Index to this page

Pheromones

Pheromones are chemicals released by an organism into its environment enabling it to communicate with other members of its own species.

Insect Pheromones

Alarm Pheromone

When an ant is disturbed, it releases a pheromone that can be detected by other ants several centimeters away. They are attracted by low concentrations of the pheromone and begin to move toward the region of increasing concentration. As they get nearer to their disturbed nestmate, their response changes to one of alarm. The higher concentration causes them to run about as they work to remedy the disturbance.

Unless additional amounts of the alarm pheromone are released, it soon dissipates. This ensures that once the emergency is over, the ants return quietly to their former occupations.

Honeybees also have an alarm pheromone (which is a good thing not to elicit around a colony of "Africanized" bees).

Trail Pheromone

Certain ants, as they return to the nest with food, lay down a trail pheromone. This trail attracts and guides other ants to the food. It is continually renewed as long as the food holds out. When the supply begins to dwindle, trailmaking ceases. The trail pheromone evaporates quickly so other ants stop coming to the site and are not confused by old trails when food is found elsewhere. And at least in one species of ant, trails that no longer lead to food are also marked with a repellant pheromone.

A stick treated with the trail pheromone of an ant (left) can be used to make an artificial trail which is followed closely by other ants emerging from their nest (right). The trail will not be maintained by other ants unless food is placed at its end. (Photos courtesy of Sol Mednick and Scientific American).

Queen Pheromone

Honeybee queens spend their lives literally surrounded by a retinue of worker bees. The queen emits a pheromone that is a complex mixture of unsaturated hydrocarbons, fatty acids, and other organic molecules. Among its effects:

Sex Attractants

Hundreds of pheromones are known with which one sex (usually the female) of an insect species attracts its mates. Many of these sex attractants — or their close chemical relatives — are available commercially. They have proved useful weapons against insect pests in two ways:

The photo (courtesy of USDA) shows the feathery antennae of a male gypsy moth. These detect the pheromone released by the females (who do not fly). In some insects, a single molecule of sex attractant is enough to elicit a response.

Sexual Deception

By an animal

Many species of spiders prey exclusively on moths of certain species and only on the males.

Studies of one species of spider, Mastophora cornigera, show that it releases a mixture of volatile compounds that mimic the sex pheromone of the moth species it preys upon. Male moths flying upwind in search of a female end up eaten instead!

By a plant

A number of species of orchids — each pollinated by the males of a single species of insect (wasps or bees) — emit the same pheromone that is the sex attractant by which females of the insect species attract the males for mating.

Pheromones in Mammals

Releaser Pheromones

Many mammals (e.g., dogs and cats) deposit chemicals in and/or around their "territory". As these vaporize, they signal to other members of the species of the presence of the occupant of the territory.

Domestic rabbit mothers release a mammary pheromone that triggers immediate nursing behavior by their babies (pups). A good thing, too, as mothers devote only 5–7 minutes a day to feeding their pups so they had better be quick about it.

Many animals, including mammals, signal with alarm pheromones. Although neither the source nor the chemical nature of alarm pheromones are known in any mammal, stressed animals release something that triggers quick behavior (e.g., flight or fight) in other members of their species. The pheromone is detected in a special cluster of cells located at the very tip of the nose and thus in a position to detected airborne molecules even before the vomeronasal organ (next paragraph) or nasal epithelium can. The detectors on these cells are primary cilia.

Primer Pheromones

Rats and mice give off pheromones that elicit mating behavior. However, the response is not immediate as it is in the releaser pheromones of mother rabbits and insects. Instead, detection of the pheromone primes the endocrine system of the recipient to make the changes, e.g., ovulation, needed for successful mating.

Primer pheromones are detected by the olfactory epithelium with which normal odors are detected and also in most mammals (but not humans) by the vomeronasal organ (VNO). The VNO is a patch of receptor tissue in the nasal cavity distinct from the olfactory epithelium. The receptors are G-protein-coupled transmembrane proteins similar to those that mediate olfaction, but encoded by entirely different genes.

Link to discussion of olfaction in mammals.

The neurons leading from the VNO take a separate path into and through the brain.

Humans may have pheromones

It has long been noticed that women living close together (e.g., college roommates) develop synchronous menstrual cycles.

This is thought to be because they release two (as yet uncharacterized) primer pheromones

Both pheromones are released from the armpits.

The pheromones are not detected consciously as odors, but presumably trigger the hormonal changes that mediate the menstrual cycle.

Welcome&Next Search

23 January 2014