Some Aspects of the Life History of the Silky Pocket Mouse, Perognathus flavus

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Silky pocket mice were examined to determine the relationship of age to size, pelage condition, sexual activity, and season. Mice were placed in juvenile, subadult or adult age classes depending on the condition of the upper fourth premolars. As the juvenile pelage of early summer-born mice is replaced by adult pelage, adult dentition is acquired and some young adult mice become pregnant. Late summer-born mice acquire adult pelage by winter, adult dentition during the winter, and become sexually active the following spring. Juveniles, young adults, adults, and old adults were present in largest proportions in summer, summer and winter, fall, and spring samples respectively. Juveniles were absent from winter samples. Pregnant mice, absent from fall and winter samples, were present in largest proportion in spring. INTRODUCTION Pocket mice of the subgenus Perognathus present several interesting ecologic and taxonomic problems. Study of these problems requires knowledge of extent of individual morphological variation attributable to age. This study was undertaken to determine the relationship of age to size, molt, pelage condition, reproductive activity, and season in the silky pocket mouse, Perognathus flavus Baird. Acknowledgments.-Thanks are due Dr. James S. Findley, who guided me in research. Clyde Jones, Eugene Fleharty, Serafin Ramon, and Dr. Findley collected the largest series of mice used in this study. Dr. James C. Underhill and Dr. David J. Merrell read the manuscript and provided suggestions helpful in its preparation. METHODS The 453 silky pocket mice used in the study were collected in New Mexico. Most were caught by hand at night using a gasoline lantern as a light source. Some were taken in museum-special traps baited with chewed, rolled oats. Study skins and the cleaned skulls of all specimens are preserved in the University of New Mexico Museum of Southwestern Biology. Measurements of total length, tail length, hind foot length, ear length, and weight were taken from the unskinned animals. Body length was computed by subtracting tail length from total length. Occipitonasal length was taken from the cleaned skull. Linear measurements were taken in millimeters, weight in grams. The relative age of each specimen was determined by examination of its upper molariform teeth. Mice designated as juveniles showed no evidence of permanent upper fourth premolars. The permanent and deciduous upper fourth premolars are differentiated by the presence of an accessory cusplet on the anterior cusp of the deciduous tooth; the permanent tooth lacks this cusp. Designated as young adults were mice whose permanent upper fourth premolars were erupting, with the deciduous teeth in place or shed, or whose per-