DISCUSSION 495 Factors Affecting Population Densities 497 Adaptations to Environment 499 LITERATURE CITED 501 INTRODUCTION Centuries ago in southwestern Colorado the prehistoric Pueblo inhabitants of the Mesa Verde region expressed their interest in mammals by painting silhouettes of them on pottery and on the walls of kivas. Pottery occasionally was made in the stylized form of animals such as the mountain sheep. The silhouettes of sheep and deer persist as pictographs or petroglyphs on walls of kivas and on rocks near prehistoric dwellings. Mammalian bones from archeological sites reveal that the fauna of Mesa Verde was much the same in A. D. 1200, when the Pueblo Indians were building their magnificent cliff dwellings, as it is today. One of the native mammals is the ubiquitous deer mouse, _Peromyscus maniculatus_. The geographic range of this species includes most of the United States, and large parts of Mexico and Canada. Another species of the same genus, the pinyon mouse, _P. truei_, also lives on the Mesa Verde. The pinyon mouse lives mostly in southwestern North America, occurring from central Oregon and southern Wyoming to northern Oaxaca. This species generally is associated with pinyon pine trees, or with juniper trees, and where the pinyon-juniper woodland is associated with rocky ground (Hoffmeister, 1951:vii). _P. maniculatus rufinus_ of Mesa Verde was considered to be a mountain subspecies by Osgood (1909:73). The center of dispersion for _P. truei_ was in the southwestern United States, and particularly in the Colorado Plateau area (Hoffmeister, 1951:vii). The subspecies _P. truei truei_ occurs mainly in the Upper Sonoran life-zone, and according to Hoffmeister (1951:30) rarely enters the Lower Sonoran or Transition life-zones. _P. maniculatus_ and _P. truei_ are the most abundant of the small mammals in Mesa Verde National Park, which comprises about one-third of the Mesa Verde land mass. Under the auspices of the Wetherill Mesa Archeological Project, the flora of the park recently was studied by Erdman (1962), and by Welsh and Erdman (1964). These studies have revealed stands of several distinct types of vegetation in the park and where each type occurs. This information greatly facilitated my study of the mammals inhabiting each type of association. The flora and fauna within the park are protected, in keeping with the policies of the National Park Service, and mammals, therefore, could be studied in a relatively undisturbed setting. Thus, the abundance of these two species of _Peromyscus_, the botanical studies that preceded and accompanied my study, the relatively undisturbed nature of the park, and the availability of a large area in which extended studies could be carried on, all contributed to the desirability of Mesa Verde as a study area.