Chemical Communication


Throughout the animal kingdom, from the simplest creature to the most complex, some form of communication takes place. Although most of us think of it in terms of sound, actually there are four methods of communication—auditory (sound), visual (sight), tactile (touch), and chemical (smell and taste).

Chirps, croaks, howls, barks, gobbles, and other such vocalizations are obvious examples of auditory communication. These sounds make it possible for animals to remain hidden while talking; however, the sounds also are heard by others and may bring a predator to lunch. Auditory communication is limited by the distance the sound can be heard.

Animals send visual messages to each other by twitching the tail, winking an eye, lowering the head, pawing the ground, bristling body hair, flashing wing color, arching the back, or making other such movements. Since animals sending visual messages must be seen while communicating, their body language also may attract the attention of a nearby predator. Communicating distance for visual messages is limited by the eyesight of the receiver. Tactile communication requires actual contact between animals and includes such gestures as a lick, nip, slap, shove, rub, or nuzzle.

Coyotes, like the family dog, use urine to mark the boundaries of their territories. These chemical messages tell other canines whether the animal that left the scent is a male or female and whether it poses a threat.

The most widely used method of communication, and the one we intend to study more closely, is the invisible language of odors. These silent messages consist of chemical substances called pheromones (FER-ah-moanz). The word pheromone is derived from two Greek words, pherein, which means “carry,” and horman, which means “excite.” Usually the message causes an immediate response.

It should be pointed out that not all odors are pheromones. Humans recognize and respond to a variety of odors, such as food, but we do not consciously communicate with each other through pheromones. In fact, we do everything possible to wash away or disguise with artificial scents any body odors we might have.

Researchers restrict the use of the word pheromone to describe chemical messages that pass between animals of the same species. Although it may seem a bit confusing, an odor can be a pheromone to one creature and merely an odor to another. For example, when a rabbit leaves a scent to convey a message to another rabbit, the scent is a pheromone. However, the coyote that follows the same rabbit’s scent cannot read the chemical message. To the coyote, the scent is an odor, not a pheromone.

Pheromones consist of many combinations of chemicals that are produced in gland found on the face, head, chest, arms, legs, back, rump, between the toes, or in several such locations. Some also are situated so they can add chemicals to body wastes. Each odor has a specific message and may travel as liquid, vapor, or gas in water or air or on the ground. Both taste and smell are used to interpret these chemical messages. Pheromones carry information about such things as identity, territory, sex, food, assembly, and danger.

Identity

Almost every species of animal has its own body scent, but there are additional odors that serve as chemical passports to identify individual animals, family groups, or members of a colony. For example, all ants have distinctive body odors that indicate the nest to which they belong and their job within the nest. When two ants meet, they use their antennae—the organs that carry their sense of smell—to establish their identities. If an ant enters the colony of a different kind of ant, its body odor identifies it as an enemy and brings on an attack.

In some laboratory tests, ants washed clean of their colony odor and dabbed with the scent of an enemy ant were attacked by their own nest mates. When another ant, painted with the scent of a dead ant, was returned to its nest, its nest mates carried it out. Each time the “dead” ant tried to return, it was removed, even though it obviously was alive and struggling. When the dead scent wore off, the ant was allowed to remain in the nest.

Territory

A personal scent often is used to establish territorial boundaries. When marking its territory, the beaver makes patties of mud, wood chips, small sticks, and pebbles and then douses them with a chemical substance called castoreum. This odorous secretion, manufactured in glands located under the tail, contains at least forty-five chemical ingredients. It is possible that different combinations of these chemicals mean different things. Other beavers may cover this mud pie with their own scented patties, causing the smelly mound of mud to grow several feet high.

Phermones gather animals together for various reasons. This large group of ladybugs probably has responded to such a chemical message.

Rabbits secrete a colorless fluid from chin glands to mark the ground and plants in their territories. This marking process is known as chinning. The male cottontail may deposit his scent on an area two acres in size, while the male snowshoe rabbit claims as many as eighteen acres.

Antelope have prominent facial glands for marking their territories. The scent is deposited at eye level on leaves and other vegetation so the odor cannot be missed by other antelope.

