[Table 11] also shows food consumption of the four females discussed above. All females, with the exception of the female with litter D, consumed amounts of food that lie within one standard deviation of the means for their species. The female with litter D had the most young, consumed the most food but drank the least water of the four females. Later, when separated from her litter and placed on the low protein diet, this female drank only .046 milliliters of water per gram of body weight per day. This figure is less than one-third of the average amount (.174) for this species ([Table 7]).
The records of water and food consumption for litters A, C, and D are given in [Table 12]; the mice in litter B persisted in placing wood shavings in the opening of the spout on their water bottle, causing loss of the water. The data show that mice in all three litters had an average water and food consumption within one standard deviation of the mean for adults of their respective species (Tables [7] and [12]). It is interesting that juveniles of both species require no more food and water per gram of body weight than adults. This indicates that if a young animal survives the rigors of postnatal life until it is weaned, it is then at no disadvantage as far as food and water consumption are concerned. This would be greatly advantageous to the species, as a population, for the young could disperse immediately upon weaning, and go into any areas that would be habitable for adults of the species.
Table 12—Food and Water Consumed by Young Mice in Litters, After Weaning. Consumption Is Calculated on the Basis of the Amount (Milliliters or Grams) Consumed per Gram of Litter Weight per Day; Total Amounts Are Shown and Can Be Divided by Litter Size for Average Individual Consumption. Litter Sizes Are as Follows: A=3; C=5; D=6.
| Litter | Total water used | Total corrected | No. days | Average total weight | ml. H2O/gm./day | Total water/day |
| P. truei (A) | 1207 | 1120 | 57 | 58.30 | .337 | 19.64 |
| P. maniculatus (C) | 1427 | 1340 | 57 | 76.14 | .308 | 23.50 |
| P. maniculatus (D) | 700 | 670 | 31 | 58.80 | .367 | 21.61 |
| Litter | Total food used | No. days | Average total weight | Gms./gm. wt./day | Total food/day | |
| P. truei (A) | 651.2 | 50 | 58.30 | .223 | 13.02 | |
| P. maniculatus (C) | 743.8 | 57 | 76.14 | .171 | 13.04 | |
| P. maniculatus (D) | 471.1 | 31 | 58.80 | .258 | 15.19 | |
The young of pregnant and lactating females are the animals in the population most likely to be affected by a deficient supply of water. Drought could reduce the water content of the vegetation to such a level that pregnant or lactating females might find it difficult, if not impossible, to raise litters successfully. If such a drought persisted throughout an entire breeding season, the next year's population would be reduced in numbers, for even under normal climatic conditions it is almost exclusively the juveniles that survive from one breeding season to the next. If such a hypothetical drought occurred, lactating females of P. truei would be in a more critical position than lactating females of P. maniculatus.
In order to determine how much water was available to mice in the peak of the breeding season, samples of the three most common plants in the study area were collected each week for analysis of their moisture content. Plants were placed in separate plastic bags that were sealed in the field. About a dozen plants of each species were used in each determination. Only the new tender shoots of the plants were collected, for it was assumed that mice would eat these in preference to the tougher basal portions of the plants. The plants were taken immediately to the laboratory and were weighed in the bag. Then the bag was opened and it and the contents placed in an incubator at 85 degrees Fahrenheit for a period of at least 72 hours. About 48 hours were required to dry the plants to a constant weight. The dried plants were weighed and their percentages of moisture were determined. Plants lose some water upon being placed in a closed bag; small drops of water appear immediately on the inner surface of the bag. Therefore, the bag must be weighed at the same time as the plants and the weight of the dried bag must be subtracted later.
The three kinds of plants chosen were among the most widely distributed species in the study area, and all three grow close to the ground, within reach of mice. Stems and leaves of two of the plants, Comandra umbellata and Penstemon linarioides, were readily eaten by captive animals. Mice also were observed to eat leaves of Comandra after being released from metal live traps. The third species, Solidago petradoria, differs from the other two in having a short woody stem that branches at ground level. The more succulent shoots arise from this woody stem. The leaves of Solidago are coarse and were not eaten by captive mice. Nevertheless, this species was chosen because it is widely distributed and has the growth form of several other species of plants in the area.
The graph in [Figure 20] shows that Comandra contains the highest percentage of water through most of the summer. Water content of both Penstemon and Comandra was greatly reduced in the dry period that occurred in early July. Solidago maintained a relatively constant percentage of moisture; perhaps its woody stem serves for water storage. The rains of July and August increased the percentage of moisture in the plants, but not to the extent expected. Neither Solidago nor Comandra reached the levels of hydration of early June. All plants were collected at or about 11 A. M. At night, when mice are active, these plants would be expected to contain a higher percentage of water than in the daytime.
The data in [Figure 20] indicate that mice probably are not endangered by water shortages in most years. The average percentage of moisture in the plants studied was as follows: Comandra umbellata 62.33 per cent; Solidago petradoria 53.0 per cent; Penstemon linarioides 49.28 per cent. If a mouse were to eat ten grams of plant material containing 50 per cent moisture, it would provide him with five grams of food and five grams of water, both of which exceed the minimum daily needs for non-pregnant adults of either species.