In North America, most wild sheep harvest regulations restrict legal harvest to males with a specified minimum horn curl. Because reproductive success is skewed toward larger males that are socially dominant, harvest regulations that target large males may select against high-quality, fast-growing males.
Given management alternatives, which harvest regulations are most effective in reducing the risk of artificial evolutionary changes in male bighorn sheep phenotype and fitness?
Read the paper on Harvest regulations and artificial selection on horn size in male bighorn sheep. Hengeveld, P.E. and M. Festa-Bianchet. 2011. The Journal of Wildlife Management 75(1): 189-197
SUMMARY OF ANALYSIS
- We analyzed horn increment measures of males harvested over 28 years (1975 – 2003) in 2 bighorn sheep (Ovis canadensis) ecotypes in British Columbia, Canada, to evaluate potential selective effects of alternative management strategies.
- We assessed variation in total horn length, early horn growth, and harvest age as a function of harvest regulation (Any Male, Three-Quarter Curl, Full Curl).
- We also assessed variation in total horn length, early horn growth, and harvest age as a function of permit type (General, LEH), hunter origin (British Columbia Resident, Non-Resident), and harvest year.
- Analyses were based on linear regression mixed-effects models computed in R statistical software packages.
KEY RESULTS / APPLICATION
- Under all regulations, males with the greatest early horn growth were harvested at the youngest ages, before the age at which large horns influence reproductive success. This was most notable in males harvested under Full Curl regulations.
- Full Curl regulations increased the average age of harvested males by <1 yr relative to Three-Quarter Curl regulations.
- Permit type (General vs Limited Entry Hunt) and hunter origin (British Columbia Resident vs Non-Resident) had little effect on horn size of harvested males.
- Age-specific horn measures in the California ecotype harvested under Three-Quarter Curl regulations declined over time but we observed no temporal declines in the Rocky Mountain ecotype, primarily harvested under Full Curl regulations.
- These results suggest that the risk of evolutionary consequences due to harvest may be greatest under the most restrictive Full Curl regulations, if harvest pressure is high. This risk is likely exacerbated for isolated populations that lack opportunities for genetic exchange from nearby protected populations.
- Management strategies that protect some males with greater early horn growth or provide harvest refuges to maintain genetic diversity are likely to reduce potential for negative effects of artificial selection.