Bears Avoid the Road Less Traveled
National Geographic has an interesting report on predator-prey issues in national parks: apparently pregnant moose in Yellowstone National Park and Grand Teton National Park tend to shift their activity closer to roads before giving birth, in order to avoid predation by grizzly bears.
According to the results of the study, bears tend to be much more wary of roadways than moose. Grizzlies usually give keep at least a 5000 meter clearance, while moose have been recorded giving birth within a scant 45 meters of a road.
One question that needs to be asked immediately is: how do we know moose are doing this to avoid predation? What if they’re just not very bright and haven’t figured out to avoid the roads themselves?
The study answers that question by showing that moose only show this pattern of behavior in areas where bears are present, and that pregnant females have shown closer and closer associations with roads over the years as bear populations have increased in the parks where the research was conducted. Also, it is noteworthy that only pregnant females showed this pattern, which also supports the hypothesis that this is an anti-predator behavior being used to decrease mortality of newborns.
I’m pursuing a career in carnivore conservation, and am extremely intrigued by any story about animals adjusting complex behaviors in response to anthropogenic influences. It would be interesting to look at why bears are so more adverse to roads than moose. The article mentions that bears are more disturbed by the noise of the traffic, but I’d be interested to learn if there is more to it than that. Could it be due to different experiences with poaching, better associative learning abilities, larger home ranges in general, foraging preferences, or something else altogether?
One of the take-home messages from this study is that it’s important to consider information like this when planning parks and other managed areas. Something as simple as the layout of access roads can have a profound impact on the dynamics of populations and how they interact with each other, in ways that can’t always be predicted. Researchers from Denali National Park have studied the moose/bear populations there and have not found the same pattern of behavior as that reported from Grand Teton. Obviously, management plans should be tailored to specific populations, as opposed to automatically assuming that what works for a species in one setting will apply equally as well in another place/time.
Another example of this is an absolutely fascinating Journal of Mammalogy paper that I read recently, Selection of den sites by black bears in the southern Appalachians. This study (which also measured other parameters affecting site selection, such as elevation and slope), done in the Pisgah Bear Sanctuary in North Carolina, showed that females with cubs actually avoid gravel roads more than paved roads, despite the fact that paved roads are usually busier. This seems counterintuitive at first, with females choosing to den much farther from quieter gravel roads and closer to the higher traffic volumes associated with the paved roads in the sanctuary.
Why would mother bears prefer to be near higher traffic paved roads than less-traveled gravel ones? The authors of the study suggest that it is due to the fact that human behavior on paved roads is much more predictable. If you’re cruising down a paved road at a relatively high speed, keeping up with the flow of other cars, chances are that you’re not going to stop, get out, and dally around the roadside.
Gravel roads, on the other hand, are more likely to be used by hikers, campers, and other people that intend to roam around the forest (maybe even pesky biologists? ;P ). Also, the authors suggest that poaching pressure (it appears that this does occur even inside the preserve, unfortunately) could be a factor as well. Male bears and females without cubs didn’t show the stronger aversion to gravel roads, but females with cubs did, suggesting that the avoidance behavior is a defensive mechanism aimed at protecting their offspring from unpredictable intruders.
If the bears have learned that human activity is less predictable when people approach on a gravel than a paved road, it makes sense that females will avoid these areas when they are choosing den sites to house their cubs. I found this to be extremely interesting; I wouldn’t have expected a stronger aversion to gravel roads, but the explanation is pretty convincing when you see all of their data.
Moral of these stories: road layouts within wildlife preserves are complicated by multiple factors, and animals sometimes react to them in ways that initially seem counter-intuitive to we mere humans.
Reynolds-Hogland, M., Mitchell, M., Powell, R., & Brown, D. (2007). SELECTION OF DEN SITES BY BLACK BEARS IN THE SOUTHERN APPALACHIANS Journal of Mammalogy, 88 (4), 1062-1073 DOI: 10.1644/06-MAMM-A-329R1.1
(Repost from my old blog archives, originally posted 10/13/2007)