Traipsing through knee-high grass and squirming through thickets of tree branches, Chris Wemmer is leading an outdoor workshop that's changing the way we look at wildlife.

Camera-trapping or "trail cams" are freeze-framing nature from the plains of Africa to backyards in suburbia. They've captured images from snow leopards striding through the Himalayas to a house cat leaping onto a bird feeder among the blue jays.

"You get these surprises. You see things you don't expect to see," said Wemmer, who is teaching a camera-trapping workshop this week in the Sierra Nevada.

The affordability and compactness of technology has made camera-trapping available to more than just scientists. Trail cameras in waterproof containers are sold at big-box retailers and speciality outdoor outlets for as little as $100 and as much as $1,000.

Remotely photographing wildlife predates the 20th century, when scientists used trip-wires connected to camera flashes to capture images of animals in the wild. As late as the 1980s, the Snow Leopard Conservancy had to rig a car battery to a camera in Nepal and wait for a big cat to step on a pad to trip the camera.

In the 1980s, an electrical engineer interested in wildlife developed a practical infrared sensor that was triggered when a moving or warm object broke the beam.

Hunters started using the cameras to find spots where deer and other game congregated.

Wemmer started camera trapping after he retired as a biologist from the Smithsonian Institution and developed a blog – Cameratrapcodger.blogspot.com – where he posts many of his photos.

On the wooded Sierra Nevada Field Campus of San Francisco State University, about two hours north of Auburn, Wemmer is teaching nine students, mostly biologists, how to use the cameras.

"We're talking about using this for research," said Kim Hastings, a biologist with U.S. Fish and Wildlife in Juneau, Alaska. "I have to figure out how to set the stage."

Tagging along with Wemmer as he scouted spots to set up his cameras, she explained how trail cameras cut costs and other problems of tracking. GPS collars are costly, and satellite tracking doesn't work in dense forests.

"Maybe we could put a sunflower seed here," Wemmer said, patting a stump. A camera here would catch squirrels, he said.

A mound of fairly fresh coyote droppings signaled a good spot to catch some wildlife, and he hammered in a metal stake and set the camera.

Before the students arrived, Wemmer had placed cameras in the woods near the school's tented encampment. He got photos of mountain beaver with widened eyes emerging from a burrow, like a movie star caught sneaking out of a nightclub.

Wemmer, 65, said camera trappers will usually get more than they expect.

One of Wemmer's students told how his attempts to see what kind of wildlife lived in culverts produced a photo of the bare rear end of a human.

In February, a student's camera in the Tahoe National Forest north of Truckee caught a wolverine, long believed to have vanished from the Sierra.

"It's changed the dynamics of getting confirmed sightings on wildlife for biologists," said Harry Morse, a spokesman for the state Department of Fish and Game. "Before, you suspected they were there, and then suddenly, boom, there it is."

The Outdoor Industry Association, which tracks participation in outdoor activities, does not follow camera-trapping, but wildlife viewing draws about 66 million people a year.

The attraction of camera tracking is bound to grow, said Matthew Lewis, a biologist with the World Wildlife Fund.

"It connects people more to wildlife and it's candid," he said. Animals are not stiff with fear because of a nearby human.

World Wildlife Fund cameras are photographing tigers in Indonesia, rhinos in Asia and swift foxes in South Dakota.

The cameras can also train on the mundane, Wemmer said.

Among the exotic shots on his blog, he also posts photos of raccoons scampering around the pool at his home near Paradise.

"If you want to see whose cat is coming into your backyard," he said, "it's good for that."

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