Acta Theriologica 44 (2): 151-160, 1999. PL ISSN 0001-7051 Discrimination between Peromyscus leucopus noveboracensis and Peromyscus maniculatus nubiterrae in the field Joseph A. BRUSEO, Stephen H. VESSEY and John S. GRAHAM Bruseo J. A., Vessey S. H. and Graham J. S. 1999. Discrimination between Peromys cus leucopus noveboracensis and Peromyscus maniculatus nubiterrae in the field. Acta Theriologica 44: 151-160. Syntopic Peromyscus leucopus noveboracensis (Fischer, 1829) and P. maniculatus nubiterrae (Rhoads, 1896) exhibit considerable overlap in external morphologies in the Appalachian mountains of the Eastern United States, making field identification of live individuals questionable in some instances. We compared two techniques for correctly identifying these live individuals in the field: the tail:body ratio and weight criterion, and discriminant function analysis of external morphological characters. Electrophoresis of salivary amylase was used to confirm species identity. The taihbody ratio and weight criterion was a poor character combination for discriminating between species, with 36% of new (unmarked) individuals misclassified. Models generated from discriminant function analysis resulted in up to 92% correct classification to species of live individuals. For any individual Peromyscus (Gloger, 1841) captured, four quantitative characters (tail length, body length, ear length, and weight) were the most useful in discrimination between species. While classification equations provided improved species identification, they still resulted in a high degree of error. Only electrophoresis of salivary amylase provided unambiguous species identification in the field, and we recommend the use of this technique. Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403, USA, e-mail: jbruseo@bgnet.bgsu.edu Key words: Peromyscus leucopus, Peromyscus maniculatus, discriminant function, field identification, electrophoresis Introduction Ir. any ecological investigation, it is critical to positively identify species. The white-footed mouse, Peromyscus leucopus noveboracensis (Fischer, 1829), and the cloucland deer mouse, Peromyscus maniculatus nubiterrae (Rhoads, 1896) coexist in the deciduous forests of the central Appalachian Mountains of the Eastern United States (Wolff 1985, Bruseo and Barry 1995). These species exhibit substantial overlap in both niche requirements and morphology (Wolff 1985). Such overlap has pronpted numerous investigations into the presence of resource partitioning and its rcle in mediating interspecific competition between these mice. Assessment of such ecological mechanisms is highly dependent on correct species identification. Misidentification of one or a few individuals can lead to commission of Type I or [151]