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Pikas (Ochotona princeps) are small lagomorphs that occupy talus slopes in western North America. During the summer they gather and store vegetation in haypiles that are actively defended, resulting in a territorial spacing system (Broadbooks, 1965; Kawamichi, 1976). Pikas do not hibernate and haypiles probably serve as a major food source during the winter. Breeding occurs in early spring (April-June) with gestation averaging 30 days (Millar, 1972;Severaid, 1950) and weaning requiring appro:dmately the same length of time (Severaid, 1950;Whitworth, 1982). Once weaned, juveniles need to establish territories and begin storing vegetation in preparation for winter (Conner, 1983).Because tundra environments are tYPified by a short growing season, selection pressures would likely favor high growth rates in pikas. Whitworth and Southwick (1981) reported that captive pikas reach minimum adult weight (150 g) at approximately 3 months of age and are among the fastest growing lagomorphs. They found that growth rates of pikas corresponded well with growth rates calculated from natural populations of the tundra hare, Lepus othus (Anderson and Lent, 1977). Anderson and Lent postulated that the high growth rates of L. othus were the result of selective pressures which affect both plant and animal species in tundra environments.The purpose of this study was to compare the growth rates of wild pikas in Colorado with those of captive pikas and to develop an equation that could be used to age animals accurately in the field. Such an equation has been developed for pikas in Alberta (Millar and Tapper, 1973).Growth data on captive pikas include data from Whitworth and Southwick (1981) and additional data collected during the spring and summer of 1981. From 1979From -1981 pregnant females (as determined by weight and palpation) 9 were captured from two sites in Summit Co., Colorado (Monte Cristo and Mt. Evans; elev. 3,960 and 3,450 m, respectively). Pikas were transported to animal-care facilities at the University of Colorado, Boulder. Female pikas and their captive-born litters were housed in large galvanized bins (0.6 by 1.5 by 0.6 m) eqUipped with metallic nest boxes (30 by 20 by 15 em). A detailed description of husbandry was provided elsewhere (Whitworth, 1982).Of the 36 pregnant females captured, 26 gave birth in the laboratory. Weights of pups were recorded on an O'halls triple-beam balance every other day from day one (day of birth) through day 60. Beyond day 60, weights were recorded weekly. Instantaneous growth rates (Kg) were calculated from the formula Kg = (1n M2 -In M 1 )/(t 2 -t l )

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