Science in Christian Perspective



Physiological Changes With Population Increase*
Marlin B. Kreider**

From: JASA 14 (September 1962): 49-51

Man, like all other forms of life, is influenced by his environment. This environment is made up of the social as well as the physical surroundings. However, man, in turn, through his superior intellect, can influence and alter this environment. He has done this frequently in the past in order to contribute to his immediate welfare, but many times, in the case of the physical environment, he has disregarded the long-term effect of his interference. Since the influence of the environment may become more critical as the human population increases, some of the more obvious limitations of the physical environment will be presented here briefly, followed by a discussion of the less obvious effects of the social and psychological environment on the physiology.

The availability of food and water has a strong influence on all living things. This factor is of most critical or primary importance in any consideration of increasing populations and is currently a serious problem for many peoples of the earth. A man can live for about 12 days without water (1) and somewhat longer than a month without food (2) under optimum conditions of temperature and rest and for varying lengths of time with inadequate or reduced diets. The availability of food is influenced by the meteorological conditions. In the extreme hot and cold barren areas of the earth the small amounts of existing life available for food will support a limited number of men.

In addition to its effect on other animals and plants, climate has a direct effect on man. Since man cannot adapt biologically to these extremes, he must construct elaborate cultural buttresses for protection. All this increases greatly the effort required to survive in such climates.

Another environmental factor is the gaseous content of the inspired air. For man, a decrease of oxygen concentration or pressure by one half, poses a serious problem as does the increase of carbon dioxide to as little as 3 percent of the inspired air for any length of time. This is not generally a problem for man except when he goes below or far above sea level but could conceivably become a problem due to large-scale manipulation by man.

The problem of adequate physical space has disturbed man thus far only when he voluntarily insisted on living in crowded areas or was forced to do so by manmade boundaries such as immigration quotas and socioeconomic patterns. Man's need for space may be more sociological and psychological than it is physiological. Certainly a high population density creates more human encounters and limits freedom, therefore it would seem axiomatic that more social and psychological problems would develop. This is borne out by studies of both animal (3) and human behavior. But in addition to affecting behavior there is evidence that these social and psychological problems created by a high population density, also create tensions that affect the physiological functions of the body. This will be discussed later. In addition to the direct effect of limited space on man is its effect on the biotic world and thus on man's food supply. An example of this effect is the'need of plants for space for roots and foliage and of some animals for more than one environment or location in order to complete their life cycles.

In the process of changing his own environment man has frequently altered the environment of plants and animals. This includes altering the water supply through massive earth removal projects or destruction of forests, cutting off the sun, poisoning the air and water with industrial wastes (4), and by numerous other means. By such acts man has hindered die growth and survival of plants and animals which at the same time decreased his own food supply. He also has upset the balances of nature by destroying certain species in an area, encouraging the population growth of another species upon which it feeds, which may then become a plant of animal pest of man (5).

The following example of man's disturbance of the intricate balance of nature is taken from a college biology text (6).

"In protecting domestic animals and expressing a dislike for certain kinds, man may have killed off as far as possible all hawks, owls, wolves, foxes, coyotes, mountain lions and snakes from an area. As a result the grass on which he sought to pasture his herd went into decline because there was no longer any appreciable check on the mouse and grasshopper population. No hawks, owls, foxes, and snakes meant more mice which scoured the region thoroughly for grass seeds to feed their increasingly numerous young and thereby stopped the grass from maintaining its normal rate of reproduction. And by killing off the wolves, mountain lions and coyotes there was not an adequate check on the deer and elk population which soon increased beyond the number for which there was food. As a result, they frantically chewed on the young growth of every tree within reach on the mountainside and killed off the forest so thoroughly that the snow melted and ran off in eroding streams early in the summer, leaving the grass lands dry and parched later in the season, because the water had

*Presented at the Sixteenth Annual Convention of the American Scientific Affiliation held at Houghton, New York, August, 1961.

**Dr. Kreider was with the U, S. Army Quartermaster Research and Engineering Center, Natick, Massachusetts. Present address is U. S. Army Research Institute of Environmental Medicine, Natick, Massachusetts.

not sunk in and become added to the water table."

By such action man has decreased the potential food supply which is already short in many parts of the earth. It is very essential that the indifference of the past concerning the destruction of other living things, be replaced by a planned farming of the land and water and proper disposal of harmful wastes. With an increasing human population, the balance of nature will become more difficult to maintain and could conceivably be the limiting factor on human population growth. Lack of food would serve as a stressful situation decreasing the general health and vitality and the reproductive potential.* This is supported by many studies concerning the effects of stress on the body (7).

