Identification of Peromyscus species in the field can often be time consuming and inaccurate. Determination of several morphological characteristics is usually required for reliable identification. Characteristics may differ only slightly, can be highly variable, or can overlap among species. In oak woodlands of the southern Sierra Nevada in Fresno County, most identifyng characteristics of the two dominant Peromyscus species, brush mouse (P. boylii) and California mouse (P. californicus) overlapped considerably. Weight, body length, tail length, hind foot length, dorsal tail-stripe width, and ear length were usually sufficient for accurate species identification. Hind foot length and dorsal tail-stripe width were the most reliable of the identification criteria examined. For hind foot length, no significant difference was found between juveniles and adults within each species. Hind foot lengths overlapped slightly between species, ranging from 20 to 25 mm for brush mice and 24 to 28 mm for California mice. Dorsal tail-stripe width had the lowest overlap between species. All of the brush mice had dorsal tail-stripes less than or equal to half the tail circumference, and 98 percent of the California mice had dorsal tail-stripes greater than half. Use of hind foot length and dorsal tail-stripe width as key field identifiers was an expedient and accurate approach for distinguishing these two species.
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