Vocalizations

A vocal mechanism typically consists of lungs to provide an air stream, a trachea to conduct the air to a larynx or syrinx, a pharynx, and the associated oral and nasal cavities. Vocalization, any sound produced by the respiratory system, implies that air flowing from the lungs has been converted into an oscillating air stream. The sound can be melodious or noisy, and when the vocal folds vibrate, the sound is termed phonation.

Animal vocalizations evolved with hearing because of the many evolutionary advantages of sound communication. Sound can be varied in pitch, duration, tonality, and repetition rate, making it possible to communicate considerable amounts of detailed information quickly. Animals may vocalize while keeping their limbs free or while hiding. Because sound waves pass readily through vegetation and around obstacles, vocalization is used among insects, frogs, and birds to indicate sexual receptivity.

Vocalizations are also an important component in the behavioral displays of reptiles, birds, and mammals. Although animals typically employ body language and nonvocal noises in their displays, vocalization provides an impressive elaboration impossible to achieve otherwise. The fearsome sight of a gorilla beating its chest and stomping the ground is enhanced considerably by its bloodcurdling roar.

There are two types of sound-generating mechanisms used for animal vocalizations. The first requires a vibrating structure, such as the voiced sounds of human speech produced by vibrating vocal folds. The second is aerodynamically excited, such as for unvoiced sounds or whistled tones. The vibrating element of voiced sounds is a flow valve, which vibrates when pressure is applied from the lungs. Two systems occur in the animal kingdom. The first is the larynx, which is common among mammals. When pressure is applied from the lungs, the vocal folds swing outward, stretching muscular ligaments, which tend to restore them. These alternating forces induce vibration in the form of air pulses, which propagate through the trachea to be emitted by the mouth or nose. The second is the avian syrinx, which is blown open by excess pressure on either side of the mechanism and restored by forces supplied by pressurized air sacs surrounding the syrinx. In both cases, pitch can be varied by the muscles associated with the valve mechanism. Airflow through the valve is nonlinear, generating a complete set of harmonics in the radiated sound.

Whistled sounds are created when an air jet impinges on a sharp edge or an aperture, creating a sinuous instability in the air stream. The sound is considerably louder when the air stream is acoustically coupled to a resonating oral or nasal cavity, the size and shape of which determines the resulting pitch.

In any acoustic communication system, signals must compete with noise in the environment. The environment has a preponderance of natural low-frequency noise, but high frequencies are produced more easily by small animals. Most vertebrates, however, do not communicate with high-frequency sounds because they are more readily attenuated and thus do not travel far. The optimum frequencies for auditory communication thus depend on the desired communication range. High frequencies are best for small animals communicating over short distances, while low frequencies better serve large animals communicating over longer distances.

Reptiles and Frogs

Although the vocal apparatus of alligators is rather primitive, they can produce noise-excited roaring and hissing vocalizations when provoked by low-frequency sounds, such as a horn or cannon. Crocodilians live as lone individuals and establish individual territories defined by their loud, vibrant roars. To roar, crocodiles tense their body muscles and raise their heads and tails high above the water. The emitted sound vibrates the animal’s flanks so violently that water is sprayed into the air. Crocodiles are also capable of deep grunting sounds used during courtship. Geckos are small, nocturnal lizards with soft skin. Their voice varies by species, from faint chirps to loud squawks.

Frog vocalizations encode several pieces of information—the species, the sex, and whether it has mated. Male mating calls attract females and indicate the number of other males nearby, critical information for females who wish to deposit their eggs in the most receptive habitat. The vocalizations are produced by primitive vocal cords, consisting of a pair of slits at the throat opening on the floor of the mouth. When the frog forces air from the lungs, the cords vibrate to produce sound. Many species also have a vocal sac, an inflatable chamber located in the throat region of males, which swells to a large size when calls are produced. Air vibrates the vocal cords while passing back and forth between the lungs and the sac, while the vocal pouch acts as a resonating chamber that amplifies the sound. The frog’s mouth remains closed while vocalizing, so it can call even while underwater.

Birds

In birds, the voice is well developed, having such distinctive sound patterns that many species are named onomatopoeically, such as the whippoorwill. Although birds use different calls for different purposes, each species has a primary song, often repeated incessantly, used for species recognition. Male birds also use vocalizations to mark their territory and to attract mates. Some species can even identify their mates by sound. During the breeding season, the male emperor penguin leaves for several days to forage for food. When he returns, he can locate his mate out of a pack of hundreds of birds from the calls emitted.

