Evolutionary demography and conservation lab - Labo de démographie évolutive et conservation


The Sheep River study area


Study area and population


Sheep River is located on the eastern slopes of the foothills of the Canadian rockies. This is a native bighorn sheep winter range and is separated from the summer range by 7 km of boreal forest. The area used by the sheep is approximately of 4-6 km2 at 1500-1700 meters of elevation. The valley is crossed by the Sheep River; its steep canyons offer access to natural salt licks and refuge from predators. In this area, escape terrain is also found in cliffs and rocky slopes. The vegetation is mostly constituted of patches of coniferous forest and alpine meadows bordered by aspen crop. In contrast to Ram Mountain, Sheep River is well connected to other populations of bighorn sheep. Although sheep are philopatric (they return to same their natal winter range each year), several males migrate to alternative populations during the rut. These breeding migrations are likely to be part of the male bighorn mating strategy as they are only observed in subordinate individuals. During the lambing season and summer, females migrate to a higher altitude in the alpine (2000-2400 meters) and during that time the ranges of females from at least three different populations overlap. One possible explanation for these movements is that predation by coyotes, wolves, bears or others, is lower at high altitude as the topography provides more secure escape terrain for females with new-born lambs.


The field camp is usually open throughout the year but most of the observations are made during spring, summer and fall. During this period, our work consists of weekly censuses of the sheep present in the study area. These censuses provide us with detailed information on population size and composition, survival, weight changes, and reproduction of marked individuals. In addition, there are several projects studying the behaviour of the sheep including: sexual segregation, social dominance, mating behaviour, breeding migration, and maternal care.


At Sheep River, lambs 4-6 months old are captured using a dart gun and tranquilizer drugs. Some older animals are also sometimes caught if they are not marked or for GPS collaring. As most individuals are caught only once in their life, compared with Ram Mountain, we do not have the same level of detail about the growth of individuals over their lifespan. However, starting in 2000, we have devised a system to attract the sheep to a platform scale (see picture). We use this system, in fall to collect data on body mass for most individuals of this population. Sheep River is one of the rare studies were it is possible to link animal behaviour with individual characteristics such as body mass or social rank. For example, individual social rank for the Sheep River rams has been monitored in the last 19 years. Work by John Hogg, has also allowed us to have information on reproductive success for this population since 1988.



Figure 1: Platform scale used to weigh the bighorn at Sheep River.


The monitoring of the Sheep River population was initiated in 1981 by Marco Festa-Bianchet. In this population, no experimental manipulation of population size has been made. In 1985-1986 there were pneumonia die-offs, with more that 50% of individuals dying over winter. In 1988, John Hogg joined the project and began monitoring male mating behaviour, social rank and paternity success. After the die-off, a high decline in population size occurred twice due to intense predation by cougars. The first predation event (93-95) was very well documented as a study on cougars was simultaneously conducted by Ian Ross and Martin Jalkotzy. From 1983 to 1995 most cougars were equipped with radio collars, and records of their kills were obtained by back-tracking their movements every week. In 1983–1992, the home ranges of 2–4 cougars overlapped the sheep winter range but they killed less than one bighorn a year. In 1993–1995, two radiocollared cougars began preying on bighorn sheep. One 10-year-old female who previously fed mostly on deer (Odocoileus spp.) switched to killing almost exclusively sheep. Her home range, which had overlapped with the bighorn winter range since her birth, shrank to include exclusively the areas used by sheep. The predation episode ended with this cougar’s death in 1995. Between 2000 and 2004, similar events have occurred. Although, we have not had as much detailed information on the cougars, we have various field evidence of elevated predation by cougar. This evidence includes recovery of cougar kills, easily recognized because the hair is stripped and remains are buried; observation of attacks and stalking; abrupt disappearance of animals that were in good condition and routinely observed; and increased wariness, agitation and location shifts by bighorn. Since 2002, John Hogg and Kathreen Ruckstuhl have expanded the monitoring of the animals outside the winter range. Using GPS technology, they are now monitoring the sheep during the breeding migration to understand movement, mating system and genetic exchange at meta-population scale.


Figure 2: Total number of sheep at Sheep River between 1982 and 2004, the red bars represents years with cougar predation.

From: Festa-Bianchet et al. 2005 Proc. Roy. Soc. Lond. B