2. Endocrines and Populations 303 



operative in natural populations of Norway rats, just as they are in freely 

 growing confined populations of house mice or voles. 



Another series of experiments with populations of rats produced results 

 which at first glance appeared to contradict the foregoing conclusions. Alien 

 rats were introduced into stationary and increasing populations to verify 

 earlier observations that such a procedure profoundly affected the popula- 

 tion (Calhoun, 1948). When stationary populations of rats were increased 

 20% by adding alien rats of one sex or the other, the populations declined 

 abruptly to about three-fifths their original values. When large numbers of 

 rats were substituted (native rats were removed and replaced by an equal 

 number of aliens) into increasing populations, the populations promptly 

 ceased growing. These procedures presumably produced severe social strife 

 with a marked increase in mortality (Calhoun, 1948; Davis, 1953). How- 

 ever, the adrenal weights of rats taken at regular sampling intervals failed 

 to reflect the increased strife and closely followed the course of the popula- 

 tion size. The explanation of this apparent paradox is clear. The introduced 

 rats were, of course, aliens — hence subordinate rats — in a strange environ- 

 ment (see Clarke, 1953; Barnett, 1955, 1958) and therefore became mor- 

 tality statistics. The native rats which succumbed were probably subordi- 

 nate animals in the original population. The adrenal weights were only from 

 survivors and therefore probably from dominant animals. It already has 

 been pointed out that adrenal weight is least in dominant animals, and so 

 the increase in adrenal weight which might have been anticipated from a 

 superficial examination of the situation obviously and simply failed to 

 appear. Had the adrenals of the rats that died been obtainable, they un- 

 questionably would have exhibited profound changes. 



Natural populations of voles (Microtus) have also been studied to deter- 

 mine the relation between adrenal function and population density. Adams, 

 Bell, and Moore (Christian, 1959b) periodically collected frequent samples 

 from a natural population of Microtus montanus in Montana for four years 

 to obtain indices of population density, adrenal weights, and data on re- 

 productive activity. The number of males collected was inadequate for 

 valid comparisons. However, population density and the adrenal weights 

 of female voles were closely correlated. The population was at peak densities 

 in late summer of 1952 after which there was a continued general overall 

 decline in the population until the summer of 1955. The adrenal weights of 

 females were maximum in the late summer of 1952 and then underwent a 

 gradual overall decline, generally following the population. However, within 

 each year there were marked seasonal changes in the population which were 

 reflected by equally marked changes in adrenal weights. The population 

 reached its annual maximum density in late summer or early fall and then 

 declined sharply to very low levels where it remained until late winter or 



