Acoustic Signals

Acoustic signals are noises that animals produce in response to a specific stimulus or situation, and that have a specific meaning. These may be vocal communications emitted from the animal's larynx, such as a wolf's howl; sounds produced by appendages, such as a cricket's chirp; or sounds created by an animal's interaction with its environment, such as a rabbit thumping the ground with its hind foot when it sights danger. The physiological characteristics of animals, such as throat shape or lung size, create constraints on the type of acoustic signals an animal produces. Similarly, the anatomical properties of the ear, and the processing capabilities of the auditory regions of the brain, can limit the range of sound that a species is capable of detecting. Compared with most mammals, humans have an abnormally complex system of vocalization that is supported by the expanded language centers of the brain, a dexterous tongue and throat, and powerful lungs. However, humans are unable to hear in the frequency range of animals that communicate at much higher pitches, such as voles, or animals that vocalize with lower pitches, such as certain species of whale.

Signal Characteristics

Several features combine to create a meaningful auditory signal. The first of these is the frequency, or pitch, of a sound. Another variable is the amplitude, or loudness. Different combinations of these two features can drastically alter the meaning of a sound. For example, a dog that whines quietly is communicating pain with a high-frequency, low-amplitude sound; a dog that growls loudly is expressing anger with a low-frequency, high-amplitude sound. The repetition rate and duration of a particular sound are likewise important. Male frogs of certain species, such as the plains leopard frog, call during breeding season to attract females; females recognize the calls of their own species by the length of the call and its repetition rate (calls per minute). Other species of frog in the vicinity use the same frequency call but vary its length and repetition rate.

The circumstances that surround acoustic signals can also alter their meaning. These include the time of year, time of day, spatial location, weather conditions, and physiological state of the organism (such as reproductive state). A mating call presented to females outside of the mating season may have no effect—the females are not hormonally prepared to respond.

The Uses of Acoustic Signals

Animals use acoustic signals in several instances: conspecific communication, sexual selection , mother-young interactions, interspecies communication, orientation, and language.

Conspecific communication.

This (intraspecies communication) occurs between animals of the same species. Although sexual selection, mother-young interactions, and language are included in the category of conspecific communication, they will be explained separately because of their ecological importance. Conspecific communication can be very complex. For example, black-tailed prairie dogs live in very structured colonies that can cover tens of acres. When a prairie dog recognizes danger, it gives a warning call to the rest of the colony. Their vocal abilities are so elaborate that these animals can communicate the size, shape, and behavior of a predator by varying the features of their call. For example, a prairie dog call signifying a running coyote would be different from one signifying a gliding eagle. Intraspecies communication can relate danger, sexual selection, state-of-being, or features of the environment. A fairly universal form of intraspecies communication is the call made by a sick or dying individual. This kind of acoustic signal refers to the animal's state-of-being.

Sexual selection.

This is the process by which species evolve characteristics that improve the chances of successful reproduction. Two facets of sexual selection involve acoustic signaling: locating and recognizing a mate, and defending territory. Acoustic signaling can improve the likelihood that an animal will find a mate of its own species over large distances or in the darkness. Sound, which travels at a rate of 340 meters (372 yards) per second in air and 1,230 meters (1345 yards) per second in water, provides an excellent means for quick signal transmission. An animal's reproductive success—the number and health of its offspring—may be seriously compromised if it cannot locate a mate of the same species. As a result of this selective pressure, many species have come to rely upon intricate calls to locate one another.

Territoriality.

Another reproductive strategy is a behavior known as territoriality, in which males guard a particular location from other males of their species. Territorial male songbirds, such as nightingale wrens, each have a unique song. They mark their territory by repeating their trademark song at the boundaries of their territory or at the nest. This warns other males away from the chosen female and the chosen tract of land, thereby increasing mating success for that male and assuring a food supply for the young. This strategy is so successful that it can be seen in such diverse animals as the midshipman fish. Males of this species growl, hum, and grunt to attract females to their nests. However, the importance of acoustic signaling to sexual selection varies widely across species.

Mother-young communication.

This exists between many vertebrate mothers and their offspring. In some species, including several groups of primates, the mother is able to recognize the distinct vocalizations of her young. Baby bird vocalizations cannot distinguish one hatchling from another, yet a mother bird responds equally to the call of any hatchling of the same age as her own young. In species where the parents invest time and energy into raising their young, it is of the utmost importance for the mother to be able to recognize the acoustic signals of her young. If she cannot distinguish her young from the young of others, she will waste precious time and resources on other individuals while neglecting her offspring. The cuckoo bird takes advantage of this fact by laying its eggs in the nests of other species. The infant cuckoo bird hatches among the young from the other species, but then it pushes the legitimate chicks or unhatched eggs out of the nest. When the mother bird finds her young have disappeared, the infant cuckoo is given all the nourishment, and the mother bird's energy is wasted on an individual that will not carry on her genes . Usually the hypersensitivity of a mother to the call of her young diminishes as her offspring mature.

Interspecies communication.

This occurs when the hallmark acoustic signal of one species is conveyed to another species and induces that species to react in a predictable manner. A common example of interspecies acoustic signaling is when a rattlesnake shakes its rattle-shaped tail tip before striking. In this case, the snake is warning offending animals of its presence and its impending strike. This type of acoustic signal is most often a warning, as in the case of the mother bear growling when an animal approaches her cubs too closely. Conversely, an unintentional form of interspecies communication occurs when a predator can track the acoustic emissions of its prey. Woodpeckers listen for the sounds of insects chewing through wood so that they know where to peck, and owls can hear the squeaking of mice in the darkness.

Orientation.

Some animals use auditory signals as their primary means of orientation. These animals emit sounds and then listen for the echoes that rebound off objects in the environment. The arrival time of the echo, its amplitude, and its divergence from the original call all give clues about the animal's environment. The best-known example of this strategy is echolocation in New World bats of the suborder Microchiroptera . The echoes from their calls are such good indicators of the environment that bats are able to fly through complicated environments in complete darkness, even when they are blinded. They also use their high-frequency echolocation to forage for fruit or flowers and to capture prey such as insects, fish, and small animals.

Language.

Humans use auditory signals to communicate with each other through language. No other animal is considered to have a system of communication complex enough to be considered language. Language differs from other kinds of communication in four features: flexibility, form, abstraction, and essentiality. Humans do not learn how to speak by memorization, but rather by learning rules for forming meaningful speech. The flexibility of language assures that despite the infinite number of experiences that influence the way people speak, they can be readily understood. Rules of sound, word, and sentence combination comprise the framework for the form of language. People express emotional and philosophical abstractions when they speak, not just the physical necessities of life. Essentiality refers to the reliance of human society on the use of language. Language is not just a tool—it shapes thinking and speech patterns and constitutes an integral part of one's sense of identity. Aside from language, humans also communicate with sounds such as laughter, screaming, and applause.

The acoustic signals of marine and terrestrial invertebrates and vertebrates depend on the physiology , social structure, and ecological niche of the organism. They are so important that the success of a species may depend entirely on acoustic signals, whether for feeding, mating, or interacting with one another. Humans inhabit every environment on Earth, and it is possible that the noise of modern human civilization is interfering with the acoustic signals of other species. A dramatic example of this is the beaching of seven whales in the Bahamas near the site of a naval sonar research site in February 2001. All of the whales showed signs of inner ear damage, and scientists hypothesized that the high amplitude noises used at the research site deafened them, causing pain and confusion. Further research into the acoustic signals used by animals could help prevent such ecological disasters.

see also Communication; Vocalization.

Rebecca M. Steinberg

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