Interannual Variation in Timing of Parturition and Growth of Collared Pikas (Ochotona collaris) in the Southwest Yukon.

The length of the snow-free season has a significant influence on reproduction and growth in northern alpine environments, and these life history traits may provide sensitive indicators of the responses of organisms to climate change. We examined growth rates and timing of parturition of collared pikas (Ochotona collaris) from 1995-2002 in the Ruby Range, Yukon Territory, Canada. Growth rates were best described using a Gompertz model, in which the asymptotic mass, determined from the average male and female weights, was 157 g, the growth rate constant (K) was 0.0557, and the age at inflection (I) was 18.12 days, for a birth weight of 10 g. The maximum growth rate for North American pikas (O. collaris and O. princeps) increased with latitude, with maximum growth rates being approximately one-third greater in northern populations where the snow-free season is less than three months long. The mean parturition date varied significantly among years from 3 June to 3 July, and delayed parturition was correlated with indices of high snow accumulation and, to a lesser extent, late spring snowmelt. However, parturition date did not significantly affect the subsequent over-winter survival of juveniles in this population, suggesting that pikas are able to adjust to seasonal uncertainty associated with highly variable spring conditions.