190 /• /• Christian 



certainty about the relationships of various environmental factors to the 

 adaptive mechanisms. Then, besides the interest in adapti\-e mechanisms in 

 relation to population growth, there is the frequently overlooked fact that 

 these same phj^siologic reactions may effect profound morphologic changes 

 in the members of a population and therefore directly affect the taxonomist 

 who must use morphologic criteria for distinguishing species and subspecies. 

 It is entirely possible that many subspecific descriptions have been based 

 on morphologic differences resultmg from differences in the densities of the 

 populations on which the descriptions are based. The mammalogist inter- 

 ested in reproduction in mammals must take adaptive mechanisms into 

 consideration, as alterations in reproductive functions are an integral part 

 of these same adaptive responses. Therefore there is adequate justification 

 for this chapter on the endocrine adaptive responses, their effects, and their 

 relationships to the densities of mammalian populations. 



No matter how well an animal may be genetically adapted to its general 

 environment, it still must have sufficient adaptive flexibility to meet the 

 daily and seasonal environmental changes, as well as emergency situations, 

 to which it will normally be subjected, and still maintain a constant internal 

 environment. Nothing in the daily life and external environment of an 

 animal remains constant; on the contrary there frequently are very sudden, 

 often extreme, shifts in the environment which are stimuli that, if unop- 

 posed, would alter the internal environment of the animals. But the internal 

 environment must remain constant if the animal is to survive. Therefore 

 there must be a constantly active system of physiologic feedback mecha- 

 nisms to compensate immediately for any tendencies to shift the internal 

 environment. However, these adaptive responses do not take place without 

 producing measurable effects in the organs and glands primarily responsible 

 for meeting the altered demands. Compensation for a life-maintaining 

 change frequently occurs at the expense of some function less immediately 

 important for survival, for example, reproduction. Consequently reproduc- 

 tive function declines measurably in the face of a need to maintain a con- 

 stant internal physiologic state in the presence of adversity. The adaptive 

 responses are changing constantly in degree to meet constantly changing 

 daily circumslances, and it is generally thought that a certain amount of 

 change is necessary to maintain the integrity of the system so that it will 

 be capable of responding to more demanding circumstances. It is not sur- 

 prising that at any given moment the physiologic status of a mammal 

 reflects its total environment and that the whole system is in a constant 

 stage of change, but for these same reasons it becomes difficult to study 

 such a dynamic system in the complex environments of natural populations. 

 Therefore a great deal of the existing evidence on the adaptive mechanisms 

 of mammals, especially in relation to population density, has been gained 

 by studies in the laboratory. 



