Effect of Adult Sex Ratio on Mule Deer and Elk Productivity in Colorado

Abstract

Mule deer (Odocoileus hemionus) and elk (Cervus elaphus nelsoni) in Colorado showed a decline in post-harvest young:female ratios during 1975-1995. One hypothesized cause of this decline in productivity is a decline in male:female ratios during the breeding period. We examined Colorado Division of Wildlife (CDOW) deer and elk population composition data obtained from helicopter surveys to see whether sex ratios explained variation in young:female ratios. Data for both deer and elk supported a response of young:100 females ratios to the male:100 females ratios during the previous year. The observed ratios were about 0.25 fawns:100 does per I buck:100 does for deer (95% CI ± 0.14) and 0.28 calves:100 cows per 1 bull:100 cows for elk (95% CI ± 0.12). However, these effects were not adequate to explain the decline in fawn:doe (1.14 fawns:100 does per year) and calf:cow ratios (0.68 calves:100 cows per year) observed during 1975-1995. Differences in the sex ratio:productivity relationship observed between populations suggest that only some areas might show an increase in young:female ratios in response to an increase in male:female ratios, and then only a small increase in young:females was predicted. We did not detect a threshold of male:female ratios for either species that precipitated a drastic decline in productivity. Based on commonly employed population composition surveys, we conclude that increasing post-season sex ratios will have little if any impact on subsequent population productivity.