The green-backed sparrow utters a hoarse scream when attack or escape is likely to occur, and medium hoarse notes when the bird’s indecision between the two courses of action make it unlikely that either will occur. To a family of migrating geese, the sounds of other geese on the ground convey the information that there is probably food and a safe shelter.

Cuckoos are a highly vocal species; they use a variety of contact calls, alarm notes, and melodious songs used to define territory or attract mates. The male’s song is characterized by a repetition of loud, short notes on a descending scale. The common cuckoo found throughout Europe, Asia, and Africa emits the well-known two-note call imitated in cuckoo clocks.

Owls produce a variety of vocalizations with a pitch, timbre, and rhythm unique to each species. Most vocalize at dusk and dawn before beginning to hunt. Their songs vary from the deep hoots of large species to the chirps and warbles of small owls. When its nest is threatened, the nestling burrowing owls emit a buzzing noise, resembling the warning sound of the rattlesnakes that frequently inhabit rodent burrows. North American screech owls begin mating when a special song, commenced by the male, is answered by a distant female. After fifteen minutes of antiphonal singing while gradually approaching, the couple meets, sings a duet with a different song pattern, and mates. Other calls of the screech owl include sounds to prompt the young to reveal their location, a food-soliciting call by the young, and barking calls used to eject the matured young from the parents’ territory.

The “voicebox” used by birds to produce birdsong is the syrinx, located where the windpipe divides into the two bronchial tubes leading to the lungs. The syrinx varies considerably among different birds. In the ordinary chicken, it is quite simple, consisting of four uncomplicated membranes, which produce the characteristic clucking sounds when activated. An asymmetric chamber at the base of the ducks’ trachea adds a noise component to its vocalization, which humans hear as “quacking.” The trachea of trumpeter swans enters the sternum, flexes twice into bony pockets, and then coils back to the lungs, somewhat analogous to bass orchestral wind instruments. This long resonator implements the production of its clarion, trumpetlike call.

The human brain can perceive speech in sounds having only the remotest resemblance to speech if the rhythm and intonation matches that of a simple sentence. Mynah birds use this phenomenon to deceive us into believing they can speak. They have a syrinx valve on each bronchial tube, which can be independently controlled to produce two simultaneous wavering tones, which we perceive as speech when they mimic the rhythm of a sentence.

Mammals

Among mammals, vocalization is used first for survival. Infants vocalize to express hunger or pain or to be located when lost. Other cries, such as the lion’s roar or the trumpeting of an elephant, mandate caution. Animal vocalizations of this type, often accompanied by an offensive posture, are used to startle or intimidate an opponent. There is a direct correlation between vocal anatomy and behavior among mammals. Social animals that readily vocalize have larynges that open less widely when they breathe, thus reducing breathing efficiency. The vocal folds must close to start phonation (wailing of cats, howling of wolves). For breathing, they must open wide to not obstruct airflow to the lungs. Horses and animals whose survival depends on running long distances while breathing aerobically have simple vocal folds that can open wide to offer an unobstructed air passage but which consequently cannot be effective phonators. The giraffe’s vocal folds are so poorly developed that the animal was long thought to be mute. In actuality, giraffes can phonate to a limited extent; they groan when injured and call their young when they stray. The more highly developed vocal folds of primates enhance phonation, but at the expense of a more constricted airway.

Vocalization is an important aspect of mammal communication. When the wild dogs of India (dholes) hunt, the leader coordinates the pack’s motions with a series of sharp yelps. The black-tailed prairie dog combines a visual and vocal display consisting of jumping into the air with its nose straight up while emitting an abrupt two-part vocalization. This display indicates that some behavior is about to be interrupted or prevented by fleeing, which usually occurs immediately thereafter. The display is only employed when an alternative to flight also exists. Hyenas have no organized social behavior but often cooperate while hunting. Their cries suggest human laughter—a low-pitched, hysterical chuckling that rises to higher tones. The female deer emits a sharp, staccato bark to warn its young when it senses danger, and lions coordinate a hunt by grunting while stalking prey.

Elephants use their trunks for communication by trumpeting, humming, roaring, piping, purring, rumbling, and chirping in Asian elephants. Their vocalizations include an assortment of trumpeting sounds ranging from outright blasts to a low groan that males use to indicate that a jousting session is finished. Elephant screams range from expressions of social agitation to the pulsating bellow emitted by a female pursued by an unwanted suitor. Babies scream when they want milk, and the scream gets progressively louder until their hunger is satisfied.

