VARIATION IN MORPHOLOGICAL-CHARACTERISTICS OF THE WHITE-FOOTED MOUSE (PEROMYSCUS-LEUCOPUS) AND THE DEER MOUSE (P-MANICULATUS) UNDER ALLOTOPIC AND SYNTOPIC CONDITIONS

White-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus) were studied under both allotopic and syntopic conditions in northeas tern Kansas. These species typically occupy different habitats with th e slightly larger P. leucopus inhabiting wooded areas, and the smaller P. maniculatus inhabiting grasslands. We recorded external measuremen ts of each species in distinct habitats where only one species was typ ically present. Results were compared to measurements obtained from a grassland area recently invaded by woody plants where both species occ ur in syntopy. Under allotopic conditions, the two species were morpho logically distinct and individuals could be readily assigned to one of the two species using a combination of measurements. Under syntopic c onditions, however, many individual Peromyscus were intermediate in ex ternal measurements when compared to the allotopic populations, and co uld not easily be classified into one of the two species. Morphologica lly intermediate animals in the syntopic population could be the resul t of smaller than average adult P. leucopus occupying the area. The ea rly successional habitat of the syntopic population is not typical for P. leucopus, but the recent invasion of woody plants may provide adeq uate habitat for smaller individuals, which may have been pushed out o f the surrounding woodlands by larger individuals.