Rapid decrease in horn size of bighorn sheep: environmental decline, inbreeding depression, or evolutionary response...

Area of Science:

• Wildlife Ecology
• Conservation Biology
• Evolutionary Biology

Background:

• Phenotypic changes in wild populations are often influenced by selection pressures.
• Desert bighorn sheep (Ovis canadensis) are a species of conservation concern, particularly regarding trophy ram horn size.

Purpose of the Study:

• To document and investigate the causes of a significant reduction in horn size in desert bighorn sheep rams over 30 years in Aravaipa Canyon, Arizona.
• To evaluate the potential roles of environmental change, inbreeding depression, and hunter selection in this observed phenotypic shift.

Main Methods:

• Analysis of horn size data from trophy desert bighorn sheep rams over a 30-year period.
• Examination of environmental factors, including winter rainfall and drought conditions.
• Assessment of demographic indicators such as the lamb-to-ewe ratio.
• Theoretical modeling to evaluate the independent and combined effects of inbreeding and hunter selection.

Main Results:

• A significant reduction in ram horn size was documented in the reintroduced Aravaipa Canyon population.
• Ram horn size reduction was positively associated with reduced winter rainfall, suggesting drought as a major contributing factor.
• The lamb-to-ewe ratio declined in the Aravaipa population, but not statewide, indicating local effects.
• Neither inbreeding depression nor hunter selection alone fully explained the horn size decrease.

Conclusions:

• Environmental deterioration, primarily drought, is a likely major driver of reduced horn size in Aravaipa Canyon bighorn sheep.
• A combination of environmental factors, inbreeding depression, and hunter selection may have contributed to the observed horn size decline.
• Management strategies such as supplemental feeding, cattle removal, and translocation are suggested to mitigate these effects.