Ecological Relationships of Peromyscus maniculatus and Perognathus parvus in Eastern Washington

Populations of the deer mouse, Peromyscus maniculatus, and the Great Basin pocket mouse, Perognathus parvus, were studied from April through September 1969 in eastern Washington. Live-traps were set along an elevational gradient from 152 to 1067 meters. Peromyscus density increased with elevation, and that of Perognathus decreased. Edaphic conditions appear to be a major factor in distribution; Perognathus, a deep burrower, finds the stoneless, loose, sandy soil of the desert floor optimal habitat, whereas Peromyscus prefers shallow holes in the rocky substrate found at upper elevations. Deer mice may also thrive only at higher elevations because more moderate summer temperatures and greater available moisture permit them to remain active and to breed during the warmest months. Analyses of cheek pouch and stomach contents showed the greatest dietary overlap to occur in early spring, prior to seed ripening, when both species feed on insects. Thereafter Perognathus is preferentially granivorous; Peromyscus continues to be largely insectivorous. Partial temporal separation exists in both daily and seasonal activity patterns. Home ranges do overlap interspecifically but fights initiated by the more dominant Peromyscus appear to be rare in the naturally open field situation.