Apart from these limiting factors of the physical environment are the physiological effects on the physiology of the sociopsychological stresses which exist in a society of high population density. No one needs to be convinced that man suffers from social and emotional tensions and stresses. The development of peptic ulcers and the ejection and riddance response of the large bowel give evidence of harmful results of nervous stresses. Wolff (8) described this ejection and riddance response. He said "a given subject confronted by overwhelming environmental demands may elaborate a pattern of ejection. Thus a person who has taken on more than he can handle or feels inadequate to the demands of his life situation or a thwarted person filled with hatred, defiance, contempt and the unconscious need to be rid of a threatening or overwhelming situation, yet passive withal, may suffer from diarrhea." This person though outwardly calm is "sitting on a powder keg" of intense hostility, resentment and guilt. This "ejection response" is a sign of mental disorder and breakdown. Dr. Wolff also states that it is the pressure of competition for social position and economic security that shortens man's life over that of his wife who is spared much of this by the pattern of our present social structure. There is also some evidence that stress hinders reproductivity in humans. Menstrual upsets during emotionally stressful periods or the development of pregnancy after a change of environment in women who previously did not become pregnant are the most obvious examples.

It would seem axiomatic that as our population density increases so would also increase these same pressures and tensions of life which would then certainly be 11stressor agents" in the General Adaptation Syndrome of Selye (7). The following is a brief explanation of the general response to stress. Normal healthy animals, including man, at rest demand of their bodies only a low level of activity. There are, however, numerous stressor agents such as nervous tensions, wounds, infections, poisons, cold, and muscular exercise which produce a high level of activity. One specific function or structure may be affected more than others, but there will also be a general increase of the level of many functions. The body attempts to overcome the stimulat ing effects of these stresses and return to the original resting level or "steady state" even though the stressor agents are still there. In this the nervous and endo crine systems play a particularly important part. The General Adaptation Syndrone is the total of the changes caused by the stress plus the body's adaptive reactions. This syndrome develops in 3 stages: (1) the alarm reaction. This is the initial response to the stress. An example of only one of the many functions that might be affected in general, is the blood pressure. A sudden sharp change, either increase or decrease, might be expected in this stage. (2) The stage of resistance. Here the individual continues to perform in spite of the heavier load and increase in the rate of "wear and tear." To follow through on our example, the blood pressure would return towards normal. If the stress persists long enough, the third stage would ensue. (3) The stage of exhaustion. Here there is failure of certain functions. It would appear that reserve energy is drawn from those functions less immediately vital to the individual, such as reproduction, growth and resistance to infectious disease and parasitism in order to supply energy for the essential functions. However, if the stressor is not withdrawn other functions may be affected and incanacita tion and death mav result.

Pure sociopsychological factors in a population of high density serve as stressor agents producing deterioration of numerous reproductive functions, decrease of resistance to disease, and actual death so as to regulate population growth. (9, 10). However, only experimental evidence for the deterioration of the reproductive capacity will be presented here. Thus as early as 1931 Crew and Mirshaia (11) compared the reproductive functions of numerous small laboratory animals in dense populations with those of animals in sparse populations, and reported that in high population densities reproduction of female mice was depressed. Also when male albino mice, which have been segregated since weaning, are placed together in groups of 1, 4, 8, 16, 32 per cage for a week, there is atrophy of the gonads and sex accessories in the dense populations, which progresses more or less linearly as the logarithm of the population increases. Similar results were obtained in populations of wild house mice (9).

Andervont (10) reported that in a group of females, estrus cycles began at an earlier age, were more frequent and lasted until a greater age in segregated mice than in their litter mates kept in groups of 8 each in a normal mouse cage. In another series of experiments, no young were produced and no females became visibly pregnant when mice were crowded 20 males to 20 females to a cage for 6 weeks, although abundant food and water were available. When the population size

*There is a contrary viewpoint that the reproductive capacity is increased in hunger as a result of the attempt of nature for preservation of the species. (De Castro, Josue, The Geography of Hunger, Little, Brown and Co., Boston, 1952)

was reduced to 10 males and 10 females all of the females became pregnant, but the number of implanted ova was reduced significantly and only 7 of the 10 females developed pups (9). The remainder lost their progeny in utero during the early stages of pregnancy. The onset of pregnancy was also considerably delayed in these animals. Therefore, there was decreased fertility, decreased implantation and a marked increase in intro-uterine mortality. Futhermore, after the young were born, there was a decrease in lactation in white mice and voles as measured by the weight of the young in comparison to the young of a group of mothers not previously crowded (9). In males where the weight of the reproductive organs is indicative of reproductive stimulation and capacity there was a decrease in organ weight. Finally the results show that some small animals respond to increased population density by a depression of reproductive function at all stages of the processes.

