Good genetic diversity is key to long term population health. Foundation Presdient John Wehausen, in collaboration with Dr. Rob Ramey of Colorado, recently began important research on the genetics of Sierra bighorn. This work is exciting because DNA for analysis is extracted from the fecal pellets of the sheep rather than from blood or tissue. Blood and tissue samples provide more and better DNA, which means less lab work, but it would never be possible to sample more than a small fraction of the sheep in each herd using such sources. The fecal source alternative used by Drs. Wehausen and Ramey offers the potential to sample all sheep in the smaller herds.
The goal of this research is to investigate the extent of losses of genetic variation due to small herd sizes, and the likelihood of future genetic problems in the population. To accomplish this goal it is necessary first to identify sheep individually by their genotypes. This allows an assessment of field data on population sizes. Very important progress on this question was made in the past year, especially regarding the Mount Gibbs and Mount Williamson groups.
The Mount Gibbs herd unit sits on the eastern border of Yosemite National Park. We have been following the dynamics of a small group of bighorn there for years. Not long ago this group had a single female as its reproductive base. We watched as she finally produced a surviving female lamb that increased the reproductive base to two ewes. Analysis of fecal samples collected in 1998 correctly identified this group as the two females and four males we sampled.
In 1999 we collected additional samples from two ewes and a lamb observed on Mount Gibbs, with the expectation of adding one new genotype from the lamb to our list. Much to our surprise, only one female matched those from 1998, while the other female was a new sheep. In field work this past summer we verified the existence of an additional female in this herd. We hope to use DNA to determine whether this additional female moved to Mount Gibbs from Mount Warren or was in the Mount Gibbs area all along.
The small herd in the Mount Williamson region (north of Mount Whitney) has been of concern for some years and has been one of our highest priorities for genetic research. This herd is one of three surviving native herds in the Sierra Nevada, but has not used its low elevation winter range since 1985. During the summers of 1997, 1998, and 1999 we collected samples high on Mount Williamson from what were clearly small groups of sheep. This area was the core summer range of this herd in the 1970s, but we have seen only a couple of sheep there during recent summers and the rarity of sign suggested very few remaining sheep. However, genotypes from these samples now indicate considerably more sheep than expected. While the genotypes show that we have sampled some individuals in multiple years, our samplings have added three new females each year, a number not consistent with the sparse sheep sign we’ve found there. It appears that we have been looking for sheep and collecting samples only on the margin of the habitat they now use. With the abandonment of winter ranges, these sheep may have shifted their core summer range. Future field work in this region will now focus on finding the core ranges of these sheep. These lab results are becoming an important tool to guide field research.