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The pain from a nettle or thorn is immediate, you see the plant and learn to avoid it even if you have the brains of a cow :) The reaction to urushiol occurs hours or days later, tough to draw a connection unless you have some cultural information to rely on. Also, it seems that humans are the only ones who have an allergic reaction, and we are not predators of the plant. I tried to find info on what plants use urushiol compounds for to no avail. My guess would be our allergic reaction is an unfortunate coincidence to a compound the plant is using for a purpose that has nothing to do with us. Anthropocentrism strikes again!
Many of the plants that cause severe contact dermatitis produce urushiol. I am also working on articles on two others, poison oak and poison sumac. Someone asked me if you could tell which plant had caused the rash, based on the reaction, which is what got me looking into this topic in the first place. The answer is that they all three produce urushiol, which is what causes the reaction, so the rashes are indistinguishable from each other.
Many sources I read in researching this article referred to the production of urushiol as a defense mechanism. I wonder if the urushiol has an unpleasant flavor, preventing grazing animals from eating it, and our allergic reaction is a secondary effect. I certainly haven't ever seen any animals eating it, or even many signs of insect damage on the leaves.
Booker, I have seen definite evidence of deer, and possibly rabbits, browsing on Poison Ivy, and there are at least two insects that eat the leaves (although I don't recall right now which ones they are). And birds definitely do eat the berries.