Certain chemicals used for communication between individuals of a single species are called pheromones.
Sensory organs inform living things about the world around them. Some senses depend on energy detection. Sight, for example, results from the sensing of light rays by light-sensitive cells in the eyes. Hearing depends on the ability of the eardrums to detect the vibrations of sound waves. Other senses, such as taste and smell, depend on the recognition of specific molecules or mixtures of molecules. Olfactory receptors, the specialized sensory cells that allow living things to “smell” molecules, are very similar in vertebrates, such as humans and dogs, and in invertebrates, such as insects. These receptors can detect incredibly small concentrations of molecules. In fact, each olfactory receptor cell of a male silkworm moth can be activated by a single molecule. Evidence discovered since the 1940’s suggests that molecular messages may be the most important means of communication in many animal species.
Certain chemicals used for communication between individuals of a single species are called pheromones. (The word pheromone comes from the Greek pherein, meaning “to carry,” and hormon, meaning to “excite.” Pheromones are used in several different ways—to attract mates, signal alarm or distress, claim territory, mark trails, and identify individuals. They can be detected at very low concentrations, so an individual need emit only a small quantity of pheromones to send a message to other individuals.
The female gypsy moth, for example, may release only 1 X ICC8 gram of sex attractant into the air, but this pheromone has the potential to attract more than a billion male moths. The olfactory receptors of the male gypsy moth, located in its antennae, are so sensitive to the female’s sex pheromone that a male can smell a female 7 miles (11 kilometers) away. The male then flies upwind toward the source of the pheromone to find the female.
When a foraging fire ant locates a food supply, it deposits a trail pheromone as it returns from the food supply to its nest site. The pheromone is excreted through the ant’s stinger. The ant touches its stinger to the ground as it marks the trail, much like a pencil making a dotted zigzag line on a page. Other fire ants then locate the food source by following the trail. The fire ant’s trail pheromone evaporates quickly so the trail disappears after a few minutes.
Scientists have determined the molecular structures of hundreds of pheromones, even though the isolation of the active chemicals from the host organisms can be extremely difficult. For example, to isolate and identify the mating pheromone of the American cockroach, chemists processed 75,000 female cockroaches. They extracted only 0.2 milligrams of active pheromone from all these insects. Then it took many years of study to find the pheromone’s formula—C15H20O3— and structure. Male cockroaches respond only to a substance of that specific formula and structure. In some insects, the active pheromone is a combination of substances. The sex attractant of engraver pine beetles is a mixture of three compounds, and all three must be present for the pheromone to have full effect
While the study of pheromones has revealed much about the way in which organisms communicate, it is also important for another reason. Pheromone research may someday lead to an environmentally friendly way of controlling insect pests. Pheromone-baited traps can be used to attract and kill insects. And, because the pheromones attract individuals of only a single species, they have no effect on other, perhaps beneficial, insects. Also, there is little danger of environmental damage, since pheromones can be used in very small amounts and are rapidly and safely degraded. In one four-year trial, pheromone-baited traps eliminated nearly 4 billion spruce bark beetles annually in the forests of Scandinavia.
Do humans use pheromones? Humans do not respond to specific chemicals in the way that insects do—nor do thev have the dog’s keen sense of smell—but the human nose is an excellent detector of certain molecules. The poisonous gas hydrogen sulfide, which is responsible for the odor of rotting eggs, can be perceived in concentrations as low as a few parts per billion. In addition, humans find the musklike odors of certain compounds very attractive: Compounds such as muscone, from musk deer, and civetone, from civet cats, are the basis of musk perfumes. The scents of both can be detected in very low concentrations, and women can generally detect them at lower concentrations than men can. Clearly, human behavior is not as affected by pheromones as insect behavior is, but the popularity of perfumes and colognes suggests that humans find some odors appealing when used by other individuals.