Study examines bighorn interactions, disease implications

A study of bighorn sheep herds in the St. Mary and Waterton Lakes drainages draws some fascinating insight into the animals’ movements and interactions through the seasons.

It also lends insight into how disease may be transmitted from one herd to another, though there hasn’t been a known major outbreak of disease in the Waterton-Glacier International Peace Park since 1983, noted U.S. Geological Scientist Tabitha Graves, one of the authors of the study.

Graves, Marie I. Tosa and Mark Biel examined radio-collared sheep data gathered over 10 years from 2002 to 2012 in the parks, an effort that was originally spearheaded by U.S.G.S. biologist Kim Keating at the time.

The original idea of Keating’s study was to map bighorn sheep habitat in the drainages, but the sheep were fitted with GPS collars, meaning they could be tracked rather precisely, giving the authors an even greater insight into their behaviors.

All told, the study looked at 97 bighorn sheep — 54 males, 43 females.

Bighorn sheep naturally break into groups different times of the year. From late spring through summer until the breeding season, males generally live separately in groups in a different mountain terrain from females as the females raise the young. The study found few male-female contacts during the summer months — June to October.

They then all come together for the breeding season and will stay together during November, December and January.

The study found, among other things, that sheep in their daily lives are willing to make tradeoffs. Disease aside, bighorn sheep are susceptible to predation, particularly mountain lions. To avoid predation, sheep are generally not very far from escape terrain, most notably cliffs.

They are willing, the GPS data found, to live in habitat that perhaps might not be as good, but had protection from predators.

The herds also appeared to be making other tradeoffs as well.

For example, in areas where bighorn sheep were vulnerable to predation, contacts between groups or pairs, was more common. In addition, contacts of all dyads (pairs) were more likely to occur where the canopy cover was higher and contacts between all dyads types (meaning both male and female) were more likely to occur when bighorn sheep were farther away from escape terrain.”

In other words, the more eyes looking for predators, the better, even if it means having to share the tender shoots in a grassy alpine meadow.

The study also found geological barriers to to bighorn sheep movements across the landscape. For example, the St. Mary drainage at the time of the study, was heavily forested and proved to be a barrier to bighorn movements.

But today, it’s far more open, as the Reynolds Fire burned the valley. Further research may be warranted to see if sheep are now crossing the valley, Graves noted, and a broader study looking at the impacts of tree canopy on sheep behavior could be helpful.

Other drainages were also barriers to movement, such as the Belly River in the north end of Glacier. The barriers could also represent important locations for disease management, should Glacier have an outbreak.

Some individual sheep still make long journeys, it seems.

For example, a ewe made a long-distance journey from west of Waterton Lake to east of Waterton Lake and returned thereafter.

Contacts between groups varied and mineral licks played a big role.

Contacts between female pairs occurred in areas close to mineral licks, in locations that had a lot of snow, high canopy cover and were rugged. Males, however, also converged at mineral licks, but with low canopy cover, low ruggedness and far from escape terrain.

Males and females, however, contacted each other at intermediate distances from mineral licks, though both sexes generally liked to be close to mineral licks.

The role of predators has a distinct impact on bighorn behavior in Waterton-Glacier.

“We found that bighorn habitat use was highly influenced by survival variables and less so by resource variables. Similar to other studies, this suggests that predation has strong selective pressures on locations of bighorn and provides evidence that bighorn operate in a ‘landscape of fear,” the study found.