SREL Reprint #1939

Population history, genetic variability, and horn growth in bighorn sheep

Nancy N. Fitzsimmons1, Steven W. Buskirk1, and Michael H. Smith2

1Department of Zoology and Physiology, Box 3166, University of Wyoming, Laramie, NW 82071, USA
2Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29801, USA

Abstract: Bighorn sheep (Ovis canadensis) are restricted in distribution and numbers relative to presettlement conditions. Some populations have allegedly suffered losses of fitness resulting from small, insular populations and a breeding system that reduces effective population size. Large horns in rams which confer breeding superiority, are absent from some populations, and this absence may result in part from loss of genetic variability. We investigated the relationship among allozyme variability, population history, and horn growth in bighorn sheep from the Rocky Mountains. Heterozygosity was higher for bighorn sheep than has been reported for Dall sheep (O. dalli). Heterozygosity and allelic variability were marginally related to effective population size for the preceeding 15 years. Horn growth was significantly higher in more heterozygous than in less heterozygous rams for years 6, 7, and 8 of life. By the end of year 8, more heterozygous rams had 13% higher horn volumes than less heterozygous rams. Most hunting of bighorn sheep involves selective removal of large-horned rams, which we hypothesize may reduce genetic variability of these populations and contribute to losses in fitness.

SREL Reprint #1939

Fitzsimmons, N.N., S.W. Buskirk, and M.H. Smith. 1995. Population history, genetic variability, and horn growth in bighorn sheep. Conservation Biology 9:314-323. 

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).