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Olfaction, the sense of smell, is the detection of chemicals dissolved in
air (or, by animals that breathe water, in water). In vertebrates it is
located in the nose. The importance and sensitivity of smell varies among
different organisms: most mammals have a good sense of smell, whereas most
birds don't. Among birds, it is important in the tubenoses. Among mammals it
is well developed in the carnivores and ungulates, who must always be aware
of each other, and in those, such as moles, who smell for their food. It is
less well developed in the catarrhine primates, and nonexistent in
cetaceans, who in compensation have a sensitive and well-developed sense of taste.
Insects have the sense of smell on their antennae. In many species it is
highly tuned to pheromones; a male silkworm moth, for example, can smell a
single molecule of bombykol.
Many vertebrates have an auxiliary olfactory sense organ called Jacobson's
organ or the vomeronasal organ, located in the vomer, between the nose and
the mouth. Snakes use it to smell prey, sticking their tongue out and
touching it to the organ. Some mammals make a face called flehmen to direct
air to this organ, which detects pheromones. In humans it is subliminal.
Mammals generally have about 1000 genes for odor receptors. Each receptor
cell in the nose expresses one of these genes. Each gene is expressed by
thousands of cells, whose axons converge in the olfactory bulb. Humans have
347 functional odor receptor genes; the rest have nonsense mutations. This
number was determined by analyzing the genome in the Human Genome Project;
the number may vary among ethnic groups, and does vary among individuals.
Some people can smell amyl acetate (which smells like bananas), for
instance; others can't.