Since a complex organism is composed of cells, many regulation and control mechanisms found at this organization level are extensions of such mechanisms as we have presented which are operative in cells.
It is also undoubtedly true of embryonic development, especially in early stages. Furthermore, genes exert their effects in the development and function of organisms through the composite effects of cells, and many effects in the more complex animals are a direct reflection of cellular control. We shall have occasion to refer to some of these effects later on.
However, certain other mechanisms are best interpreted as emergent qualities which come about as a result of increased specialization of cells and a coordination of their activities in tissues, organs, and organ systems.
In other words, a homeostatic mechanism is not necessarily a mere summation of many homeostatic mechanisms at the cellular level; it may be an expression of the entire organism taken as a whole. Withal, the examples which are presented below fall into this category.
One of the clearest examples of homeostasis in animals is seen in the birds and mammals with regard to the regulation of body temperature. You will recall that only these animals are warm-blooded. The homeostatic mechanism involved is a temperature-regulating center located in the hypothalamus of the brain.
It is a small mass of nerve cells which connect with effectors at the surface of the body, and it is extremely sensitive to changes in temperature. Let us suppose that a person is situated in a room where the temperature is 100 degrees.
As soon as body temperature increases, the temperature-regulating center sends nervous impulses to the sweat glands of the skin, causing them to secrete moisture to the outer surface of the body. Evaporation of the moisture lowers the body temperature, and a drastic rise is thus prevented. When the internal temperature falls below 98.6 degrees the center causes the sweat glands to be less active, and loss of heat from the body is prevented.
Here we have a homeostatic system. Of course, there are many other factors involved in temperature homeostasis of the human, such as shivering, stimulation of the conscious areas of the brain in regulating clothing, stimulation of the conscious areas of the brain in regulating clothing, and so on, but even with these added complications, the temperature regulating center is a homeostatic system. We have oversimplified it only for purposes of illustration.
The tendency of a population to cluster around a height mean is best interpreted homeostatic ally. Assuming that its chance for survival is best at the mean weight, and this is apparently the case, mortality increases as one departs from the mean in either direction. Because animals near the mean weight become more involved in reproduction than do those which fluctuate from it, there is continued tendency for the population to remain clustered around the mean.
However, this raises an important question. Why the less fit weights are not eliminated altogether? This question can be answered in terms of genetics and evolution, and here again; we lack sufficient insight at this point to appreciate most of the implications.
Suffice it to say at this point that genes which influence the development of less fit traits in an organism may persist for a long time in a population, and for all practical purposes, we can always expect come variation.
Perhaps you have heard the expression “balance of nature”. This is a clumsy and oversimplified statement of a homeostatic mechanism which is indeed operative in nature, but which hardly keeps populations in balance.
Actually, nature as a whole has never been in balance, and it never will be. Only isolated segments of interacting populations demonstrate anything resembling a balance, and even here, close studies reveal continuous fluctuations.
Nevertheless, interacting species demonstrate some rather interesting homeostatic mechanisms. Like the factors which operate in stabilizing a population at a mean weight, these mechanisms tend to maintain the status quo. Without them, there would be no semblance of a balance at all.
Let us consider a particularly well-studied case of interspecific homeostasis that of the Canadian lynx and the snowshoe hare. Beginning in the year 1736, the Hudson’s Bay Company kept records on the number of lynx pelts taken by trappers in Canada, and it soon became apparent that peak numbers occurred every ten years.
Subsequent observations revealed that the snowshoe hare population peaks stayed slightly ahead of those of the lynx population. It has been determined that the hare population fluctuates at ten-year intervals with or without the lynx, and apparently this is a result of a complex of factors. However, the relationship of the lynx cycle to that of the hare is readily explained.
Since the lynx preys chiefly upon the snowshoe hare, its fortunes rise and fall with the hare population. In this fashion, the lynx population is subject to a form of homeostatic control.