304 /. ,/. Christian 



early spring when breeding recommenced and the populations increased 

 sharply. Changes in adrenal weight closely followed these animal cycles in 

 population density. Peak adrenal weights were found in early fall and coin- 

 cided with a cessation of reproduction and the beginning of the autumnal 

 decline in the populations. Adrenal weights were minimal by November 

 and remained there until March, ^ when the population again began to in- 

 crease. Adrenal weights were maximum in the last summer of 1952, aver- 

 aging about 15% greater than in any other year for a comparable time of 

 year. The mean weights in winter were roughly 60% less than the maxi- 

 mum. These results indicate that the supposedly severe winter conditions of 

 the Rocky Mountain Region of Northern Montana do not impose physio- 

 logic hardships on voles. Climatic conditions, if they affected the animals 

 at all, apparently were unimportant compared to changes in population 

 density. The sharp declines in the population every fall evidently were due 

 to a cessation of breeding and continuing random mortality, not to a de- 

 clining environment, otherwise the adrenal weights would not have de- 

 creased so spectacularly (Christian, 1959b). It is possible that physiologic 

 responses to high densities in late summer contributed to the annual de- 

 clines in population density, but it seems unlikely except in 1952, In that 

 year it is probable that physiologic adaptive responses were at least partly 

 responsible for the decrease in the size of the population. It is noteworthy 

 that the rapid decline from peak densities in the late summer and early 

 fall of 1952 was not caused by increased mortality from disease (Adams et 

 al.; cited in Christian, 1959b). 



Adrenocortical-density relationships were studied by Louch (1958) in 

 two natural populations of voles (Microtus pcnnsylvanicus) . He used 

 eosinophil counts, supplemented by adrenal weights, as indices of adreno- 

 cortical activity. Relative adrenal weights were high and eosinophil counts 

 remarkably low in one population at peak density. Following a sharp de- 

 cline in this population, indicated by an 85% drop in the population index, 

 the eosinophil counts rose sharply about 500% and continued to rise more 

 gradually for the subsequent nine months of the study. Adrenal weight 

 averaged 59% lower during the period of low density than during the pre- 

 ceding period of high density: 14.5 mg./lOO gm. compared to 23.0 mg./lOO 

 gm. The size of the second population remained low tliroughout the experi- 

 mental period. The eosinophil counts began high and gradually rose to 

 higher levels. Adrenal weight declined about 27% (from a mean of 19.4 to 

 a mean of 15.2 mg./lOO gm.) in the second population over the same period 



3 Subsequent work has suggested that the low winter adrenal weights are partly due 

 to sexual inactivity and regression, although not entirely as a number of the voles were 

 not sexually regressed. In other words the adrenal weights should be low due to density 

 factors (cf. October), but these were too low [see Christian (1961, 1963b)l. 



