Hunting and mountain sheep: Do current harvest practices affect horn growth?

Tayler N LaSharr; Ryan A Long; James R Heffelfinger; Vernon C Bleich; Paul R Krausman; R Terry Bowyer; Justin M Shannon; Robert W Klaver; Clay E Brewer; Mike Cox; A Andrew Holland; Anne Hubbs; Chadwick P Lehman; Jonathan D Muir; Bruce Sterling; Kevin L Monteith

doi:10.1111/eva.12841

Hunting and mountain sheep: Do current harvest practices affect horn growth?

Evol Appl. 2019 Jul 29;12(9):1823-1836. doi: 10.1111/eva.12841. eCollection 2019 Oct.

Authors

Tayler N LaSharr¹, Ryan A Long², James R Heffelfinger³, Vernon C Bleich⁴, Paul R Krausman⁵, R Terry Bowyer⁶, Justin M Shannon⁷, Robert W Klaver⁸, Clay E Brewer⁹, Mike Cox¹⁰, A Andrew Holland¹¹, Anne Hubbs¹², Chadwick P Lehman¹³, Jonathan D Muir¹⁴, Bruce Sterling¹⁵, Kevin L Monteith¹⁶

Affiliations

¹ Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology and Physiology University of Wyoming Laramie WY USA.
² Department of Fish and Wildlife Sciences University of Idaho Moscow ID USA.
³ Arizona Game and Fish Department Phoenix AZ USA.
⁴ Department of Natural Resources and Environmental Science University of Nevada Reno Reno NV USA.
⁵ School of Natural Resources and the Environment University of Arizona Tucson AZ USA.
⁶ Institute of Arctic Biology University of Alaska Fairbanks Fairbanks AK USA.
⁷ Utah Division of Wildlife Resources Salt Lake City UT USA.
⁸ US Geological Survey, Iowa Cooperative Fish and Wildlife Research Unit, Department of Natural Resource Ecology and Management Iowa State University Ames IA USA.
⁹ Western Association of Fish and Wildlife Agencies-Wild Sheep Working Group Texas Parks and Wildlife Department Rochelle TX USA.
¹⁰ Nevada Department of Wildlife Reno NV USA.
¹¹ Colorado Parks and Wildlife Fort Collins CO USA.
¹² Alberta Environment and Parks Rocky Mountain House AB Canada.
¹³ South Dakota Game Fish and Parks Custer SD USA.
¹⁴ Oregon Department of Fish and Wildlife Lakeview OR USA.
¹⁵ Montana Fish, Wildlife, and Parks Thompson Falls MT USA.
¹⁶ Haub School of Environment and Natural Resources, Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology and Physiology University of Wyoming Laramie WY USA.

Abstract

The influence of human harvest on evolution of secondary sexual characteristics has implications for sustainable management of wildlife populations. The phenotypic consequences of selectively removing males with large horns or antlers from ungulate populations have been a topic of heightened concern in recent years. Harvest can affect size of horn-like structures in two ways: (a) shifting age structure toward younger age classes, which can reduce the mean size of horn-like structures, or (b) selecting against genes that produce large, fast-growing males. We evaluated effects of age, climatic and forage conditions, and metrics of harvest on horn size and growth of mountain sheep (Ovis canadensis ssp.) in 72 hunt areas across North America from 1981 to 2016. In 50% of hunt areas, changes in mean horn size during the study period were related to changes in age structure of harvested sheep. Environmental conditions explained directional changes in horn growth in 28% of hunt areas, 7% of which did not exhibit change before accounting for effects of the environment. After accounting for age and environment, horn size of mountain sheep was stable or increasing in the majority (~78%) of hunt areas. Age-specific horn size declined in 44% of hunt areas where harvest was regulated solely by morphological criteria, which supports the notion that harvest practices that are simultaneously selective and intensive might lead to changes in horn growth. Nevertheless, phenotypic consequences are not a foregone conclusion in the face of selective harvest; over half of the hunt areas with highly selective and intensive harvest did not exhibit age-specific declines in horn size. Our results demonstrate that while harvest regimes are an important consideration, horn growth of harvested male mountain sheep has remained largely stable, indicating that changes in horn growth patterns are an unlikely consequence of harvest across most of North America.

Keywords: artificial evolution; bighorn sheep; harvest‐induced evolution; horns; selective harvest; trophy hunting.