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Pheromones

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

Insect Pheromones

Most of the pheromones discovered so far have been found in the social insects, e.g., ants and honeybees. Many of these are releaser pheromones; that is, they trigger an immediate behavioral response.

Examples:

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.

A stick treated with the trail pheromone of an ant (left) can be used to make an artificial trail with 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 mandibular pheromone

Honeybee queens spend their lives literally surrounded by a retinue of worker bees. The workers are attracted to her by a pheromone that she releases from her mandibular glands. The pheromone is a mixture of unsaturated fatty acids.

Sex attractants

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

Male Confusion

Distributing a sex attractant throughout an area masks the insect's own attractant and thus may prevent the sexes getting together. This "communication disruption" has been used successfully against a wide variety of important pests. The sex attractant of the pink bollworm has reduced the need for conventional chemical insecticides by 90% in some cotton-growing areas.
Insect Monitoring

Insect sex attractants are also valuable in monitoring pest populations. By baiting traps with the appropriate pheromone, a build-up of the pest population can be spotted early. Even if a conventional insecticide is the weapon chosen, its early use reduces
- the amount needed
- damage to the crop
- cost to the grower
- possible damage to the environment.
Early detection of pest build-up is a key ingredient in the system known as integrated pest management (IPM).

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.

Games Predators Play

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!

Pheromones in Mammals

Identification 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.

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 insects. Instead, detection of the pheromone primes the endocrine system of the recipient to make the changes, e.g., ovulation, needed for successful mating.

Detection of these pheromones requires a functioning vomeronasal organ (VNO). This is a patch of receptor tissue in the nasal cavity distinct from the nasal epithelium through which normal odors are detected. The receptors are 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 have pheromones

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

It turns out that this is because they release two (as yet uncharacterized) primer pheromones

one prior to ovulation that tends to speed up the onset of ovulation in others
one after ovulation that tends to delay the onset of ovulation in other women.

Both pheromones are released from the armpits.

The pheromones are not detected consciously as odors, but presumably are detected by the human VNO and trigger the hormonal changes that mediate the menstrual cycle [link to discussion] unconsciously.

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17 June 2001