Why do caribou calve where they do?

A female Western Arctic Herd caribou with her newborn calf, Alaska.

NPS/Matt Cameron

Caribou are well-known for returning to calving areas each spring, but why do they use those particular places? Researchers at the National Park Service and University of Alaska - Fairbanks set out to investigate this question for Alaska’s largest caribou herd, the Western Arctic Herd, that lives in northwest Alaska. They determined where females delivered their calves using GPS collar data from 2010 through 2017, calculated the calving area for each year, and then compared each calving area to every other year to see how consistent they were. Lastly, they used satellite data to test what characteristics of the landscape caribou selected for calving. They found that calving areas typically differ from year to year, but share a common area of consistent overlap. This general area, called the Utukok Uplands in the North Slope, has not changed since the first study to document calving areas for the herd in the early 1960s. They also found that calving areas are characterized by high-quality vegetation at the average time when females have their calves. This finding suggests that caribou are using memory to guide them to the calving area each year, and then once in the general area, are searching for high-quality forage such as the flowering heads of cottongrass. Read the manuscript, linked below, for a more detailed account of the study and the implications for migratory species around the world.

Pronounced Fidelity and Selection for Average Conditions of Calving Area Suggestive of Spatial Memory in a Highly Migratory Ungulate

Abstract

A distinguishing characteristic of many migratory animals is their annual return to distinct calving (birthing) areas in the spring, yet the navigational mechanisms employed during migration that result in this pattern are poorly understood. Effective conservation of these species requires reliable delineation of such areas, quantifying the factors that influence their selection, and understanding the underlying mechanisms resulting in use of calving areas. We used barren-ground caribou (Rangifer tarandus granti) as a study species and identified calving sites of the Western Arctic Herd in Alaska using GPS collar data from 2010–2017. We assessed variability in calving areas by comparing spatial delineations across all combinations of years. To understand calving area selection at a landscape scale, we performed a resource selection analysis comparing calving sites to available locations across the herd’s range and incorporated time-varying, remotely sensed metrics of vegetation quality and quantity. We found that whereas calving areas varied from year to year, this annual variation was centered on an area of recurring attraction consistent with previous studies covering the last six decades. Calving sites were characterized by high-quality forage at the average time of calving, but not peak calving that year, and by a narrow range of distinct physiographic factors. Each year, calving sites were located on areas of above-average conditions based on our predictive model. Our findings indicate that the pattern of spring migration for pregnant females was to migrate to areas that consistently provide high-quality forage when averaged across years, and then upon arriving at this calving ground, refine selection using their perception of annually varying conditions that are driven by environmental stochasticity. We suggest that the well-documented and widespread pattern of fidelity to calving grounds by caribou is supportive of a navigational mechanism based on spatial memory at a broad scale to optimize foraging and energy acquisition at a critical life-history stage. The extent to which migrants depend on memory to reach their spring destinations has implications for the adaptability of populations to changing climate and human impacts.

Cameron, M. D., K. Joly, G. A. Breed, C. P. H. Mulder, and K. Kielland. 2020. Pronounced fidelity and selection for average conditions of calving area suggestive of spatial memory in a highly migratory ungulate. Frontiers in Ecology and Evolution 8: e564567.