Animal song

Functions of vocalizations

Vocalizations can play a wide variety of different roles. In groups such as anurans and birds, several distinct types of notes are incorporated to form songs, which are sung in different situations and serve distinct functions. For example, many frogs may use trilling notes in mate attraction, but switch to different vocal patterns in aggressive territorial displays. In some species, a single song incorporates several note types which serve different purposes, with one type of note eliciting responses from females, and another note of the same song responsible for warning competitor males of aggression.

Mating and courtship

Vocalizations play an important role in the mating behaviour of many animals. In many groups (birds, frogs, crickets, whales, etc.), song production is more common in males of the species, and is often used to attract females.

Bird song is thought to have evolved through sexual selection. Female songbirds often assess potential mates using song, based on qualities such as high song output, complexity and difficulty of songs, as well as presence of local dialect. Song output serves as a fitness indicator of males, since vocalizations require both energy and time to produce, and thus males capable of producing high song output for long durations may have higher fitness than less vocal males. It is thought that song complexity may serve as an indicator of male fitness by providing an indication of successful brain development despite potential early-life stressors, such as lack of food. Social transmission of songs allows for development of local dialects of song, and female songbirds also typically prefer to choose mates producing local song dialects. One hypothesis for this phenomenon is that selecting local mates allows the female to choose genes specially adapted to suit local conditions.

Frog song also plays a prominent role in courtship. In túngara frogs (Engystomops pustulosus), male frogs increase the complexity of their calls, adding additional note types when greater numbers of competitor males are present, which has been found to attract greater numbers of female frogs. Some species change their courtship calls when females are especially nearby. In male glass frogs (Hyalinobatachium fleichmanni), a long frequency-modulated vocalization is produced upon noticing another nearby frog, but is changed to a short chirping song when a female approaches. Several species (e.g., dendrobatid frogs (Mannophryne trinitatis), ornate frogs (Cophixalus ornatus), splendid poison frogs (Dendrobates speciosus)), switch from long-range loud trilling sounds to short-range quieter chirps when females move closer, which is thought to allow mate attraction without alerting competitor males to female locations.

Although highly complex song-like production has been identified in whales, the function is still somewhat elusive. It is thought to be involved in courtship behaviour and sexual selection, and singing behaviour becomes more common during the breeding season.

Aggression and territorial defense

Another major function of song output is to indicate aggression among males during breeding seasons. Both anurans and birds use singing in territorial displays to confer aggressive intent. For eastern smooth frogs (Geocrinia victoriana), for example, courtship songs involve shorter notes to attract potential mates, and are followed by longer tones to repel males. Frequency of sounds produced generally negatively correlates with body size both within and among species, and allows competing males to assess body size of vocalizing neighbouring frogs. Male frogs typically approach higher frequency sounds more readily than lower frequencies, likely because the frog producing the sound is assessed to be a smaller, less dangerous competitor.

In territorial birds, males increase song production rate when neighbouring males encroach on their territory. In great tits (Parus major), nightingales (Luscinia megarhynchos), blackbirds (Turdus merula) and sparrows (family Passeridae), playing song recordings slows the rate at which males establish territories in an unoccupied region, suggesting these birds rely on song output in establishing territorial boundaries. Experimentally muted Scott's seaside sparrow (Ammodramus maritimus) lose control of their territories to other males. Thus, territorial birds often rely on song production to repel conspecific males.

Individual recognition

Like the human voice, bird song typically contains sufficient individual variability to allow discrimination of individual vocal patterns by conspecifics. Such discrimination is important to mate recognition of many monogamous species. Seabirds, for example, often use vocalization patterns to recognize their mate upon reunion during the breeding season. In many colonial nesting birds, parent-offspring recognition is critical to allow parents to locate their own offspring upon return to nesting sites. Cliff swallows (Petrochelidon pyrrhonota) have been demonstrated to preferentially respond to parental songs at a young age, providing a means of vocalization-based offspring recognition.

Social transmission and learning

Learning and development of birdsong

Learned vocalizations have been identified in groups including whales, elephants, seals, and primates, however the most well-established examples of learned singing is in birds. In many species, young birds learn songs from adult males of the same species, typically fathers. This was first demonstrated in chaffinches (Fringilla coelabs). Chaffinches raised in social isolation develop abnormal songs, however playing recordings of chaffinch songs allows the young birds to learn their species-specific songs. Song learning generally involves a sensitive learning period in early life, during which young birds must be exposed to song from tutor animals in order to develop normal singing as adults. Song learning occurs in two stages: the sensory phase and the sensorimotor phase. During the sensory phase, birds memorize the song of a tutor animal, forming a template representation of the species-specific song. The sensorimotor phase follows and may overlap with the sensory phase. During the sensorimotor phase, young birds initially produce variable, rambling versions of adult song, called subsong. As learning progresses, the subsong is replaced with a more refined version containing elements of adult song, called plastic song. Finally, the song learning crystallizes into adult song. For song learning to occur properly, young birds must be able to hear and refine their vocal productions, and birds deafened before the development of subsong do not learn to produce normal adult song.

The sensitive period in which birds must be exposed to song tutoring varies across species, but typically occurs within the first year of life. Birds in which song learning is limited to the initial sensitive period are referred to as closed-ended learners, whereas some birds (e.g., canaries; Serinus canaria), continue to learn new songs later in life, and are called open-ended learners. Some species of birds, such as the brown-headed cowbird (Molothrus ater), parasitize other bird species, laying their eggs in the nests of other birds such that the heterospecific bird raises the chicks. Although most birds acquire song learning within the first year, brown-headed cowbirds have a delayed sensitive period, occurring approximately one year after hatching. This may be an adaptation to prevent the young birds from learning the songs from the foreign bird species. Instead, the young birds have a year in which to find conspecifics, and learn their own species-specific song.

Birds are generally predisposed to favour learning of conspecific songs, and will typically preferentially learn the song form conspecific animals rather than heterospecifics. However, song learning is not completely restricted to within-species songs. If exposed to heterospecific birds of another species in absence of same-species birds, young birds will often adopt the song of the species to which it was exposed. Although birds are capable of learning song production purely from audio recordings of birdsong, tutor-student interaction may be important in some species. For example, white-crowned sparrows (Zonotrichia leucophrys) preferentially learn the songs of song sparrows (Melospiza melodia) when exposed to recordings of white-crowned sparrows and live song sparrows. In other words, the interactive nature of a live tutor seems to trump the familiarity of the recordings from conspecifics.

Cultural transmission of whale song

While vertical transmission (parent-offspring) is a common element of song learning, horizontal transmission among animals of the same generation can also occur. Male humpback whales produce various songs over their lifetime, which are learned from other males in the population. Males in a population conform to produce the same mating song, consisting of a highly stereotyped vocal display involved in mate attraction. The cultural transmission of these songs has been found to occur across great geographic distances over years, with one study noting song transmission across the western and central South Pacific Ocean populations over an 11-year period.