Genetic Decline in Bighorn Sheep Reversed

An isolated population of Rocky Mountain bighorn sheep suffered from decades of inbreeding until the herd was genetically rescued. A detailed study of the herd's genetic makeup discovers that reproductive declines due to the limited genetic variability of a small population can occur quickly. But such declines can also be rapidly reversed.

Bighorn sheep from Alberta's Banff National Park started a new herd at National Bison Range of northwest Montana in 1922 with the release of 12 animals. Initially the wild herd grew rapidly, but soon peaked at 90 animals and ultimately averaged 42 sheep over the ensuing decades.

Genetic diversity of the insular group was exceptionally low, restricted to the limited gene pool of the dozen founding animals. For 11 generations the Bison Range sheep endured a population bottleneck.

Then wild sheep, captured mainly from other Montana herds, were transplanted to National Bison Range on several occasions from 1985 to 1994, totalling 15 animals. The recent migrants soon brought about dramatic changes to the herd's reproductive fitness.

The genetically impoverished Bison Range herd quickly rebounded. By 1993 genes from the new introductions predominated and the herd's population growth rate reversed its decline.

Animals carrying the most genes from the new migrants were noticeably fitter. These females bore lambs averaging one kilogram heavier and gave birth nine days earlier than the most inbred ewes. Both characteristics contribute to greater reproductive success. Ewes with lots of migrant genes were 2.2 times more successful at producing healthy young each year compared with the relatively inbred females.

Rams possessing the highest proportion of migrant genes also showed 2.6 times greater reproductive success, siring more lambs than the most inbred males. Additionally, mature bighorn sheep lived two years longer on average if they had plenty of migrant genes.

Scientists say the genetic rescue of the Bison Range sheep was surprisingly swift. The rapid rate at which reproduction improved indicates that the original population was overwhelmed by deleterious genes. Not only did additions of sheep augment genetic diversity, the new genes enhanced survival and reproduction. Since the founders' harmful genes were largely recessive, the newly-contributed genes overshadowed any negative effects.