Patience is a virtue: How northern Minnesota wolves ambush their prey
Wolves that roam northern Minnesota’s forests hunt moose and deer in packs, often by outrunning — and outlasting — their prey.
But new research shows that wolves in the region around Voyageurs National Park also employ sophisticated ambush hunting tactics to catch and kill beavers — an important source of food for wolves during the summer and early fall.
In a study published Tuesday in the journal “Behavioral Ecology,” researchers from the Voyageurs Wolf Project, a collaboration between the University of Minnesota and the national park, document how wolves will often quietly wait eight to 12 hours in a single spot for a beaver to pass nearby, and have evolved other hunting tactics specifically tailored for hunting beaver.
It’s the first study to systematically analyze how wolves ambush their prey, and the latest in a string of surprising findings from the project in and around Voyageurs National Park that have turned upside-down assumptions and stereotypes of how wolves in northern Minnesota survive during the summer.
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Earlier studies have shown that wolves consume large quantities of blueberries, and for the first time documented wolves hunting freshwater fish as a seasonal food source.
The researchers have also quantified just how much wolves rely on beavers for food. They discovered beaver can constitute up to 42 percent of a pack's diet from April until October. While one wolf on average kills around seven beavers per year around the park, researchers found one wolf that had eaten 28 beavers in one season.
This new study shows just how they do it.
“What we have found by doing this work is that where wolves wait [in ambush] seems almost perfectly honed to actually take advantage of beavers,” said Tom Gable, Voyageurs Wolf Project lead researcher and the study’s primary author.
The findings are only possible because of new technology. Since 2015, researchers have outfitted wolves from seven different packs in and around the park with GPS collars that transmit location data every 20 minutes. That allows them to zoom in on wolves’ predation habits at a much finer scale than previous collars.
Whenever a wolf lingers at a spot for more than 20 minutes, researchers mark the place, and then visit it on foot to look for clues of what may have happened there— the place where the wolf bedded down in the grass, or the remains of a kill. When a beaver is killed, often all that’s left are the two large incisors they use to cut down trees.
Over the past five years, researchers estimate they’ve searched nearly 12,000 locations. At those spots they documented nearly 750 places where wolves waited to ambush beavers but were unsuccessful, and another 214 locations where they succeeded in killing a beaver.
Among their findings:
Wolves almost always choose ambushing sites downwind from beavers. Beavers have extremely poor eyesight, so they rely almost exclusively on their keen sense of smell to detect predators. Wolves appear to have learned this through time. “The results are very clear” said Gable. “89 to 94 percent of the ambushing sites were downwind, where beavers were likely unable to smell wolves.”
Wolves choose ambushing sites just a few feet from where beavers are active on land, again suggesting that wolves have learned beavers cannot see them.
Wolves often choose ambushing sites along beaver trails — where they travel to cut down trees for their dams — as far away from the water as possible, suggesting that wolves have learned that if beavers are able to make it back to water, they have a much better chance of escaping.
Wolves are extraordinarily patient. “Wolves waited an average of four hours during each stakeout. But they often waited eight to 12 hours or more, and one wolf even waited-in-ambush for 30 hours,” said Austin Homkes, a co-author on the study.
Beavers are a lot harder to kill than scientists previously thought. Despite their poor eyesight and their apparent clumsiness on land, wolves succeed in killing beavers in fewer than a third of their ambush attempts. Many successful beaver kills appeared to be the result of chance encounters.
Taken together, their findings show that “when wolves choose [ambush] spots, they seem to have a very good understanding of beaver behavior, which is quite fascinating,” said Gable. “And I think it does speak to their general intelligence, of how they figure out patterns and figure out ways to get food in unique places and through unique methods.”
The hunting behaviors the researchers documented were also widespread. Wolves from multiple packs across several years used the same ambushing tactics, suggesting wolves use similar strategies across the larger ecosystem surrounding Voyageurs National Park, and likely across a wide swath of North America where wolves and beavers coexist.
“There's something bigger going on here,” Gable said. “It's not just a few individuals that figured this out. We think the patterns we've identified speak to this larger behavior in wolf ecology.”
Gable said researchers are still trying to tease out just how important beaver are to wolf diets. While wolves clearly eat a lot of beavers during the summer, Gable said all that indicates is that beavers are the best available food option to them at that particular time.
The late summer and early fall, when wolves eat the most beaver, is the leanest time of the year for wolves. Deer fawns have grown and can more easily escape wolves. Adult deer are the healthiest and hardest to catch all year. Other food they might rely on, like blueberries, are also gone.
So beavers are one of the only food sources remaining. Gable surmises that could be part of the reason why wolves are willing to devote so much time to ambushing them.
“They can spend a bunch of time and effort chasing after deer fawns and adult deer with probably very limited success,” Gable says, “or they can hardly move, and just [lie in wait] where beavers are going to be active on land.”