2. Eudocrines and Populations 319 



microtines discussed (Pitelka, 1957b). Pitelka (19o7a, b) also makes the 

 point that the cessation of reproduction at peak densities may occur and 

 produce a decline in the lemmings before their predators have a chance to 

 exploit the lemming population. Lemming populations have not been 

 studied for a long enough period of time or with sufficiently detailed evalua- 

 tion of the various factors which might inhibit reproduction to warrant any 

 conclusions at present. However, it is apparent that there can be inhibition 

 of reproduction with increased density, but the extent to which food sup- 

 plies may also contribute has not been sufficiently appraised. The evidence 

 presently available suggests that failure of reproduction in response to ex- 

 treme densities is the major cause of the cyclic declines in lemming popula- 

 tions. 



Many studies indicating a decline in reproduction with high population 

 densities in other species, such as hares (Preble, 1908; MacLulich, 1937), 

 but in none of these is it possible to ascribe the alteration in reproduction 

 solely to density factors, even though it may well be that such is the case. 

 Many other studies could be listed which illustrate the same sort of thing, 

 but no useful purpose would be served by doing so here. More recent studies 

 with Microtus pennsylvamcus indicate that in this species the annual re- 

 productive cycle is governed by population density, but this does not appear 

 to be true for Peromyscus manicidatus (Christian, 1961). 



The main point of interest is that reproduction does respond to changes 

 in population density in natural populations of voles and rats and probably 

 in other .species as well. In some studies there was a reciprocal relationship 

 between adrenocortical and reproductive function, in others only one or 

 the other of these general responses was studied, but the results conform 

 to this general relationship. It seems undeniable, especially in the light of 

 studies on laboratory populations, that physiologic adaptive responses are 

 operative in natural populations and are reflected by curtailment of repro- 

 duction and growth and stimulation of increased pituitary-adrenocortical 

 activity in natural populations. However, detailed and well correlated 

 studies on a variety of species still are required and for a variety of different 

 populations in order to assess the relative importance of these mechanisms 

 in curtailing free-for-all population growth. 



3. Disease Resistance and Mortality 



One of the major points of the hypothesis that physiologic mechanisms 

 could control population growth as a feedback system acting in response to 

 changes in density was that the sudden mass mortality seen at the end of a 

 build-up of populations to peak densities could be accounted by exhaustion 

 of the adaptive responses (Christian, 1950b). It was suggested that the 