The effect of social stress within a population was also studied by comparing the socially dominant and subordinate animals. When house mice are placed together there is immediate fighting, which soon ceases with the establishment of a social hierarchy with one mouse dominant over the other and another subordinate to all of the others. The remaining mice arrange themselves in some sort of hierarchy in between. The subordinate animals were frequently placed under stress of having to cower before the more dominant animals. It was found that the reproductive organs were heavier and reproductive performance was best in the socially dominant females in each population even though they were obliged to fight more to maintain their status (9).

Christian (9) studied these effects on reproductivity and also the evidence of decreased resistance to disease and increased mortality which result from sociopsychological stresses in a population of high density, and concluded that the underlying mechanism involves the neuroendocrine system. He describes the process as follows: Social pressure works through the hypothalamus of the central nervous system and the anterior lobe of the pituitary gland to stimulate three basic actions of the "alarm reaction." They are as follows: (1) decrease of production and release of growth hormone, (2) decrease of production and release of gonadotrophins, and (3) increase of production of the adrenocorticotrophic hormone (ACTH). These responses represent the "stage of exhaustion" and produce their effects in the following way (Figure 1): As a result of the decrease in growth hormone there is a withdrawal of the stimulus for growth and metabolic activity which in turn suppresses antibody production, phagocytosis, inflammation and granulation eventuating in decreased resistance to disease and increased mortality. This then has the effect of slowing or limiting the population increase. Through another basic response to stress, a decrease in gonadotrophins, there is a withdrawal of stimulus to the reproductive organs so that there is a suppression of spermatogenesis, estrus and sex steroids which decreases fertility, fecundity, maturity, and lactation. As a result, the population increase is checked by a second effect, a decrease in the birth and survival of new individuals. Through the third mechanism, an increase in ACTH stimulation, there is an increased production of corticosteroids by the adrenal cortex. This potentiates both of the previous effects: the suppression of metabolism, which decreases the resistance to disease, and, at the same time, depression of reproductive function. The result is further decrease in population through a decrease in birth and survival of new individuals and an increase in the death rates.

Much evidence for the above pattern is found in animal studies (9) but little definitive knowled ge is available for humans. Even though there are many parallels in the responses of animals and humans, caution must be exercised in applying these mechanisms to man. However, even if they do apply to man it is hoped that the human population will never become so dense that these mechanisms will be called into operation to any large extent since it would result in an increase in the incidence of nervous and degenerative diseases and in early death.

From the foregoing discussion it may be concluded that as the population increases the physical factors of the environment such as food, water, atmosphere, and living space will become more critical and may limit the population. At the same time social and psychological stresses will increase and may produce a reduction of reproductive capacity, resistance to disease and longevity through neuroendocrine pathways as implied from animal studies.


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2. Benedict, F. G. A Study of Prolonged Fasting. Carnegie Institute of Washington, 1915.

3. Clarke, George L. Elements of Ecology. Wiley, 1954.

4. Prindle, R. A. In Farber, S. C., and R. H. L. Wilson. The Air We Breathe - A Study of Man and His Environment. Thomas, 1961.

5. Andy, J. R. In Farber, S. M., and R. H. L. Wilson. The Air We Breathe - A Study of Man and His Environment. Thomas, 1961.

6. Milne, L. J., and M. Milne. The Biotic World and Man. Prentice-Hall, 1958.

7. Selye, H. The Stress of Life. McGraw-Hill, 1956.

8. S. Wolff, H. G. Stress and Disease. Thomas, 1953.

9. Christian, J. J. In Gorbman, A. Comparative Endocrinology. Columbia U. Symposium, Cold Spring Harbor, Wiley, 1959.
10. Andervont, H. B. "Influence of environment on mammary
cancer in mice," 1. Nat. Cancer Inst. 10,579. (1950)

11. Crew, F. A., and L. Mirskaia. "The effects of density on
an adult mouse population." Biol. Geaeralis. 7,239. (1931)