Q: How do birds so casually eat hot chili peppers, which give humans fits?
A: They don't feel a thing, say UC San Francisco researchers. The culprit in the peppers is capsaicin, a chemical known to bind to a receptor on the human tongue called vanilloid receptor 1, or VR1. VR1, which produces pain when stimulated, is also triggered by heat. Biologist David Julius and his colleagues isolated the VR1 from birds and found that capsaicin does not bind to it and thus does not produce a painful sensation. The difference between birds and mammals yields an evolutionary advantage for the pepper plant. Enzymes in the stomachs of mammals break down the seeds, and the hot taste discourages them from consuming--thus destroying--them. But the seeds pass through the intestines of birds undamaged and are dispersed widely.