Urushiol, the irritating component from Poison Ivy and its kin, is not a defensive chemical. Its action is too slow to deter animals from eating the plant, and many animals do consume it with no problem. Many humans are allergic to urushiol, and some other animals too. Urushiol allergies have been found in some guinea pigs, mice, sheep, dogs, and rhesus monkeys, but only when applied to bare skin, not fur. If urushiol gets on an animal's fur, it does not bother the animal, but it can trigger a reaction in an allergic human who pets the animal.