Pheromone

A pheromone is a chemical substance secreted or excreted by an organism to trigger a social response in members of its own species. These chemical messengers are transported outside the body to affect the recipient. They can impact neurocircuits, including the autonomous nervous system. This can lead to physiological changes mediated by hormones or cytokines. It can also cause inflammatory signaling, immune system changes, or immediate behavioral shifts. Pheromones are used by a vast range of life, from unicellular prokaryotes to complex multicellular eukaryotes.

Scientists categorize pheromones based on how they affect the recipient. Releaser pheromones cause an immediate change in behavior. These often act as powerful attractants that can draw mates from over two miles away. Because they trigger rapid responses, they are typically quickly degraded. In contrast, primer pheromones have a slower onset and a longer duration. Instead of changing behavior, they trigger changes in developmental events. Maud Norris first described these primer pheromones in the species Schistocerca gregaria in 1954. There are also signal pheromones, which cause short-term changes like the release of neurotransmitters.

One major category is aggregation pheromones, which cause individuals to gather in one location. This group is called an aggregation. These chemicals help with mate choice, defense against predators, or overcoming host resistance through mass attacks. Many insect groups use them, including Coleoptera, Collembola, Diptera, Hemiptera, Dictyoptera, and Orthoptera. In recent decades, these have been used to manage pests like the boll weevil (Anthonomus grandis) and various weevils. Because they are effective at very low concentrations and are non-toxic, they are highly selective suppression methods.

Alarm pheromones serve as urgent warnings. Some species release volatile substances when attacked by a predator to trigger flight or aggression. For example, the milkweed aphid releases pheromones and sticky wax when attacked, causing nearby aphids to flee.

In bees, ants, and wasps, these chemicals can trigger aggression. The species Vespula squamosa uses them to alert others to threats. Even plants use them; some emit alarm pheromones when grazed upon. This causes neighboring plants to produce tannins, which makes them less appetizing to herbivores.

Trail pheromones are essential for social insects like ants. Ants use volatile hydrocarbons to mark paths to food sources. As long as the food is available, visiting ants continuously renew the trail because the pheromone evaporates quickly. If the food supply dwindles, the trail-making stops. Interestingly, Pharaoh ants (Monomorium pharaonis) use a repellent pheromone to mark trails that no longer lead to food. This helps the colony engage in more efficient collective exploration. Other species, like the army ant Eciton burchellii, use pheromones to maintain foraging paths.

Sex pheromones are vital for reproduction across many species. In animals, they often indicate that a female is available for breeding. Some insects, like certain moths and butterflies, can detect mates from great distances. At a microscopic level, bacteria like Bacillus subtilis release chemicals to induce a "competent" state in neighbors. This allows bacteria to take up DNA from other cells through a process called transformation. Even sea urchins use pheromones to coordinate the simultaneous ejection of sex cells into the water.

The history of pheromone research involves many notable scientists. The term "pheromone" was a portmanteau coined in 1959 by Peter Karlson and Martin Lüscher. They based the name on the Greek words for "to carry" and "apart." Earlier researchers included Jean-Henri Fabre and Karl von Frisch, who called them "alarm substances." A major breakthrough occurred when the German biochemist Adolf Butenandt characterized bombykol. This was the first well-characterized pheromone, released by female silkworms to attract mates. This discovery helped define how chemical signals elicit innate behaviors in conspecifics.

Pheromones also play roles in territory and social maintenance. Dogs and cats deposit territorial pheromones in urine to mark boundaries. In social seabirds, the preen gland is used to mark nests and territories. Other specialized signals include necromones, which are released by decomposing organisms. These allow crustaceans and hexapods to identify the presence of dead members of their species. There are even chemicals like TAA in rabbit milk that induce suckling behavior in newborns. This shows how chemical ecology connects to almost every aspect of biological survival.