Caribou are complicated. Across the species Rangifer tarandus (which includes reindeer) there is huge variety in size, color, and behavior. For instance, some caribou generally stay put, while others migrate vast distances. Even the way the migraters migrate has perplexed some scientists.
“There is an incredible amount of unexplained and unexplainable mystery to the drivers of caribou migration,” said Elie Gurarie, an assistant professor at SUNY College of Environmental Science and Forestry. Some aspects of this great migration, however, are becoming better understood.
The map above shows the range of seven major caribou herds in northern Alaska and Canada. These are barren-ground (migratory tundra) animals. This “ecotype” of caribou migrates hundreds of miles each spring, moving toward the continent’s northern coast, where they birth their young. In contrast, woodland caribou live throughout the boreal forests and mountain ranges of North America. They are less social and do not migrate.
The timing of the migration—when the caribou depart their wintering ranges and when they arrive at their calving grounds—was thought to depend on factors such as the timing of snowmelt and the availability of vegetation for forage. In research published in 2019, Gurarie (previously at the University of Maryland) and colleagues showed that other factors mattered more. “Nothing that we expect to be important—snow, vegetation, etc.—ended up being important,” he said.
The research, funded in part by NASA’s Arctic-Boreal Vulnerability Experiment (ABoVE), involved tracking more than 1,000 caribou from seven herds between 1995 and 2017. Their movements were correlated with factors such as local weather, the timing of melting snow, vegetation availability, and global climate patterns.
Gurarie and colleagues observed that the start of the spring migration each year was generally synchronized among all herds across the continent. This coordinated departure in March or April, they found, depends primarily on large-scale climate cycles. You can see the synchronous departure in the video above, which shows the animals’ tracked movements between February and July in 2013 and 2014.
The arrival of caribou at their calving grounds, however, is much more staggered and depends on weather conditions during the previous summer—namely, whether it was a warm and windless summer that favors insects. Harassment by insects such as mosquitoes, black flies, and botflies can have a detrimental effect on the health of female caribou, so a buggy summer this year can mean a delayed arrival at the calving grounds next spring.
Gurarie said he was equally surprised by the massive, continental-wide synchrony of the start of the migration, and the delayed effect of last summer’s insects on the current year’s migration arrival time. What this relationship between climate, weather, and migration means for the future of caribou herds amid warming Arctic summers remains to be seen. As Gurarie and colleagues wrote in their paper: “A central challenge in arctic ecology is understanding the potential effects of a changing climate on caribou and reindeer, the most widespread terrestrial herbivore in the Arctic.”
NASA Earth Observatory image by Joshua Stevens, using data from Gurarie, E., et al. (2019). Video/animation by Elie Gurarie. Story by Kathryn Hansen.