Influence of Water Balance and Microclimate on the Local Distribution of the Redback Vole and White‐Footed Mouse

A comparison was made of the importance of water balance, as influenced by microclimate, upon the local distribution of the white—footed mouse, Peromyscus leucopus, and the redback vole, Clethrionomys gapperi. The characteristic habjtats of the two species in southern New England are dry upland woods and low swamps, respectively. Temperature and relative humidity in the habitats of the two species differed only from 0800 to 1600 on clear days; at other times they were the same. Daytime relative humidities averaged 75 to 85 and 70 to 80% in swamp and upland sites, respectively. There was no consistent difference in the absolute humidities between the swamp and upland at any season. Air temperatures during the summer were 2 and 3ᵒC lower in the swamp than in the upland; during the winter they were the same in both. Water turn—over rate of C. gapperi is appriximately 2.2 times that of P. leucopus (10.46 and 4.82 g/day, respectively). The difference results primarily from a more dilute urine from C. gapperi; the urine of P. leucopus is 2.2 times as concentrated as that of C. gapperi. Restriction of C. gapperi to low, wet areas in southern New England is correlated with the availability of standing water or an accessible water table, not with micorclimate. Evaporative water losses of C. gapperi are significantly greater than those of P. leucopus at absolute humidities of 6.0—12.8 mg/l; they were essentially the same at 16.3 mg/l. C. gapperi is only slightly diurnal; living in the drier uplands would increase its water requirements by only 0.02 g/day. The low water requirement for kidney function of P. leucopus permits it to live in drier upland wooded situations where free water is normally restricted to that available in food (fruits and insects). P. leucopus is strictly nocturnal; mcroclimates in swamps and uplands are similar at these times and would not be a factor in the water balance or local distribution of this species.