A few weeks ago Yellowstone National Park
officials discovered the carcass of one of the park’s deadliest wolves,
an aging male that scientists knew to be an aggressive bison hunter.
Wolf No. 495 died naturally, but his body bore
bruises consistent with injuries inflicted in an encounter with large
game, according to Dan MacNulty, an assistant professor of wildlife
ecology at Utah State University.
The wounds found on No. 495 help explain
MacNulty’s latest findings that wolves’ hunting success bears little
correlation to the size of the hunting party. Wolves hunt in groups
because taking down large hoofed animals is not only challenging but
dangerous.
But if the attack party exceeds four animals,
the chance of success can actually diminish, according to research
MacNulty and colleagues published this week in the journal Behavioral
Ecology.
“Wolves aren’t as effective hunters as we
think they are. That perception is premised on the notion that each
individual contributes to the hunt, so there is an additive effect when
the group is bigger. That is just not the case,” said MacNulty, an
assistant professor of wildland resources. “Individuals are responding
to the threat of injury and death that large prey poses, so they are
pulling back, making decisions to avoid the cost of injury.”
The new research is based on eight years of
observations in Yellowstone’s Northern Range involving 94 wolves from
five packs, including the late No. 495 from Mollie’s pack.
Once eradicated from the Northern Rockies,
wolves were reintroduced at Yellowstone in 1995. These animals and their
descendants are among the most closely studied populations in the
world. The observations track behavior of individual animals over the
course of their lives, creating a powerful data set for understanding
this controversial social predator.
The findings suggest group hunting is not the
main reason wolves live in packs, according to co-author David Mech of
the U.S. Geological Survey’s Northern Prairie Wildlife Research Center.
“Looking to lions and other social predators, it provides further
insight into the evolution of living in groups,” he said.
Hunting is a four-stage process for wolves that
grows increasingly dangerous. First the group approaches a prey herd,
then chases it and singles out an individual before taking it down.
MacNulty’s data tracked how many wolves were
involved at each stage of particular hunts and their outcomes. But his
team also knew a lot about each wolf involved, which gave the team
insights into wolf behavior.
“We knew the gender, the age, whether they were
breeding,” said MacNulty. “Parents generally take the lead because they
have offspring to provide for. … Given a choice, wolves will stay out
of harm’s way until it’s safe to enjoy the spoils of the hunt. They’re
opportunists. And this challenges the popular belief that wolves are
highly cooperative hunters.”
MacNulty has been involved with the Yellowstone
Wolf Project from its inception. Co-author Doug Smith, of the National
Park Service, leads the program and conducted some observations
aerially, although most were done on the ground by volunteers.
According to its 2010 annual report, the
project recorded 268 wolf-related kills in the park that year. Elk
comprised 79 percent and bison nine percent. Of the elk, only 18 percent
were bulls.
Researchers hope the livestock industry can use their findings to reduce wolf depredation.
Real wolves bear almost no resemblance to their
fairy-tale caricatures. In previous research, MacNulty demonstrated
that wolves’ hunting prowess peaks at age two or three, then declines
rapidly. These archetypal killers aren’t well-built for killing big prey
anyway.
Cougars’ claws and powerful forelimbs are not
only effective tools on large prey, but also enable the predator to kill
without being killed. By contrast, wolves’ tools are teeth and jaws,
but to put them to use, they must expose themselves to serious harm.
“Wolves are risk averse. They are cautious
hunters,” MacNulty said. “Hunting success also peaks in small groups
with other social predators. But our study is the first to rigorously
test this pattern and demonstrate that it’s likely due to individuals
switching from cooperation to ‘free riding’ as group size increases.”
Co-authors include John Vucetich of Michigan
Technical University and Craig Packer of the University of Minnesota.
Funders include the National Science Foundation, National Geographic and the Yellowstone Park Foundation and USGS.
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