Peromyscus maniculatus is ubiquitous, occurring in habitats ranging from mesic boreal forests to arid southwestern deserts. Most subspecies of P. maniculatus live in moderately mesic or near-mesic environments, but a few have adapted to arid conditions. It has been assumed that the success of P. maniculatus in inhabiting such diverse habitats is associated with its adaptability to different kinds of food and varying amount of available water (Williams, 1959b:606).
Throughout its range P. maniculatus coexists with one or more other species of Peromyscus that are more restricted in distribution. Peromyscus truei is one such species.
Both species live under xeric or near-xeric conditions, for the climate of Mesa Verde is semi-arid. Other than a few widely-scattered springs, there are no sources of free water on the top of the Mesa Verde land mass; thus animals inhabiting the park must rely upon moisture in the plants and other foods they eat, or upon dew.
Several investigators have studied water consumption in mice of the genus Peromyscus ([Table 7]). Dice (1922) did so for the prairie deer mouse, P. m. bairdii, and the forest deer mouse, P. leucopus noveboracensis, under varying environmental conditions. He found that both species drank about the same amounts of water per gram of body weight, and that food and water requirements did not differ sufficiently to be the basis for the habitat differences between these species. Neither of his samples was from an arid environment. Chew (1951) studied water consumption in P. leucopus, and recently reviewed the literature on water metabolism of mammals (Chew, 1965). In his studies of five subspecies of two species of Peromyscus, Ross (1930) found significant differences in water consumption between species but not between subspecies within a species. One of the subspecies of P. maniculatus tested was from a desert region, whereas the other two were from mesic areas along the coast of California.
Lindeborg (1952) was the first to measure water consumption of both P. m. rufinus and P. t. truei, the species and subspecies with which my experiments are concerned. Lindeborg also tested the ability of five races of Peromyscus to survive reduced water rations. Unfortunately, the subspecies chosen for these experiments did not include P. t. truei or P. m. rufinus. Lindeborg (1952:25) found that the "amounts of water consumed by various species of Peromyscus from different habitats within the same climatic region were not conclusively different." However, he did find significant differences between some subspecies from different geographical areas. For example, he found no significant difference in water consumption between P. m. bairdii from Michigan and either P. m. blandus or P. m. rufinus from New Mexico, but he found a highly significant difference between P. l. noveboracensis from Michigan and P. l. tornillo from New Mexico. Lindeborg also found that the subspecies of Peromyscus that consumed the least water, and that were best able to survive a reduced water ration, were those from the more xeric climatic areas.
Some mammals may be able to change their diets in times of water stress, and thereby compensate for a shortage of water. At such times, Dipodomys selects foods with high percentages of carbohydrates and conserves water by reducing the amounts of nitrogenous wastes to be excreted (Schmidt-Nielsen et al., 1948).
Williams (1959b) found that P. m. osgoodi from Colorado drank more water on a diet rich in protein than on one rich in carbohydrates. But, her mice on a high carbohydrate diet used less than a normal amount of water for a period of only five weeks; at the end of the five weeks they were drinking about as much as they had been when on the control diet of laboratory chow. Likewise, mice adjusted to the high protein diet by consuming more water; but by the end of the fifth week their daily water consumption approximated the amount drunk when fed on laboratory chow. Because of these results, Williams questioned the validity of the assumption that P. maniculatus is able to inhabit a diversity of habitats because of its adaptability with respect to food and water requirements.