Communication

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The world is full of sights, sounds, and smells that organisms use to communicate with each other. Because humans are diurnal (active during the day) and have well-developed eyes and ears, we tend to think of communication in terms of vision and acoustics. However, other animals, plants, fungi, and even microorganisms can communicate, and do so using a variety of different methods. Communication is defined as any signal from one organism that influences the behavior of another organism. The type of signaling an organism uses depends on the reception abilities of the receiver.Nocturnal animals use sound and smells to communicate with each other, flowers attract pollinators by smell and sight, microorganisms communicate through touch and chemicals, and aquatic organisms can use electricity.

Communication between different individuals enhances the chances of survival by the sender. The sender may successfully defend a territory, thus ensuring a food supply, or successfully attract a mate, thus increasing reproductive success. Parents communicate with offspring, increasing the probability that the offspring will mature and reproduce.

While the sender of a signal usually benefits, predators have learned to exploit these signals. The calls of male Tungara frogs can be received by bats. Fireflies have learned to prey on other fireflies by mimicking mating signals. There are five modes of communication used by animals: visual, acoustic, chemical, tactile, and electrical.

Visual communication is transmitted by light, ranging from infrared to ultraviolet, and is detected by photoreceptors. Only vertebrates and arthropods have photoreceptors advanced enough to be useful in communication. In vertebrates, the receptors are located in the retina of the eye, but in arthropods, they are encased in each of the miniature "eyes" that form their compound eyes. The eyes of arthropods and vertebrates are very different, having independent evolutionary origins. In general, vertebrates have sharper vision than arthropods, but this clarity is due to the larger size of vertebrates rather than a more advanced eye.

Some visual signals are exhibited simply through color patterns. One example is aposematic coloration, in which an animal advertises that it is toxic, distasteful, or otherwise dangerous, through bright colors. Like many signals, aposematic coloration can be deceitful. Coral snakes and scarlet king snakes have yellow, red, and black bands, but only coral snakes are venomous. King snakes mimic coral snakes in order to appear venomous and escape predation. Similarly, a group of harmless flies is marked with yellow and black banding, mimicking the various bees and wasps that actually do pose a danger.

Sometimes visual cues evolve when other forms of communication are ineffective. The semaphore frog in Borneo lives on rocks next to raging waterfalls, so the normal croaks and whistles of frog communication would be ineffective. This species has instead evolved visual signaling, which involves flashing white-spotted feet.

Visual signals, which are most effective in daylight, are usually used by diurnal animals. However, some insects, such as the familiar firefly, have evolved ways to communicate visually in darkness. Male fireflies flash bioluminescent abdomens in a particular pattern, hoping to elicit a similar flash sequence in some female on the ground. After receiving a signal, the male will join the female and mate. One genus of firefly, Photurus, has discovered the flash signal of a different genus, Photinus. Photurus females mimic Photinus females, calling in the Photinus males. When the male of the wrong species arrives expecting a mate, the female eats him.

Acoustic signals are produced in a variety of ways, from striking objects to vibrating vocal cords. A sound is heard when vibrations in air or water are detected by mechanoreceptors, which vibrate in response. In mammals, reptiles, birds, and amphibians, the receptors are located in the inner ear. Arthropod receptors are variable, and may be found on the legs, thorax, or abdomen. The only fishes that can detect sound are those with modified flexible air sacs.

Since sound is carried farther in water than in air, aquatic mammals can communicate over great distances. Orcas (so-called "killer whales") use an elaborate system of cries to establish dominance, find offspring and mates, and even express contentment. Each pod of orcas develops its own dialect of cries, allowing pod-mates to recognize each other.

Birds use songs to declare territories and enhance their chances of survival by reducing harmful encounters with birds of the same species. When male birds establish territories for the mating season, they often come to physical blows with each other to defend their boundaries. Once the territory has been settled, the birds reinforce their boundaries by singing rather than fighting. If a bird ceases calling, other birds will immediately take over the space.

Most organisms can use molecules to communicate. Animals concentrate chemical receptors in the nose, mouth, and antennae. Vertebrates have the most developed senses of smell and taste because the receptors are kept moist and isolated. Plants cannot sense chemical signals in the same way that animals do, but plants do emit them in abundance. For example, pine trees emit terpenes, sharply odoriferous chemicals that communicate distastefulness to herbivorous insects. This signal is an allomone, a term for any chemical used to communicate between members of different species.

Chemicals that communicate between members of the same species are called pheromones. Female moths use these powerful signals to attract mates. Male moths, with their fantastically plumed antennae, are able to detect just a few molecules in a square kilometer (approximately 0.4 square miles). Honeybees use pheromones in conjunction with visual cues to communicate to other workers where food sources are located. Mammals rub scent glands on objects to mark their territories, and on each other during the mating season to act as aphrodisiacs.

Touch is detected by proprioceptors on pliable body surfaces of the receiver. Proprioceptors respond to temporary changes in the shape of the surface or the movement of sensory structures such as hairs, whiskers, and bristles. Structures that have a large number of receptors are tactile organs. Human fingertips are tactile organs, having about 100 receptors per square centimeter (0.15 square inch). The tentacles of octopuses, antennae of some insects, and bills of sandpiper birds are also tactile organs.

Touch is a less informative means of communication than sight, sound, or chemicals, but can be crucial. Male and female crane flies must touch legs before either animal will accept the other as a mate, and human infants must be held and cuddled to develop properly and to recover more quickly from illness.

Communication by electrical current has evolved only in fishes, but within this group it has arisen several times independently. The fish generate an electrical charge in specialized cells called electrocytes. Electrocytes are arranged in columns and surrounded by insulating cells. Electric eels can generate charges up to 720 volts, but these strongly electrical fish use their charge to capture prey, rather than for communication.

Weakly electric fish, such as skates and knifefishes, evolved the use of their signals for social communication because they are either active at night or in murky water. Electrical signaling is highly versatile. A single fish can communicate territory boundaries, advertise for a mate, or show aggressiveness just by changing the strength and pattern of pulses. Wave fish use their signals to establish a social hierarchy. Dominant fish reinforce their position by matching their charge frequency to submissive wave fish, forcing the submissive fish to shift their frequencies away. Male Nile fish spend days building a suitable nest and then send out invitations to females by emitting pulses of electricity.

Communication is essential to any form of social interaction, and so all living things have developed some way to transmit and receive information. The few examples provided in this article do not come close to demonstrating the diversity of communication that exists in the natural world.

Acoustic Signals; Aposematism; Courtship; Vocalization.

Bradbury, Jack W., and Sandra L. Vehrencamp. Principles of Animal Communication. Sunderland, MA: Sinauer Associates, 1998.

Kirchner, Wolfgang H., and William F. Towne. "The Sensory Basis of the Honey-bee's Dance Language." Scientific American 270, no. 6 (1994):52-59.

Moller, Peter. Electric Fishes: History and Behavior. New York: Chapman and Hall, 1995.

Morton, Eugene S., and Jake Page. Animal Talk: Science and the Voices of Nature. New York: Random House Publishers, 1992.

Narins, Peter M. "Frog Communication." Scientific American 273, no. 2 (1995):78-83.

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