Mule Deer Survival in Colorado, Idaho, and Montana

Abstract

We examined survival rates of mule deer (Odocoileus hemionus) fawns (1 Jan-31 May) and adult (≥1 vr old) females (1 Jun-31 May) from Colorado, Idaho and Montana to assess the influence of survival on population dynamics over a broad geographic area. Survival rates were estimated from 1,875 radiocollared fawns and 1.536 radiocollared adult female-vears. We found significant year-to-year differences in overwinter survival rates of fawns among states (P < 0.001), while annual survival rates of adult females showed less variation across vears (P < 0.256). Sampling distributions of survival rates by age class were modeled with the beta-binomial distribution (BBD) and not found different among states (ad F: P = 0.118; fawns: P = 0,856), The mean overwinter survival rate for fawns was 0.444 (SE = 0.033), with SD = 0.217 (SE = 0.019). The mean annual survival rate for adult females was 0.853 (SE = 0.011), wiht SD = 0.034 (SE = 0.014). All 3 states exhibited differences in body size of fawns at the start of winter across years, and body size was a predictor of overwinter survival (P < 0.001). Fawn sex ratios in December at time of capture were not different from 50;50 (P = 0.729). However, a sex differential in overwinter survival of fawns was observed (P = 0.002), but beta-binomial models of survival distributions were not different between sexes (P = 0.458). Frequencies of 3 categories of proximal causes of fawn mortality predation, winter malnatrution other) differed among states (X 2 4 = 41.24, P < 0.001 ). A deterministic model with a mean winter survival rate of 0.444 for fawns and an annual rate of 0.853 for adult females predicted December fawn:doe ratios would have to be at least 66: similar processes regulate these populations hence results from specific study areas are generally more applicable than commonly thought.