenditure is proportionately greater with the swallow. Its efforts to catch its daily needs of flying insects require miles of flight with continual changes in direction of flight and with accompanying greater energy needs. Among the mammalia the domestic animals probably expend the least effort to obtain food. They are cared for by man, and are given food garnered, prepared, and presented by man. The hog, for example, needs to exert no effort to get its food, it has no struggle against enemies, and it has a wonderful digestive activity enabling it to digest a high proportion of its food. Of the large mammals perhaps the elephant makes the highest expenditure for food. It eats incessantly, and prehension and digestion are continual processes. The great energy demands incidental to securing food are not determined solely by the elephant's enormous size, but by its poor digestive capacity. In the wild it must often travel miles and trample down vast areas in getting its food, which is not usually sufficiently supplied by simple browsing. Here is an herbivorous animal having the highest heat production of any animal that has been studied, which must get its food by muscular effort not in simple grazing but in tearing up roots and young growth and in pulling down twigs and branches of trees, and which after all this effort has such a poor digestion that its intake of food must be tremendous in order to supply enough for life. In view of these greatly varying daily activities among the different animals there is obviously little likelihood of finding any uniformity in the total heat production for the day of these various animals. Their daily requirements for securing food differ enormously, as exemplified by the snake that eats once and then may have complete repose for days or weeks and the swallow or bat incessantly flying after insects and the elephant incessantly tearing through the forest in its search for food, only a small proportion of which is of value to it. It is, therefore, inconceivable to think of comparing the energy needs or energy output of these animals with varying activity during the whole day. TIME OF DAY The question arises as to whether the comparison of these animals should be based on their metabolism at night, when major intensive muscular activity does not exist. Is there any similarity in the heat production of the various animals at night when in muscular repose? Obviously in this type of comparison the nocturnal animals that invert the daily rhythm of activity and sleep during the day should be disregarded or else measured during the daytime. Rats, which are nocturnal animals, have been found to be quietest during the day, especially with a bright light playing on them. The cat has not been studied to determine whether its metabolism is lower in the daytime than at night, but from its nocturnal habits one would predict that its metabolism is lower during the daytime. The metabolism of the goose is not significantly different at the two times of day, provided it is kept in darkness. On the other hand, both the cock and the hen have a lower metabolism at night. Whether this is due solely to the darkness (this is doubtful, for darkness can be secured during the daylight by keeping the animal in a light-free chamber) or to a decrease in external noises (a sound-proof chamber was not available) is not known, although undoubtedly the external stimulus plays a role. Even animals that might sleep in the daytime might have a higher metabolism than when sleeping at night because susceptible to external noises, which are more prevalent during the daytime. This susceptibility to external stimuli has been noted with semi-dormant animals, such as the marmot. The chimpanzee is particularly prone to remain extremely quiet in complete darkness, but even when apparently asleep is extraordinarily sensitive to external conditions, even to the creaking of a board in the floor, and the ingenuity of the operator is taxed to maintain complete quietness in the experimental room where such animals are kept. This characteristic of the chimpanzee is probably a retention of the habits of wild animals in the forest, which are awakened from sleep at any time during the day or night by external noises and are instantly on the alert. As the object is to measure the metabolism unaffected by external factors, the time of day can not be disregarded in careful basal metabolism studies. DEEP SLEEP Although animals vary in their external muscular activities (which result in heat production) and in their internal digestive processes (which also result in heat production), is it possible to eliminate both of these and then to compare the various animals when there is no external muscular activity and no digestive activity? This search for a basis of comparison of the metabolism of different animal species is essential, for this comparison has reference primarily to the endogenous cell processes normally occurring in animals. When these cell processes are whipped into increased action by excessive activity, one can not expect the increased cell action to be uniform for any two species of animals, because the activity itself is not uniform. Sleep favors complete relaxation, although even during sleep there may be some digestive activity from the after-effect of previously eaten food. An animal lying down, motionless, is in the ideal condition so far as absence of major, visible muscular activity is concerned. However, even under this condition there still may be a considerable amount of tension, a slight interplay of muscles, not sufficient to be considered a tremor but nevertheless an existing play of muscles. During sleep this tension is obviously somewhat less, although muscular movements are not entirely ruled out. It has been clearly demonstrated that the metabolism of humans even when well-conditioned, post-absorptive, and immobile is about 10 per cent lower during sleep than when the same individuals are awake. This point has been further accentuated by the fact that a group of women who have normally the lowest heat production of any of the human race thus far studied when awake, that is, the Tamils in India, likewise have a still further lowering in heat production of 10 per cent when they go to sleep. 1 The exact definition of sleep with animals is somewhat uncertain. There is clear evidence that the elephant usually goes to sleep almost instantly on lying down. When it is standing, it may doze, but whether it sleeps deeply while standing is uncertain. The respiration rate is one of the best indices of this condition. When the elephant is standing quietly and awake, the rate may be 11 per minute, but when it is lying and is asleep the rate will drop to 5 per minute. Sleep may result in a decrease in the heat production of the mouse of circa 25 per cent. If for comparative purposes measurements of the heat production must be restricted to those periods when animals are asleep, then the study of comparative metabolism will have to stop, for the animals that can be studied while asleep are extremely few. Consequently sleep is not insisted upon as a prerequisite, but if the factor of sleep enters into the measurements then there should be something simulating a rhythm in the metabolism, which should decrease below the average minimum level when the animal is awake and then return to this level. But then the difficulty is that we are counting upon a super-basal activity to counteract the depressant influence of sleep. In any event so far as this somewhat subtle factor is concerned we have nothing but a mélange, still definitely unclassified or undefined although technically termed basal metabolism. The so-called "basal metabolism" of a human being, which by common consent today is measured 12 hours after the last meal and in a state of complete muscular repose, is not by any means the lowest metabolism compatible with human life. Other factors appear to lower the metabolism. In any metabolism measurements the operator always has to deal with a metabolism above basal, rarely if ever below basal. In recent years there has been a trend in certain German laboratories to emphasize the finding of a negative phase of metabolism, that is, a metabolism much lower than normal or basal. That it is possible to lower the metabolism below basal is, however, not denied. An especially careful study on a group of people with a normally low metabolism, that is, the Tamils in India, has shown a tendency for the metabolism to be lowered 10 per cent by sleep. On the other hand, so-called "hypnotic sleep" did not result in any appreciable lowering of the metabolism. Only in recent years, however, has a technique been available for studying accurately the metabolism during sleep. Most of the earlier studies were based upon mouthpiece or face mask measurements. The tendency for the mouth to relax and the mouthpiece to loosen during sleep, thus making complete closure questionable, is only too easily recognized. With the advent of the helmet apparatus, however, studies during sleep are easily and rapidly made. BODY POSITION With some animals the body position plays an especially important role in basal metabolism measurements. Humans and most other animals are studied while lying. The dog, for example, when lying, is unquestionably under less muscular tension than when standing. On the other hand, it is believed that the horse stands with a minimum amount of exertion, possibly no greater than when it is lying. The elephant stands during much of the day, often when sleeping. It may be assumed that the position assumed by animals when taking their rest or sleep is probably the position with least muscular strain. However, for such animals as are studied in the standing position (the cow, the steer, the horse, and other large domestic animals) it is customary to correct the metabolism measurements for the extra energy required in changing the body position from lying to standing or vice versa, if such movements took place during the period of measurement. The correction factors to be applied in such cases have already been discussed in detail elsewhere.