Why do humans have a sense of smell? And why are some things (like skunk spray) smelly, while other things (like aluminum cans) are not?
William Etges, associate professor of biological sciences, replies:
A scent or odor is a chemical or mixture of chemicals that can be detected by smell or physical contact. We are 10,000 times more sensitive to smells than taste. Humans can detect thousands of different odors because they possess about 10 million receptors for these chemicals in the olfactory lining of the nose. Dogs have 200 million!
Everything from the thousands of varieties of perfumes and the smell of fresh mown grass to road kill on a hot summer’s day and a skunk’s spray can be detected by these olfactory receptors. Organisms produce these chemicals for a wide variety of reasons — some are known as pheromones like musk to attract members of the opposite sex for mating. Migrating salmon can “smell” the stream they hatched in from the ocean. Plants produce compounds to attract insect pollinators. Not all are perceived by humans as “pleasant” — the world’s largest flower (2 meters wide) produced by the tropical Titan arum plant smells like decaying flesh to attract pollinating flesh flies. Predators will remember to avoid skunks on their menu once they have been sprayed. Why these smells are generally perceived as “bad” and others are pleasant is probably the way this information is perceived by the brain. It is not well understood but varies among organisms. The smell of rotting meat or vegetables has probably been associated with the health risks of eating them since man’s early history.