2. Endocrines and Populations 191 



Past research on the adaptive mechanisms has emphasized the adrenal 

 cortices and to a lesser extent the thyroid gland and their hormones. Un- 

 questionably the adrenal cortex is essential to life and plays a basic role in 

 the adaptive responses, nevertheless there is a real tendency to overlook 

 the paramount importance of other systems and organs which also respond 

 to adverse circumstances. In fact their actions are simultaneous with and 

 inseparable from the actions of the adrenal cortex in many instances. A 

 great many responses on the part of the organism act in concert to prevent 

 any alteration in the basic physiology of the animal and to meet emergency 

 needs. The central nervous system is a major and integral part of these 

 adaptive mechanisms. Our understanding and interpretation of the physio- 

 logic changes taking place under a given set of circumstances too frequently 

 suffer from a tendency to think statically and in terms of isolated organs, 

 systems, or hormones — a result of the kind of experimental approach 

 necessary to understand the actions of various glands and their secretions. 

 The isolated organ concept must give way to thinking in terms of dyna- 

 mically interacting systems. However, to describe these mechanisms and 

 their effects in dynamic inclusive terms is extremely difficult and has been 

 made even more so by the recent elucidation of the key role played by the 

 central nervous system in regulating the activities of the glands of internal 

 secretion, as well as by the realization that we are dealing with an enor- 

 mously complex interacting system further complicated by the complex 

 temporal relationships of the responses of these systems to the applied 

 stimulus and to each other. 



The balance of this chapter will be devoted to a more detailed discussion, 

 first of the physiologic adaptive mechanisms themselves, and then with 

 particular attention to the evidence implicating these responses in the 

 regulation of the growth of mammalian populations. An attempt will be 

 made to clarify some of these responses and to indicate areas where further 

 research is needed, especially in relation to behavior and population density. 

 Subjects which are adequately covered in the usual textbooks of physiology 

 will be omitted or only briefly summarized. 



Part 1. The Endocrine Adaptive Mechanisms 



I. Introduction 



When a mammal is subjected to a stimulus which, if unopposed, would 

 result at least in a change in its internal physiology, and more likely produce; 

 a circulatory collapse, a series of neural, neuroendocrine, endocrme, and 

 vascular responses follow which counteract the deleterious effects of the 



