It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact. Capsaicin and several related compounds are called capsaicinoids and are produced as secondary metabolites by chili peppers, probably as deterrents against certain mammals and fungi.
According to World Book Encyclopedia, “So far scientists have named and classified more than 1 1/2 million animals. Over half of these are types of insects and other species are discovered each year. Scientists believe there may be from 2 million to as many as 50 million kinds of animals alive today.”