Water Stress of Free‐Living Peromyscus Truei

The water relations of a Peromyscus truei population from a xeric environment, chamise chaparral, were studied in the field during two periods, midsummer and late summer, and in the laboratory. Animals were maintained in the laboratory on three water regimens that represented a gradient of water stress, but food was available ad libitum. When subjected to increasing degrees of water stress, urine osmotic concentrations increased, feces became drier, and body—water turnover rates, determined with tritiated water, decreased. When subjected to the severest water stress, diurnal torpor occurred and apparently caused a decline in urine concentration and an increase in fecal water content during the daytime. Urine osmotic concentrations and fecal water contents of midsummer mice in the field were similar to values for mice subjected to severe water stress in the laboratory. In late summer, however, the values indicated low water stress. The lower water exchange during midsummer, as determined by tritiated—water turnover, indicated that the greater water stress at this time was at least partly due to a reduced availability of water compared to late summer. During both periods food was the only apparent source of water. Mice in midsummer were feeding primarily on insects, foliage, and fungi, but during late summer, when acorns became available, they switched to this as the predominant dietary component. Acorn utilization was accompanied by onset of female reproductive activity and body weight increases for both sexes. It is suggested that water stress was alleviated by the acorn diet and was responsible for the lack of reproductive activity and depressed body weights for several months during summer.