So how can we keep animals out of our picnic baskets? The National Park Service has come up with all kinds of strategies to try – from installing animal-proof trash bins to educating visitors to lock up their food in special canisters. Visit Yosemite and you'll get a personal lecture from a ranger on how to store your food to keep from tempting the black bears who lurk near campsites.

According to a study in the March issue of the journal Frontiers in Ecology and the Environment, these kinds of measures seem to be paying off (in Yosemite, at least). Researchers at the University of California, Santa Cruz wanted to find out whether the proportion of human food in the diet of the black bears there had declined since the park received a big grant to manage the food issue in 1999.

To find out, they gathered samples of bear hair and bone dating back to 1915 and measured the chemical isotopes in them. If you've read The Omnivore's Dilemma or seen the documentary King Corn, you might remember that human hair samples reveal a lot about our diet today – namely, that we eat a lot of corn. Corn has a heavy carbon and nitrogen isotopic signature, and it goes into sodas, pastries, and even our meat, since it's a big part of what livestock are fed.

Here's the really fascinating thing we learned from Jack Hopkins, lead author of the paper. (He did the Yosemite bear research while he was an ecology Ph.D. student at Montana State and a research fellow at UC-Santa Cruz.)

"A bear that eats like us is going to look like us" in an isotopic analysis, he says. In other words, the corn shows up in the hair of bears who've learned to love and steal human food.

The good news, though, is that Hopkins and his co-author, Paul Koch, a professor of earth sciences at UC Santa Cruz, found that the human foods in Yosemite bears' diets had decreased by about 63 percent between 1999 and 2007. In other words, teaching park visitors to lock down their food has succeeded at keeping our snacks out of bears' bellies, they conclude.

Still, Hopkins says, even if Yosemite can get 99 percent of its visitors to comply with the policy, with 4 million visitors a year, there will always be several who forget or don't pay heed.

"The bears are still there, just waiting for someone to be careless," he says. "There are still a lot of problems."

Copyright 2014 NPR.

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How Yosemite Keeps Its Bears' Paws Off Campers' Hamburgers