Urine, with its added chemical scents, is used quite frequently for indicating territorial boundaries. Bears plaster mud—often made by first wetting on the ground—on trees and then rub against it. Hairs come off and stick in the mud adding more bear scent to the tree. Bison also urinate on the ground, roll in the resulting mud, and then rub against trees.

Coyotes, wolves, and the family dog always use urine to identify their territories. You probably have watched a male dog sniff a tree, trash can, or fence post to read the scent message left by others. Reactions are varied. If he scratches the ground vigorously with his back legs or growls, he probably has identified the scent of a male dog that could be a possible challenger. If he whines and sniffs again, the scent probably was left by a female. If he displays no reaction, the scent probably was left by a smaller male or a friendly one that he knows poses no threat. Once he has read the messages and reacted to them, he usually wets on the object to leave his own scent. Although the first dog that wet on the object claimed the territory, the following dog may claim it too. By reinforcing the scent with frequent visits, a dog establishes his territorial boundaries, but one day he may be called upon to defend them.

When two greyhounds, housebroken for ten years, suddenly began wetting on the furniture, rugs, and corners of the rooms, their master was astonished. He couldn’t figure out what was wrong until he realized that their unusual behavior began shortly after his parrot learned to mimic a dog’s bark. The well-behaved dogs were completely confused by the invisible, scentless, but audible dog in their midst. Unable to find or confront the intruder, they were attempting to communicate with it by marking their territory and extending a challenge.

Male squirrels are among those animals that sprinkle their mates with urine to mark them and warn other males to keep their distance.

Sex

Of all the different messages that are sent by pheromones, the most basic ones, except those for alarm, are related to sex

Insects probably have the most effective long-range sex attractants. For example, a female moth’s mating pheromones are so powerful she can attract males from miles away. The male moth’s antennae may have as many as 150,000 scent receptor cells, 60 to 70 percent of which are sensitive to female sex pheromones.

In one experiment, male silkworm moths were marked and released from a moving train at various distances from a caged female. Her chemical sex messages brought some of the males to her from a distance of seven miles. Some researchers doubt the pheromones were entirely responsible, but everyone agrees the female moth’s odor will attract males for a distance of one to three miles with no difficulty. These potent sex pheromones are released from a pair of tiny glands located near the tip of the female moth’s abdomen. Although the amount of scent she is capable of producing is extremely small, it is estimated that if it were released all at once it would attract as many as a trillion males. Once she has mated, she stops producing pheromones that attract males.

The strength of these insect sex pheromones has been the subject of many studies. In one five-day field test, a caged female pine sawfly attracted more than 11,000 males. Other researchers found the sex odor of the female cockroach strong enough to cause the male to respond when fanned with a piece of paper the female had merely walked across. It also has been observed that the pheromone the queen honeybee releases during her mating flight is strong enough to attract males from hundreds of yards away.

When the young male alligator reaches maturity, it will be able to eject a powerful scent from glands located under its jaws to attract a mat during the spring breeding season.

Bull alligators eject a jet of powerful scent from glands under their jaws to attract their mates. During the spring breeding season, this odorous vapor hangs over an alligator swamp like a fog.

Barnacles discharge pheromones into the salt water to attract their free-swimming larvae. This chemical attractant leads the larvae to the established barnacle colony to enlarge it and provide more mates.
In addition to its uses for territorial marking, urine also is used by many animals to transmit sex attractants. Female crabs secrete a chemical in their urine to attract the males. We can be sure that scent, not sight, is involved because male crabs, when placed in water that has previously held a ready-to-mate female, become excited by her lingering scent in the water. They assume mating positions even though no female crab is present.

Many female fish also release chemicals into the water with their urine before they are ready to lay eggs. By the time the male fish catch the scent and follow it back to the female, they are ready to mate. In some fish species, the males release a scent that lures the females for courtship or triggers other males to seek mates.

Researchers have determined that a female dog may have as many as fifty-two different chemical variations in her urine during a single year. By smelling and tasting her urine, male dogs are able to calculate her readiness to mate to the day, and her scent usually attracts more than one willing male.

