When predators become prey

What happens when a predator becomes prey? Evolution has provided animals that are on the menu for bigger, toothier animals with a suite of abilities to help keep them from winding up in someone else’s metaphorical stewpot. But predator avoidance is something that the predators themselves haven’t really had to worry about. But thanks to human hunting, many predators have, in a real sense, become prey.

How does the pressure from hunting affect a top carnivore like a bear? Can they learn to adapt? That’s what evolutionary ecology graduate student Milena Stillfried from Germany’s Leibniz Institute for Zoo and Wildlife Research wanted to find out. Together with her colleagues, Stillfried assessed whether human behavior altered black bears’ movements, and if different types of human activities did so differently.

Black bears in Michigan have to deal with two different forms of human presence, one of which is fairly low-risk and one is higher risk. Before the hunting season begins, human hunters and their dogs spend time in the forests near the bears, because the dogs need to be trained for the hunting season. Despite the fact that they’re being trained to help chase bears, this sort of activity is classified as low-risk for the bears. Once the bear season begins in the fall, the bears are obviously at greater risk from any nearby human presence. To see whether the bears changed their movement patterns in response to the humans, the researchers set ethical traps for them (often baited with donuts) and after being sedated for a health assessment, the bears were outfitted with GPS collars and let go.

Stillfried found that the bears’ movements were not random. Instead, they changed their movement patterns over time, especially as dog training season transitioned into hunting season. Throughout most of the year, the bears didn’t pay all that much attention to roads, though they tried to avoid crossing paved ones. In a fight between a bear and a speeding vehicle, the car usually wins, after all. But when human disturbance was higher – whether due to dog training or to hunting – the bears kept away from non-paved roads, which are favored by hunters and dog trainers, who tend to be on foot. The problem with that is by keeping away from non-paved roads, the bears wind up being closer in proximity to paved ones.

Even more dangerous, the hunting season caused bears to cross both types of roads more frequently – though they were often smart enough to do it at night, rather than during the day. That might seem like a foolish gamble, but it turns out to be quite rational. Between 2009 and 2011, only eight bears in the area were killed by vehicles, while 590 were killed by hunters.

Combined, these results suggest that the bears can not only perceive the increased risk from humans and try to adapt to it, but that they can also do so on a fairly fine temporal scale.

The autumn is a time when bears need to be bulking up to survive their winter hibernation. If they spend all their time avoiding humans, dogs, and the threat of hunting, it might mean that they’re unable to feed effectively enough to survive the winter. That could explain why they were moving around so much in the dark of night. Bears typically forage during the day, when food is more visible then and they can feed more efficiently. But having their daytime attention switched from foraging to survival, the bears seem to be forced to forage at night instead, a time that’s more difficult to find adequate nutrition.

What’s not yet clear, and what the researchers hope to tackle next, is just why the presence of humans caused the bears to change their travel patterns. If it’s predator avoidance per se (a top-down process), then their movements might indicate that they’re looking for safe shelter, for example. But if their altered movements are instead an attempt to forage optimally given their circumstances (a bottom-up process), then their movements could be explained by their hunger.

Either way, by being forced away from non-paved roads and closer to paved ones, these bears are—quite literally—caught between a hunter and a hard place. – Jason G. Goldman | 28 October 2015

Source: Stillfried, M., Belant, J. L., Svoboda, N. J., Beyer, D. E., & Kramer-Schadt, S. (2015). When top predators become prey: Black bears alter movement behaviour in response to hunting pressure. Behavioural Processes 120, 30-39. DOI: 10.1016/j.beproc.2015.08.003.

Header image: Black bear via shutterstock.com