Table 10—Weights of Mice at Start and Finish of Experiments, Showing Changes in Weight and Mean Weights, and Means of Changes in Weight (mean delta).
| Peromyscus truei truei | |||||||||
| No. | Lab Chow | Hog Chow | Corn | ||||||
| Start | End | Δ | Start | End | Δ | Start | End | Δ | |
| 1 | 31.0 | 31.3 | 0.3 | 31.3 | 32.3 | 1.0 | 32.3 | 29.0 | 3.3 |
| 5 | 31.1 | 30.5 | 0.6 | 30.5 | 32.8 | 2.3 | 32.8 | 28.7 | 4.1 |
| 6 | 27.6 | 27.1 | 0.5 | 27.1 | 29.5 | 2.4 | 29.5 | 27.3 | 2.2 |
| 7 | 28.0 | 26.3 | 1.7 | 26.3 | 27.5 | 1.2 | 27.5 | 22.2 | 5.3 |
| 13 | 25.8 | 30.6 | 4.8 | 30.6 | 27.0 | 3.6 | 27.0 | 22.2 | 4.8 |
| 14 | 26.9 | 30.7 | 3.8 | 30.7 | 31.4 | 0.7 | 31.4 | 27.3 | 4.1 |
| 15 | 25.4 | 29.4 | 4.0 | 29.4 | 29.8 | 0.4 | 29.8 | 24.0 | 5.8 |
| 16 | 33.0 | 32.9 | 0.1 | 32.9 | 30.5 | 2.4 | 30.5 | 26.0 | 4.5 |
| 19 | 37.6 | 38.1 | 0.5 | 38.1 | 31.8 | 6.3 | 31.8 | 22.0 | 9.8 |
| 20 | 23.5 | 25.8 | 2.3 | 25.8 | 26.2 | 0.4 | 26.2 | 22.9 | 3.1 |
| Ȳ | 28.9 | 30.2 | 1.8 | 30.2 | 29.8 | 2.0 | 29.8 | 25.2 | 4.7 |
| Peromyscus maniculatus rufinus | |||||||||
| No. | Lab Chow | Hog Chow | Corn | ||||||
| Start | End | Δ | Start | End | Δ | Start | End | Δ | |
| 2 | 23.0 | 20.7 | 2.3 | 20.7 | 21.1 | 0.4 | 21.1 | 18.6 | 2.5 |
| 3 | 22.7 | 23.1 | 0.4 | 23.1 | 23.8 | 0.7 | 23.8 | 20.7 | 3.1 |
| 4 | 22.0 | 21.1 | 0.9 | 21.1 | 21.8 | 0.7 | 21.8 | 21.3 | 0.5 |
| 8 | 26.3 | 28.1 | 1.8 | 28.1 | 15.8 | 2.3 | 25.8 | 23.8 | 2.0 |
| 9 | 21.5 | 24.0 | 2.5 | 24.0 | 25.1 | 1.1 | 25.1 | 21.8 | 3.3 |
| 10 | … | … | … | … | … | … | 22.5 | 20.0 | 2.5 |
| 11 | 21.0 | 22.1 | 1.1 | 22.1 | 20.8 | 1.3 | 20.8 | 19.0 | 1.8 |
| 12 | 22.3 | 23.2 | 0.9 | 23.2 | 21.3 | 1.9 | 21.3 | 20.4 | 0.9 |
| 17 | 18.9 | 20.0 | 1.1 | 20.0 | 19.2 | 0.8 | 19.2 | 19.4 | 0.2 |
| 18 | 17.0 | 17.5 | 0.5 | 17.5 | 19.5 | 2.0 | 19.5 | 17.3 | 2.2 |
| 21 | 18.9 | 18.1 | 0.8 | 18.1 | 20.2 | 2.1 | 20.2 | 17.3 | 2.9 |
| Ȳ | 21.4 | 21.8 | 1.2 | 21.8 | 21.8 | 1.3 | 21.9 | 19.9 | 2.2 |
Lindeborg (1950:76) found that 15 days before parturition, pregnant and non-pregnant females of P. m. bairdii drank about the same amounts of water, that females consumed more water after the young were born and until they were weaned, and that water consumption increased with an increase in weight in young, growing individuals. He found that in the later stages of pregnancy, females of P. m. bairdii required 36 per cent more water than non-breeding females; at 14 days after parturition, nursing females required 111 per cent more water than non-breeding females, and at weaning time, 158 per cent more water. Dice (1922:35) reported a 217 per cent increase in drinking of P. m. bairdii before parturition, and 171 per cent increase while nursing.
Several females of both species were bred prior to the start of the experiments described herein. As a consequence, it was possible to determine water and food consumption for lactating females of each species, and later, for their litters. Pregnant and lactating females, and newly-weaned litters, were fed laboratory chow throughout this experiment. The litters were separated from their mothers as soon as the young were observed to be eating, or no later than 33 days after birth.
[Table 11] shows the amounts of water and food consumed by two females of each species while they were either in the later stages of pregnancy, or were nursing. Although the data in [Table 11] do not cover the full developmental time of the litters involved, it is obvious that both lactating females of P. truei and one female of P. maniculatus consumed more water than the average for their species ([Table 7]). Water and food consumption was measured for both females of P. truei while they were nursing. The female that gave birth to litter A was left in the cage with the male for several days after the litter was born, resulting in another litter being born about 27 days after the first. Therefore, the record of this female represents an extreme case of stress (probably a common occurrence in nature) in which a female is nursing one litter while she is pregnant with a second.
The record of the female of P. truei that gave birth to litter B is the most complete, including data from the fifth day after parturition until the young were weaned on the thirty-third day after parturition. The record of the female of P. maniculatus that gave birth to litter C covers the last 10 days of nursing before the young were weaned. After being separated from her litter, this female drank more than the average amounts of water, on both high and low protein diets. Although the food and water were lost several times for the female of P. maniculatus with litter D, the period of time covered by the 14 days when water and food consumption were measured includes times just prior to parturition and to weaning of the young.
Table 11—Water and Food Consumed by Nursing Females of P. truei and P. maniculatus. Consumption Is Calculated on the Basis of Amount (Milliliters or Grams) Consumed per Gram of Body Weight per Day, as well as Total Amounts Used per Day.
| Female | Water used | No. days | Average weight | ml. H2O/gm./day | Total water/day | No. in litter |
| P. truei (A) | 447 | 17 | 33.00 | .796 | 26.29 | 3 |
| P. truei (B) | 676 | 28 | 32.70 | .738 | 24.14 | 3 |
| P. maniculatus (C) | 191 | 10 | 19.45 | .983 | 19.10 | 5 |
| P. maniculatus (D) | 133 | 14 | 24.35 | .224 | 5.46 | 6 |
| Female | Food used | No. days | Average weight | gms. food/gm./day | Total food/day | No. in litter |
| P. truei (A) | 214.7 | 26 | 33.00 | .250 | 8.26 | 3 |
| P. truei (B) | 120.5 | 24 | 32.70 | .153 | 5.02 | 3 |
| P. maniculatus (C) | 47.8 | 10 | 19.45 | .246 | 4.78 | 5 |
| P. maniculatus (D) | 180.1 | 21 | 27.42 | .312 | 8.58 | 6 |
It is interesting that the female of P. maniculatus with litter C used much more than the average amount of water for the species, and even more per gram of body weight than lactating females of P. truei. Conversely, water consumption of the female with litter D was within one standard deviation of the mean for all adults of P. maniculatus. I infer that at least some lactating females of P. maniculatus are better adapted to aridity than are some lactating females of P. truei.