2. Endocrincs and Populations 305 



as the 59% decline in the first population. However, the decline was not 

 nearly so great and the initial weight was 19% less than in the first popula- 

 tion. The lower adrenal weight was reflected by a higher mean eosinophil 

 for the same period of time. Even though the reason for the lower adrenal 

 weight in the second population during the second period is not known, 

 adrenal weight and eosinophil counts in these studies generally reflected 

 differences in population density (Louch, 1958). 



Methods for estimating most natural populations of mammals are at 

 best relatively crude, insensitive, and subject to many errors. At best they 

 can detect only relatively large changes in population density. The catch 

 per trap-night or any comparable index of population density obtained by 

 some sort of uniform trapping effort is probably as good an index as any 

 readily usable procedure. The catch per unit of time was used in these 

 studies of vole populations by Louch (1958) and Adams et al. (Christian, 

 1959b) , as well as in the studies of rural populations of Norway rats (Chris- 

 tian, 1959b) . Errors in estimating population density in this fashion may 

 account for most of the observed discrepancies. If the existence of physio- 

 logic responses to population density is established for a variety of species, 

 it is likely that the magnitude of these responses will provide a much more 

 precise index of relative population density (the important figure for practi- 

 cal purposes) than any existing indirect method for determining population 

 size.^ 



Limited studies with natural populations of other species have been made. 

 A 68% decline in the index of the size of a population of white-footed mice 

 {Peromyscus leucopus) from one July to the next was accompanied by a 

 58% decline in their mean adrenal weight (Christian, 1959b). Similar, but 

 less conclusive, results have been obtained with natural populations of 

 short-tailed shrews [Blarina) (Christian, 1954, and unpublished). Beer 

 and Meyer (1951) studied the seasonal changes in the endocrine and 

 i-eproductive organs of muskrats and found a marked peak in adrenal 

 weight in adults of both sexes in early fall and a second minor peak in 

 March and April, especially in adult females. These seasonal changes are 

 similar to those we have noted in Microtus montanus and may be related 

 to behavioral and social changes. 



Preliminary work on the relationships of the weights of deer adrenals to 



" The persistence of immature zonation in the adrenals of males and lack of hyper- 

 trophy due estrogenic stimulation in females may explain some of the discrepant re- 

 sults with Microtus, and possibty Lemmus, as these would seriously confound adrenal 

 weight relationships if the population under study is composed of an appreciable per- 

 centage of sexuallj' immature, regressed, or otherwise sexually inactive animals. Winter 

 or high-density populations will consist largely of such animals [see Christian (19(il, 

 1963b)]. 