Elephants also communicate with infrasonic frequencies (below the range of human hearing), which humans detect as an air pulsation accompanied by low-frequency rumblings. Rumbles constitute most elephant vocalizations and explain the uncanny ability of widely separated groups to coordinate their activities. There are rumbles of reassurance, rumbles to say, “Let’s go,” rumbles to maintain contact, rumbles to cry, “I’m lost,” courtship and mating rumbles, and a humming rumble produced by mothers for newborn calves. Rumbles also coordinate activities within a given group when preparing to fight for dominance with another group, and mothers use a special rumble to reassemble the younger members of their families. About fifteen of the known rumbles have an infrasonic component, which enables elephants to maintain contact over long distances. Because low frequencies dissipate less rapidly in air, they can travel up to 10 kilometers (6.2 miles). Elephants also emit infrasound to alert others to listen carefully for faint, higher-frequency sounds containing more detailed information.

Research indicates that elephants have a more complex and diverse vocal communication system than previously believed. Their calls were long known to transfer information concerning emotions, relationships, and intentions between large elephant groups. However, scientists assumed these vocalizations to be context-dependent and behavior-related. However, further research asserts that while context plays a role in elephant communication, they also understand one another and communicate without context clues. A four-year study of elephants in Kenya found that these animals use between 469 and 470 distinct calls. These include unique vocalizations for the animal making the initial call and a unique noise from the receiving animal in response. Using the machine learning algorithm called Random Forest to analyze over one hundred elephants’ calls, data revealed that elephants use specific vocal labels for one another the same way humans use names. Additionally, this communication did not appear to rely on imitation, which is observed in parrots and dolphins.

Highly territorial mammals, such as lions, coyotes, and wolves, vocalize extensively at dawn and dusk to establish and maintain territory. Some species vocalize to attract mates and to intimidate rivals. Male moose give hoarse, bellowing cries during mating season to locate cows; the cows respond with a softer, somewhat longer lowing sound. To collect as large a harem as possible, the male elk challenges competitors by emitting a buglelike sound. This vibrant call begins in the low register, ascends to a high pitch, and then abruptly drops into a scream. Bull seals, arriving at breeding grounds before the females, attempt to obtain as many cows as possible for their harems by frightening away competitors with loud roaring.

Bats and dolphins utilize high-frequency ultrasonic vocalization, or echolocation, as animal sonar to find prey and navigate in the dark. High frequencies are desirable for locating small targets, as high-frequency waves are more directional, and a wave cannot “see” an object smaller than its wavelength.

Primates

Primates communicate by various vocal sounds as well as by facial expressions. Apes and monkeys use growls, grunts, twitterings, chirpings, whispers, barks, screams, and cries to warn of danger, indicate alarm or distress, and keep the members of a clan together. Velvet monkeys are known to have three alarm calls: One warns of eagles, one of snakes, and one of leopards. Howling monkeys emit loud, disconcerting, barking roars. The sound is produced by air passing through a resonating chamber in the throat. Rival groups fighting over territory engage in a duel of roaring until one group retreats. While roaring, all other activity, such as feeding, playing, or exploring, comes to a halt. A female howler may also wail in distress when one of her young falls from a tree, while the youngster emits diminutive cries to indicate its position.

Primates lower the natural resonant frequencies of their vocal tracts when faced with danger to project the sonic aura of a larger animal. Apes and monkeys achieve this by protruding and partly closing their lips to generate low-pitched, aggressive sounds.

Among the great apes, gibbons are the most vociferous. Their raucous cries, especially boisterous at sunrise and sunset, can carry more than a mile. Apes only vocalize to express an emotional state. The ability to produce vocal sounds not linked to instinct or emotion is the primary difference between human speech and ape calls. The changing sound patterns of human speech represent abstract concepts, while apes produce simple melodies tied to their mood. The seat of human language is the cerebral cortex, while ape vocalizations are controlled by the subcortical neural structures involved in emotion. Emotionally based human vocalizations, such as sobbing, crying, laughing, giggling, or shouting in pain, are also controlled subcortically.

Principal Terms

Larynx: The vocal mechanism of mammals, consisting of a structure of cartilage at the upper end of the trachea containing the vocal folds

Pharynx: Lower part of the vocal tract, connecting the mouth and nasal cavities to the larynx

Syrinx: The vocal mechanism of birds, consisting of one or more membranous structures at the lower end of the trachea, where the windpipe divides into two bronchial tubes leading to the lungs; the membranes vibrate due to pressure differences when air streams across their surfaces

Trachea: A cartilaginous tube that transports air from the lungs to the pharynx

Vocal Folds: Small, laminated sheets of muscle which meet at the front of the larynx; they are open for breathing and are brought together to vibrate for voiced sounds

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