2. Endocrines and Populations 285 



eosinophil counts and population density, indicating a progressive decline 

 decline in the number of circulating eosinophils with increasing population 

 density, but the correlation between eosinophil counts and population 

 density was not significant in the third population. The latter population, 

 however, never reached 30 animals in size. The published figures indicate 

 a striking parallel between the rate of population growth and eosinophil 

 counts for all three populations, and in all three there were significantly 

 fewer circulating eosinophil when the populations of these voles were greater 

 than 30 than when they were less than 30 (Louch, 1956). These results 

 indicate that adrenocortical function increased progressively with increasing 

 population density. The variability in Louch's data reflect to some extent 

 the difficulties in obtaining precise eosinophil counts, even though he was 

 fully aware of these problems and took every step possible to a\^oid the 

 usual pitfalls (Louch et al, 1953; Louch, 1956). The problems involved in 

 using eosinophil counts as indices of adrenocortical function have been dis- 

 cussed fully elsewhere (Thorn et al, 1953; Louch et al, 1953; Rosemberg 

 et al, 1954; Speirs, 1955; Visscher and Halberg, 1955; Louch, 1956; and 

 earlier in this chapter) . Handling will cause an adrenal medullary and corti- 

 cal discharge due to fear, excitement, and possibly rage in wild mammals 

 and therefore can effect eosinophil counts (Southwick, 1959) . Conseciuently 

 it is possible for variability to result from handling, as well as by individual 

 and perhaps more importantly, by unknown factors, unless appropriate 

 precautions are taken (Louch, 1958). However, in spite of these problems, 

 eosinophil counts offer a promising means of investigating adrenocortical- 

 population density relationships until simple, more direct tests become 

 available. Probably the best procedure at the present time is to follow the 

 population with eosinophil counts and to substantiate the changes with 

 adrenal weights at the termination of the study. Louch's results provide 

 evidence of increased adrenocortical function in voles with increased popu- 

 lation density. 



Li a similar experiment, a marked increase in adrenal weight, as an index 

 of cortical activity was found in both male and female voles (Microtus 

 pennsijlvanicus) from a freely growing confined population which had 

 reached its maximum size (Christian, 1959b) . The adrenals of mature male 

 voles were increased 39.6%, and those of mature females 36.6% over their 

 segregated controls. It was pointed out earlier that adrenal weight-body 

 size relationships may vary with species and with sex; therefore, if animals 

 covering a wide range of sizes are to be used, the correct relationship in 

 these must be determined. It was found that in male voles over 115 mm 

 long, adrenal weight did not vary with further increases in the size of the 

 animal, therefore the absolute adrenal weights could be used for compara- 



