Published online 10 October 2007 | Nature | doi:10.1038/news.2007.155

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Animals capitalize on predators' fear of people to protect their young.

Some animals seem to take comfort in roads. US National Park Service

A drive through Yellowstone National Park these days is like going on a photo safari: elk, bison, deer, big horn and moose hang out so close to the roads that it’s easy to spot them. Research now shows this isn’t because the area is jam-packed with animals — some mammals seem to be attracted to the roads as a shield against predators.

The idea that prey protect themselves by hanging around roads and people has been suspected for some time. “We have seen elk congregating around human areas in Banff National Park in Canada, and deer clustering along the edges of roads in the Rocky Mountains, and always thought there was something more to this,” says Dave Augeri, a conservation biologist at the Denver Zoological Foundation in Colorado. Now, research conducted by Joel Berger of the Wildlife Conservation Society, in Teton Valley, Idaho, puts hard evidence behind these suspicions.

In the Yellowstone area, bears tend to stay clear of roads because rangers have taken active measures to make them wary of people. Berger set out to see whether other animals would take advantage of this situation, particularly when at their most vulnerable: when giving birth.

Between 1995 and 2004, Berger monitored births from 25 female moose in and around Grand Teton National Park, inside the Yellowstone ecosystem. He reports in Biology Letters1 that pregnant moose moved an average of 122 metres closer to roads each year of their birthing period, bringing them from 1,000 metres to 380 metres of the road; non-pregnant moose, and those in bear-free areas, did not move.

Baby moose and their mothers are vulnerable to attack. US National Park Service

Berger suggests that the pregnant moose move because their calves are heavily predated on by bears, and the bears keep away from roads. “It would seem that moose mothers are using the fear that bears have of roads to protect their young when they are most vulnerable,” says Berger.

Not so pristine

Park managers for Yellowstone and the Grand Tetons have struggled to keep their environments as untouched as possible. Bans on feeding wildlife are in place, litter control has been honed to a fine art and vast sections of the parks have no human structures at all — visiting them requires days of travel on foot.

The fact that a few roads can disrupt the ecosystem so greatly creates a dilemma for the conservationists responsible for keeping the areas wild. “This just points out that ecotourism is not some gilded pathway,” says Fred Bercovitch, a behavioural biologist at the centre for Conservation and Research for Endangered Species in San Diego, California. Even watching animals discreetely from the safety of a car can change their behaviour.

Fortunately, Berger notes that the redistribution of moose mothers in this instance could be a unique and possibly temporary phenomenon. Before 1950, bears were hunted in the Grand Tetons. When the park was established and hunting ceased, bears migrated in from the neighbouring countryside. “These back-country bears had little or no experience with humans and it’s entirely possible that they may still be adapting to the roads they are encountering,” Berger says.

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The bears in Grand Teton National Park could well grow more comfortable with roads in the future, putting an end to the protection that the roads currently grant to prey. In Alaska and the northern rockies, bears seem to be used to roads, and pregnant moose don’t seem to be attracted to them.

“This is an intriguing phenomenon with essential implications for conservation,” says Augeri. “Clearly, we have to look much more closely at the unanticipated influences our activities are having on predator–prey relationships.”