VOYAGEURS NATIONAL PARK— Last winter’s record mild conditions presented an unprecedented challenge for gray wolves here, and a unique research opportunity for the Voyageurs Wolf Project, or VWP.
Researchers with the project closely monitored two of the seven packs they study to get a clearer picture of how effectively wolves can hunt white-tailed deer when the agile ungulates aren’t weakened and bogged down by deep snow. The winter of 2023-24 was the mildest ever recorded across Minnesota, including at Voyageurs National Park, and the comparatively warm temperatures and very limited snow cover presented a challenge for wolf packs that rely heavily on deer for food in winter.
While it may not be widely known, extensive research shows that wolves in northern Minnesota kill few adult deer outside of late winter, when deer are typically most vulnerable due to the effects of normal winter conditions. They can be exceptionally vulnerable during winters with deep and prolonged snow cover, as the region experienced in the winters of 2021-22 and 2022-23.
Researchers with the VWP used GPS data to determine when the wolves from two packs, the five-member Thuja pack and the seven-member Stub-Tail pack, had brought down a deer, an event that is usually readily recognizable from pack movements, or lack of movement in the aftermath of a kill.
During last winter’s mild conditions, the research team found that wolves struggled to bring down deer and likely failed to maintain their body weights at a time when wolves are typically in peak condition.
“Indeed, the kill rates of wolves on deer we observed were some of the lowest kill rates of wolves on deer during winter that have been documented,” wrote Tom Gable, the lead author on the study. “This result is unsurprising because numerous studies have shown that the magnitude of wolf predation on deer during winter is primarily modulated by winter conditions, in particular the depth and duration of snow cover.”
The researchers say the results suggest that most adult deer are not vulnerable to wolf predation during mild winter conditions.
While both packs studied struggled to bring down deer, the Stub-Tail pack went nearly a month, or 26 days, in March, without killing a deer. While the Thuja pack was somewhat more consistent, it typically went about five days between kills in February and March.
From April through May of that year, researchers reported no successful deer kills by either pack. According to the researchers, that is not atypical for snow-free periods of the year.
From February through the end of March, the two packs killed a combined total of 23 deer, or an average of just under one deer per wolf per month.
The study found that scavenging comprised about a quarter of the calories that the wolves consumed during the two winter months, primarily in March. A moose calf, that apparently died of disease in the Stub-Tail pack’s territory and a dead horse that had been dumped in the Thuja pack’s territory provided an important supplement for the two packs under difficult conditions.
The implications
The research suggests that while adult white-tailed deer are a critically important food source for wolves in northern Minnesota from mid-to-late winter, wolves’ ability to catch deer is largely dependent on winter conditions, and may be quite limited in mild winters, particularly winters with limited snow cover.
This finding, according to the researchers, confirms that the severity of winter conditions is the primary factor determining population trends among white-tailed deer in Voyageurs National Park and the surrounding area, which is on the northern fringe of white-tailed deer range.
The study also concluded that wolves, on average, appear to take fewer adult deer than is commonly believed. As the researchers noted, “wolf predation on adult-sized deer primarily occurs during October–April with predation generally peaking in February–April…notably, predation on adult deer from May to August is rare.”
The two packs involved in the study showed limited success hunting deer outside the narrow window in the late winter of 23-24, when the region still had some snow cover, even though it was limited.
The study looked at the two packs’ hunting success in the fall, beginning in October and found very limited success during that period. That could be a reflection of the difficulty of hunting adult deer during periods without snow cover as well as the relatively low deer density in and around Voyageurs National Park.
Even so, the study concluded that the two wolf packs likely took an average of 6.2 adult deer per pack member over the course of the entire year. That number could well have been much higher in a severe winter with abundant snow, but appears significantly lower than common belief about the impact of wolf predation on deer in the region.