Howdy, Matt and the Elves:
I'm planning a trip to Yosemite and reading up on safety in bear country. Apparently the main reason bears encounter humans in the first place is because their sense of smell is so acute that they can pick up even residual traces of human food. The Yosemite website implies that bears have been known to rip a car door off its hinges just to get at an empty soda can or a stick of gum. So if the bears' sense of smell is that sharp, hasn't anybody found a smell that repels them? I should think that a whiff of badly cooked cabbage or something could be sprayed on one's clothing to keep the bears away.
-- Kevin Wohlmut, San Diego
You'll probably have the campground all to yourself if you take a bath in old cabbage water, but of course the gum-chewing bears will be lined up at the tent with their plates and forks. If Dumpsters smell like a banquet hall, there's not much future in pursuing a repellant smell that would be a practical bear-chaser. Most animals repelled by smells react to the smells of natural predators, not noxious odors. Bears should be wary of human smell, so maybe a really ripe T-shirt and old underwear would do the trick. But most bears that hikers encounter have learned otherwise. While the animals don't see us as a primary food source, they also know we won't charge at them screaming "ooga-booga-booga!" and hack up their loved ones. Another problem with smell research is that bear behavior varies a lot; if Test Bears A through F flee at the smell of a shot of bus exhaust from an aerosol can, when you encounter Random Bear G in the woods, there's no telling what he'll do. The final problem may be getting scientists to volunteer for the field testing.
The newest wrinkle in Bear-B-Gone technology goes by the coy name of "bear spanking." For years the town of Mammoth Lakes has been plagued with the bear equivalent of beer runs. A naturalist finally discovered that bears will retreat if they're shot with nonlethal rubber bullets. Pain seems to be the universal deterrent. I doubt that's feasible in Yellowstone, so go with the traditional remedies: hike on-trail, in the daytime, wear "bear bells" or otherwise make a lot of noise so you don't startle them, steer way clear of cubs, and as a last resort carry a can of large-animal pepper spray. Of course, for the spray to work the bear has to be (at most) 30 feet from you, and in your hysterical state you have to hit him in the eyes, nose, and/or mouth. But I'm sure you can handle it.