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

Area of Science:

• Ecology
• Evolutionary Biology
• Wildlife Management

Background:

• Human harvest of secondary sexual characteristics in wildlife, such as mountain sheep horns, raises concerns about evolutionary consequences.
• Selective harvesting of males with desirable traits can alter population age structures and genetic selection pressures.

Purpose of the Study:

• To investigate the influence of harvest, age structure, and environmental conditions on horn size and growth in North American mountain sheep (Ovis canadensis ssp.).
• To determine if selective hunting practices are driving evolutionary changes in horn morphology.

Main Methods:

• Analyzed horn size and growth data from 72 mountain sheep hunt areas across North America (1981-2016).
• Assessed the effects of age structure, climate, forage availability, and harvest metrics on horn morphology.
• Utilized statistical models to disentangle the impacts of age, environment, and harvest on horn size.

Main Results:

• Changes in mean horn size were linked to age structure shifts in 50% of hunt areas.
• Environmental conditions explained horn growth changes in 28% of areas.
• After accounting for age and environment, mountain sheep horn size remained stable or increased in approximately 78% of hunt areas.
• Age-specific horn size declined in 44% of areas with harvest regulated solely by morphological criteria.

Conclusions:

• While harvest regimes are a factor, mountain sheep horn growth has largely remained stable, suggesting evolutionary changes due to harvest are unlikely in most of North America.
• Phenotypic consequences of selective harvest are not inevitable, as many hunt areas with intensive and selective harvest showed no age-specific horn size decline.