Why Some Caribou Calves Don't Survive
Close monitoring of 50 newborn woodland caribou calves for their first two months of life provides some insights into caribou behaviour and the factors that influence calf survival.
Northern ecotype caribou when bearing young need to balance avoiding predators with accessing nutritious vegetation.
Calf mortality peaked among caribou of northeastern British Columbia in the second week of life, with wolverines, unexpectedly, a main predator. A second and higher peak in calf deaths occurred near the end of their first month. This time gray wolves were the main cause. Grizzly bears and eagles took a smaller number of the calves studied.
Pregnant caribou choose landscapes where they avoid wolves and bears, but at different scales. Overall, the calving areas support relatively sparse vegetation, yet are far from wolves. Within these areas, caribou cows give birth at sites relatively safe from grizzly bears and near high-quality browse. More newborn calves survive in areas with plenty of willow, bog birch and other shrubs to conceal young from predators.
For cow-calf pairs that migrated away from the birth site within 2 to 7 weeks, calf survival was 200 percent better. Migrating enabled access to better forage and possibly avoiding wolves.
The three calving areas studied southwest of Fort Nelson vary in vegetation, elevation and topography, ranging from forested valleys to rugged alpine. They also differ in the risk of encountering predators, but still produced similar calf survival rates of around 70 percent.
David D. Gustine, Katherine l. Parker, Roberta J. Lay, Michael P. Gillingham and Douglas C. Heard. 2006. Calf Survival of Woodland Caribou in a Multi-Predator Ecosystem. Wildlife Monographs. 165(1): 1-32.