Moose Moms Prefer Traffic to Grizzly Bears, Study Says

Scott Norris
for National Geographic News
October 12, 2007
Mother moose use roads as a kind of human-made shield to protect their newborn calves from predatory bears, a new study has found.

In Wyoming's Grand Teton National Park, pregnant moose move closer than normal to paved roads before giving birth.

By cozying up to highway corridors, the moose can birth and raise their calves in relative safety.

"Bears tend to be more averse to roads than moose," said lead study author Joel Berger, a biologist with the Wildlife Conservation Society in Teton Valley, Idaho, and a professor at the University of Montana in Missoula.

Previous studies have shown that grizzly bears—the main threat to newborn moose—seldom venture within 1,640 feet (500 meters) of roads.

(Related photo: grizzly vs. moose [warning: graphic content].)

The bears' fear of whizzing traffic gives a huge advantage to moose moms willing to tolerate a little noise.

"The closest we've had an animal give birth to a road is less than 50 yards [45 meters]," Berger noted, although distances of a few hundred yards were more common.

"[Mothers and calves] hang out in these areas for several weeks until the calves are old enough to run from a bear," Berger said.

His study appears online this week in the journal Biology Letters.

Roads as Refuges

Berger's team tracked the locations of individually marked moose living in the Grand Teton region over a ten-year period.

Non-pregnant females showed no interest in being near roads.

But the distance between roads and the birthing sites used by pregnant moose shrank by about 410 feet (125 meters) a year.

An expanding bear population in the region is probably the main driver behind this steady movement toward areas of high human activity, Berger said.

In areas without bears, the moose moms didn't seem to gravitate toward roads—supporting the idea that moose actively seek out human-built corridors as refuges from predation.

Grizzly bears have only recently moved back into the Grand Teton region, Berger noted. (related photos: survival of the grizzly bears.)

Over time, he predicts, the bears will become increasingly bold in pursuing moose into areas close to roads and the shield effect will diminish.

Observations from the past few years suggest the predators are starting to lose their fear.

"The bears are starting to figure this out," Berger said.

Getting Accustomed

But the new finding contrasts with results from other locations.

Terry Bowyer, a biologist at Idaho State University in Pocatello, studied moose-bear interactions in Denali National Park in Alaska.

"Moose [in Denali] do not give birth near roads, and bears are not traffic-averse," Bowyer said.

Both he and study author Berger agree that the disparity is probably due to the Alaskan populations' more continuous association with humans, as well as with each other.

The new finding from the Grand Tetons is fascinating, Bowyer said.

"It illustrates how humans may influence predator-prey dynamics, something that previously had been underappreciated."

Berger added that national parks are often used to study species in a natural setting.

But animal behavior may be influenced in unexpected ways by the millions of visitors flooding the parks each year and the facilities built to accommodate people.

"We need to better understand the consequences of the infrastructure we put in place," Berger said.

"Just by being there we have changed the distributions and behavior of some species."

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