Wolves (Canis lupus) are predators. Beavers are foragers. They both use their keen sense of smell to survive. That much is clear. But wolves have the overwhelming advantage in speed and you might think they take advantage of that to hunt all prey. How can prey ever hope to escape these beasts? Beavers – the prey that accounts for 42% of wolves’ diets in spring and fall – use their sense of smell to detect wolves in the area and try to avoid getting eaten. With poor eyesight, smell is their primary sense. But it turns out that instead of relying on speed, wolves systematically use wind direction and patience to get within a few feet of beavers (1-5 meters) in order to catch them. A recent study entitled “Wolves choose ambushing locations to counter and capitalize on the sensory abilities of their prey” conducted by Thomas Gable, Austin T Homkes, Sean Johnson-Bice, Steve Windels, and Joseph K Bump supports the idea that wolves can, and do, select a spot to hunt from based on certain environmental factors such as wind direction in order to bypass identification by the beavers. This study took place in the Greater Voyageurs Ecosystem in Minnesota.
Figure 1: This map shows the Greater Voyageurs Ecosystem in yellow. The star on the larger map of the United States marks this spot as well. This image was taken from The Voyageurs Wolf Project at https://www.voyageurswolfproject.org/greater-voyageurs-ecosystem.
Background
There are two broad categories of predators: ambush predators who use sit-and-wait or sit-and-pursue strategies to catch their prey, and cursorial predators who use the wide-range, active-hunting strategy to chase and outrun prey. Wolves are usually cursorial predators, but now we see they can switch things up. When hunting beavers and certain other prey, wolves will shift to using ambush tactics. While we know this generally, there is not a lot of research to describe exactly how predators hunt, because it is hard to study. What we know of ambush behavior comes primarily from observations or correlative data (data gathered that shows two things are related, but does not show how and therefore can not prove they directly affect one another). The researchers of the study note that, “To our knowledge, this is the only study that demonstrates that carnivores can choose ambush locations with olfactory concealment.”
As for the prey, beaver location on land is reasonably predictable, as they don’t travel too far from their lodges and they repeatedly visit the same habitat features, including foraging trails they develop themselves. Researchers identified six common features with beaver habitats: dams, foraging trails, forest interior, lodges, shorelines and waterways. They tend to stay close to the water for a quick escape route should they run into or smell a predator. But they do verge into the forest to forage for trees.
Methods
Research methods were nearly as interesting as what they uncovered, revealing wolf ambush bed hideouts. First, researchers had to follow the wolves. From 2015-2019, foothold traps and cable restraints were used to temporarily capture wolves in order to attach GPS collars on them. The GPS coordinates were used to ascertain where wolves chose to wait to ambush beavers as well as how they chose their ambush locations in relation to the sites on land they most likely expected beavers to be.
Wind direction was also a major player in conducting the research. Wind direction collected hourly from local weather stations was used to estimate wind direction during hunting and calculate whether wolves would have been detected by the beaver’s sense of smell from either the beaver trail or the pond. As odor can waft over water, the beavers would have been able to smell the wolves from the water and not come out, so this was an important factor in the study. The study assumed that beavers could smell the wolves from any distance downwind from the wolf. The wolf hunting attempts were put in three categories: “wolf likely undetected by beaver, wolf likely detected by beaver, or unknown.”
Figure 2: This diagram demonstrates how the researchers found evidence regarding the general distance the wolf bed is from the pond and the feeding trail the beaver uses as well as where the wolf could be detected by the beaver’s olfactory system. Panel (a) shows that the wolf generally waits-in-ambush father from the water than the beaver feeding trail. The researchers would find the wolf bed, then record and chart the different ecological features they studied, such as the distance the wolf bed was from the pond vs the feeding trail. In panel (b), the researchers used wind direction collected from weather stations to determine if the wolf would likely be detected by the beaver via it’s olfactory system. The black arrows represent the wind directions that the wolf would be detected by the beaver during, and the white arrows represent the wind directions that the beaver would be unable to detect the wolf during.
Also, wolf beds – where the wolf was waiting-in-ambush – were described as “a circular area of depressed vegetation or earth with wolf hairs scattered on the ground.” Wolf beds could occur where a beaver was killed, aka a “kill” site, or where there was no kill made, aka a beaver-hunting attempt or an “ambush site.” Bed sites were determined by first tracking the wolves with GPS’ and following up in the field with detailed mapping and vide0.
Now for the gory part: evidence that a wolf killed a beaver was determined to be any cluster of fresh beaver remains, meaning bones, fur, contents of the stomach, or caster glands (an inner body part of the beaver associated with urine). When a beaver kill was identified, features of the habitat were documented, such as proximity to feeding trails and dams.
Interestingly, by studying the location of ambush bed sites with no associated kill, the researchers could determine that a wolf was in fact hiding lying in wait and where, relative to beaver trails, pond and wind direction. GPS tracking provided data to indicate length of time spent in the bed. If a kill was associated with a bed location, researchers did not exclude the possibility that the wolf encountered the beaver by chance and simply sat down to eat it after. Kill sites and wind direction were documented regardless, but this prevented them from analyzing how often ambush hunting worked (capture rate).
Results
Researchers found that wolves positioning themselves downwind to prevent detection was systematic. “To prevent detection, wolves predominantly chose ambush locations that had olfactory concealment (89–94% of attempts) where beavers likely could not detect them.” That’s persistence.
Wolves spent anywhere from 20 minutes to 30 hours waiting at an ambush site. On average, they spent around 4 hours waiting for their prey. That’s patience.
Figure 3: This image shows an unsuspecting beaver strolling right in front of a cutout image of a wolf. Due to beavers’ poor eyesight, wolves can get a lot closer to beavers during hunting as long as they stay still. The movement will alert beavers to their presence. The image was a resource used at: https://www.mprnews.org/story/2021/02/10/patience-is-a-virtue-how-northern-minnesota-wolves-ambush-their-prey.
The study also supported the hypothesis that wolves take advantage of beaver’s bad vision by positioning themselves at ambush sites closer to beaver features, the six habitat features identified. Wolves likely have learned that beavers cannot see them when they remain still and adapted to this by waiting between 1-5 meters away from beaver features with “little to no visual cover”. This may increase their hunting success as beavers will have less chance for escape from a shorter striking distance. That’s up close and personal.
The distance a wolf waited from the beaver ponds seemed to be random, surprisingly. Researchers had expected to see wolves waiting further from water most often, as there is a consensus within scientific literature that beavers would have a harder time escaping when traveling away from their ponds, but they found no pattern in the distance from ambush location to pond; and state in their study that there is no real outside data that supports the consensus, “it is merely a hypothesis.” Because the wolf selected ambush locations appear to be random with respect to distance from ponds, this suggests that either, “1) wolves do not understand beavers are more vulnerable with increasing distance from water or 2) beavers are not more vulnerable at increasing distances from water and, thus, wolf ambush strategies reflect this fact.” More research is needed to decipher which is true.
The ability of predators and prey to adapt to each other’s behaviors is extraordinary and evolutionary. What’s more, to understand sensory strengths and weaknesses, and simultaneously account for wind direction is another level up. Wolves and beavers have demonstrated this sensitivity and ability, clearly. This research is the evidence. It gives way to many more questions, but a significant one we must ask ourselves is: What will beavers do to outsmart wolves in the future? As the song goes, the answer, my friend, is blowing in the wind.
Figure 4: A wolf from the Greater Voyageurs Ecosystem. https://www.voyageurswolfproject.org/peer-reviewed
