2. Endocrines and Populations 247 



ably from individual to individual in the same species, as well as with sex, 

 size, and therefore presumably age, in most mammals. Consequently, it is 

 necessary to obtain large enough samples of each sex in each age category 

 (at least mature and immature) to have reliable criteria for evaluating 

 adrenal changes in a population and with time. 



In most studies of natural populations the investigator does not have 

 the privilege of selecting animals for size, weight, and sex each time a 

 sample is collected, and it is therefore necessary to determine adrenal 

 weight-body weight, adrenal weight-body length or some similar relation- 

 ship for the species and population with which he is working in order to 

 establish a common baseline for all samples. For example, it has been found 

 that the logarithm of adrenal weight in milligrams on body length (exclu- 

 sive of the tail) gives the best straight-line relationships for all sizes and 

 for both sexes of Norway rats (Christian and Davis, 1955). In other 

 species, for example Microtus pennsylvanicus, there are marked differences 

 in body-adrenal relationships between the sexes. The oft-used relationhsip 

 of milligrams of adrenal weight per 100 grams of body weight is quite 

 arbitrary and usually overcorrects for lighter and undercorrects for heavier 

 animals, therefore this relationship is useful for only a very narrow range 

 of body weights. Too often adrenals are collected as an afterthought along 

 with other data for which the investigation was originally designed. As a 

 result the data on adrenal weights usually are inadequate. A study of 

 changes in adrenal weight must be designed specifically to obtain this and 

 the necessary related data. Samples should be consistent with respect to 

 time, size, and sex and age composition. Many potentially useful field 

 studies involving an immense amount of effort have been of little value 

 because of poor sampling, usually a result of treating the collection of data 

 on adrenal weights as a secondary consideration in the investigation. 

 Sampling usually does not alter the populations, as the normal rate of re- 

 cruitment more than offsets losses due to sampling, in addition to which 

 compensatory changes in mortality rate will usually compensate for sam- 

 pling losses, unless the sampling is very frequent and intense. 



b. Routine Idstologic measurements. It is very useful in evaluating adrenal 

 activity to measure the widths of the various zones and to coimt the number 

 of cells in cortical cords in an area where cords are in straight columns 

 (Zwemer, 1936; Zwemer et al., 1938; Christian, 1956). A variation of this 

 is the somewhat more precise technique of projecting a section through the 

 center of the gland and outlining the various zones, cutting them out and 

 weighing them or else measuring their areas with a planimeter. It is possible 

 with serially sectioned glands to repeat such a procedure every so many 

 sections and in this way obtain precise information on the contributions 

 made by the individual zones to the total weight of the gland. These tech- 