When the female weasel is ready to mate, she leaves a scent trail on the ground. If a male weasel discovers this trail, he follows, depositing his own scent in hopes that other males will be discouraged from following her too.

With the aid of scents, ants are able to recognize nest mates and enemies, mark trails to food sources, give alarm signals, and relay many other messages required for the efficient management of the nest.

Some animals, such as rabbits, squirrels, foxes, and porcupines, sprinkle their mates with urine to mark them and warn other males to keep their distance.

Food

Scent glands on the ant’s abdomen are used to establish a chemical trail from the food source to the nest. The scout that discovers the food lays down the first trail. Other ants then follow it to the food and reinforce the scent on their way back to the nest. As the food supply grows smaller, less scent is left and fewer ants return to the food. When the food is gone, the last ant leaves no scent trail on the way back to the nest.

Several experiments can be made with these ant trails. If you brush away a portion of one, the ants will scramble about in confusion until they locate it again. If you place a piece of paper between the nest and a food source, the ants will walk across it, depositing a chemical trail on the paper. By pivoting the paper, you will lead them down their trail and away from the food, but only for a short while. If there is no food at the end of the trail, the trail will be abandoned and the scouts will begin another search for food. You might also try shifting the food to see how long it takes the ants to relocate it.

Houseflies do not make food trails, but they do leave a scent on food that will attract any other flies that are in smelling distance.

Assembly

Pheromones gather animals together for many different reasons. Young fish, although reared separately, use scent to identify members of the same species and will form schools when placed together. Like silent cries, chemical pheromones secreted by larvae honeybees assemble workers to feed the developing young. Female earwigs emit a pheromone that keeps the young nearby while they are immature. Some newborn fish release a chemical that keeps their parents close by for protection and may prevent the parents from eating them. When it is hibernation time, ladybugs send out a chemical signal that brings the beetles together into large groups.

The bark beetle scout uses an assembly pheromone to summon other bark beetles once a desirable tree has been found. While scouting bees search for a place to establish a new hive when the old ones become too crowded, the queen bee holds the swarm together by emitting an assembly pheromone.

Among the simplest forms of life is a substance known as slime mold. Some biologists class it as a plant, others as an animal, and still others as a mixture of the two. Regardless, it begins life as a single-celled organism capable of discharging a chemical substance in rhythmic puffs. This chemical and its method of release serve as an attractant which causes several of the single-celled organisms to gather together and fuse into one mass so reproduction can occur.

Danger

Hurt fish release a chemical that communicates alarm to other fish. If an injured minnow is placed in a minnow school, the whole school quickly flees in alarm. Pouring the water from an injured minnow’s tank into a school of minnows also causes alarm, so we know the warning is delivered by a pheromone in the water.

When an attack occurs on an ant nest, the guard ants in that area release an alarm scent. Those ants nearest the danger point respond to the alarm and rush to defend the nest. If more help is needed, additional scent is released. This selective alarm system does not interrupt the duties of the entire nest.

Did you ever wonder why more than one wasp, hornet, or bee rushes out and stings the person who disturbs a nest? Pheromones play a part. The first wasp deposits a pheromone on the enemy with its sting. The odor identifies that person as the enemy to others of the hive, causing them to attack too.

Many times silent alarms go out without our being aware of them. For example, if you disturb a plant that is covered with aphids, more than likely the insects will send out an alarm signal that causes most of them to drop to the ground for safety. When danger is past, they resume their places on the plant.

When you think of an alarm odor, the skunk probably comes to mind. However, since the skunk uses its spray as a defense weapon against anything that threatens it, not to communicate with other skunks, its potent spray is not a pheromone.

The uses of pheromones mentioned in this article are but a few examples of the ways the silent language of the animal world touches almost every phase of most animals’ lives. One day, through continued chemical research, we may understand more about how to use them to attract, repel, or control the creatures that inhabit our world.


Additional Information:

Ilo Hiller
1983 Chemical Communication. Young Naturalist. The Louise Lindsey Merrick Texas Environment Series, No. 6, pp. 55-58. Texas A&M University Press, College Station.

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