Simple. When an animal perceives any person, animal, or it's own kind as threatening their status in the established pecking order at any given moment in time, they in turn will react by any means at their disposal (making threatening noises or gestures, growling, barking, hissing, attacking, flailing, pecking, biting , etc) making sure that they retain or raise their own status of dominanace over the individual involved. Some animals have very sensitive olfactory capabilities, and can indeed smell minute differences in odors of chemicals and/or perfumes, hormones or pheramones and may react favorably or unfavorably when they come upon them based on their experiances to them.