Did a Pneumonia Die-Off Leave Rocky Mountain's Bighorn Sheep Inbred?

More varied genes can mean less disease. NPS-RMNP.

The question

The bighorn sheep (Ovis canadensis canadensis) of the park's Mummy Range declined in a pneumonia die-off in the 1990s. Managers wondered whether it had caused a population bottleneck: a sharp loss of genetic variety that leads to inbreeding because there are fewer possible mates. A smaller gene pool can lower fitness, such as lamb survival and breeding, and the ability to adapt to disease or other stress. If a severe bottleneck were found, the park could add sheep to the herd to improve its genetic health and chances of survival.

The study

In 2005, Gordon Luikart of the University of Montana and park volunteers collected fecal pellets near the Sheep Lakes mineral lick and in the Mummy Range, taking care not to disturb the animals. In the lab, Luikart extracted DNA from the pellets and measured its variation, using statistics to test for a bottleneck and for recent newcomers that might add variety. The pellets were also checked for lungworm larvae, a heavy load of which may make bighorns prone to pneumonia outbreaks.

The results

• The herd's genes looked like those of a population without a bottleneck.
• But no genes from outside the herd turned up, so no recent immigrants were adding to its gene pool.
• Lungworm larvae, which likely contribute to pneumonia die-offs, were in more than half the samples.
• Sheep with more heterozygosity, two different versions (alleles) of a gene at the same place on a chromosome, had fewer parasites. The sample was small, but it suggests that less genetic variety (inbreeding) can weaken resistance to parasites.

Studies like this, done without handling animals, can answer questions that once needed intensive capture; in future the park may track herds by sampling droppings instead of fitting collars.

This summary rests on reports available when it was written (September 2005, updated May 2009) and is not a statement of park policy or a definitive account of the research. More on the park's research: nps.gov/romo.

Sources

• Colleen Flanagan, "Population Genetics of Bighorn Sheep," National Park Service, Rocky Mountain National Park. https://www.nps.gov/articles/population-genetics-of-bighorn-sheep.htm
• A textbook diagram of a population bottleneck, from Pearson Education, is not reproduced here.