The data in Table 7 show that both species vary their food intake with changes in diet. Table 10 shows weight changes that took place in individual mice when fed each of the three diets. A change in weight of one gram cannot be considered as important, for the weight of an individual mouse fluctuates depending upon when he last drank, ate, defecated or urinated. The only significant changes in weight occurred when mice were fed low protein food (Table 10). Individuals of _P. truei_ lost 15.72 per cent and individuals of _P. maniculatus_ lost 10.03 per cent of their total body weights on this diet. This indicates that food having a protein content of more than 10 per cent but less than 23 per cent is required for maintenance of weight in these animals. Although knowledge of the amount of water consumed, _ad libitum_, by adult mice is valuable information, maintenance of the population depends upon reproduction and dispersal of young individuals. My trapping data indicate that only two to three per cent of the adults live long enough to breed in consecutive breeding seasons. In spring, the breeding population is composed largely of mice that were juveniles or subadults during the latter parts of the breeding season. Therefore, the critical time for the population may well be the time when the season's young are being produced. Any unfavorable circumstances, such as a shortage of food or water, that would affect pregnant or lactating females would be of primary importance to the integrity of the population. TABLE 9--A Comparison of Mean Daily Water Consumption of Mice on High Protein Diets. Numbers in Parentheses Are Average Values; All Others Are Ranges of Values. Column headings: A: Temperature B: Relative humidity C: Investigator ================+===========================+=========+=======+========= | Mean daily H_{2}O | | | | consumption | | | Species +-------------+-------------+ A | B | C | cc./gm. wt. | Total cc. | | | ----------------+-------------+-------------+---------+-------+--------- _P. m. osgoodi_ | (0.27-0.54) | (4.6-9.3) | 18-22 C | 10-20 |Williams, | | | | | 1959 ----------------+-------------+-------------+---------+-------+--------- | (0.496) | (10.74) | | | _P. m. rufinus_ | 0.186-0.764 | 4.54-16.57 | 20-23 C | low |Douglas ----------------+-------------+-------------+---------+-------+--------- | (0.653) | (19.57) | | | _P. t. truei_ | 0.429-1.031 | 13.28-30.28 | 20-23 C | low |Douglas ----------------+-------------+-------------+---------+-------+---------