Synonyms
Definition
Stereotyped vocalization: Evolved, species-typical vocal act with signal character.
Introduction
How did human communication evolve from an animal-like vocal system to spoken language? Research on primate communication in particular has produced a wealth of data, which highlight the various evolutionary continuities and discontinuities between animal and human communication. Research topics include the mechanisms and flexibility of sound production, how different call units are assembled into complex utterances, how meaning is extracted from vocal structures, and how the psychological mechanisms underlying animal call production and comprehension compare to those of humans. The purpose of this chapter is to present the state-of-the-art of these various strands of research and to show how methodological tools of linguistics can help decipher animal communication.
Animal Vocal Behavior
Vocal Control
Mammalian vocalizations are produced by a larynx that oscillates in response to airflow from the lungs. This creates a basic acoustic signal further shaped by the supra-laryngeal vocal tract. Many species can actively change the geometry of their vocal tracts, which determines the call’s resonance properties and acoustic quality. Within the primates, humans are somewhat special in that the larynx is in a permanently low position, which results in a characteristic perpendicular two-tube vocal tract. There has been much debate as to whether this anatomical specialization is crucial for speech production, but current opinion suggests that this is not essential. Humans are also unusual in their high degree of motor control of both larynx and vocal tract, mostly due to specialized cortical innervations (Ackermann et al. 2014). Great apes also have relatively good motor control over the facial musculature, including some of the speech articulators, suggesting that the transition to speech was largely due to control of the larynx (Lameira et al. 2014). One consequence of the lack of voluntary control over sound production is that nonhuman primates are essentially prevented from acquiring new sound patterns by vocal learning. As a result, each species communicates by means of species-specific vocal repertoires that develop under relatively strong genetic control.
Acoustic Flexibility in Animal Calls
Although the differences to humans are vast, primates exhibit limited vocal flexibility in some call types. At the most basic level, many call types carry individual signatures, which enable receivers to extract identity information. For example, chimpanzees (Pan troglodytes) recognize each other by their pant hoot vocalizations, a long-distance signal, and discriminate the calls of neighboring males from the calls of unknown stranger males (Herbinger et al. 2009). Another interesting example is found in bottle-nosed dolphins (Tursiops truncatus) where individuals match each other’s individual signature whistles, although they are not produced by the larynx (King and Janik 2013). Although there is widespread evidence for individually distinct calls, not all call types convey identity cues, Gelada baboons (Theropithecus gelada) being a striking example (Bergman 2010). In several primates, there is also evidence for acoustic convergence, usually within the contact calls. The general pattern is that calls of closely affiliated individuals are more similar to each other than calls of less affiliated group members. Another source of acoustic flexibility is found in calls given to external events as, for example, in Japanese macaque (Macaca fuscata) and chimpanzee food calls, Rhesus macaque (Macaca mulatta) and chimpanzee agonistic calls, or baboon (Papio ursinus) alarm barks (Zuberbühler 2015). A related but somewhat more complex way of creating acoustic variation is by assembling discrete acoustic units within the same calls. For example, male Campbell’s monkeys (Cercopithecus campbelli) combine three basic alarm calls with an acoustically invariable vocal suffix. Suffixed calls are typically given to situations that do not warrant direct or strong antipredator responses, while unsuffixed calls are almost always given to dangerous predators, such as leopards or crowned eagles. Diana monkeys (Cercopithecus diana), a frequent association partner of Campbell’s monkeys, discriminate this subtle feature, suggesting that suffixation is an evolved function in primate communication. Another relevant example is Diana monkey contact calls, which consist of individually distinct arched structures that convey identity information, which are regularly combined with three other call types linked with specific external events (Zuberbühler 2015).
Animal Call Sequences
In a number of species, semantically relevant units can be at the level of call sequences, rather than individual calls (Kershenbaum et al. 2016). A recent example is the vocal system of pied babblers (Turdoides bicolor), a social passerine that combines alert and recruitment calls into sequences when encountering terrestrial predators. Recipients respond differently to sequences than components calls, and it has been argued, controversially, that these birds show compositionality, as they communicate both context and requested action (Engesser et al. 2016; but see Schlenker et al. 2016). Related examples in primates are the call sequences of black-and-white Colobus monkeys (Colobus guereza), with sequence length and structure relating to predator type. In putty-nosed monkeys (Cercopithecus nictitans) call combinations encode both predator class and travel intention and in Campbell’s monkeys predatory and non-predatory dangers (Schlenker et al. 2016). For apes, gibbon duet songs broadcast social information relevant to neighboring individuals but lar gibbons (Hylobates lar) also sing when encountering predators. Predator-induced songs and duet songs are assembled from the same unit repertoire but according to different patterns. Another example is bonobos producing sequences of acoustically variable calls when finding food, which are linked to the value of the food, something that listeners use to base their foraging decisions (Zuberbühler 2015).
Meaning in Animal Calls
There is overwhelming evidence that primates can extract information from others’ calls, sometimes by taking into account the pragmatic context or reason for calling. The basic pattern is that specific events tend to trigger specific vocal responses, which are recognized and interpreted by listeners, a sort of by-product semantics. Hence, one hypothesis is that there is a profound discrepancy between signalers (uttering vocalizations for specific functions, e.g., to recruit a nearby ally), and recipients, who can infer the call causing events, even if not participating in the event. According to this model, nonhuman primate communication is profoundly different from human language, where speakers are interlinked at a deep psychological level. Humans communicate, verbally or nonverbally, to be understood and have repair mechanisms in case of failure. Recipients, on the other hand, assume that signalers intend to communicate something meaningful and use common ground to infer meaning, beyond signal-event contingencies and direct linguistic meaning. An important problem in current research is therefore the degree to which primates communicate “intentionally,” i.e., with the goal of informing about events relevant to their listeners and whether they experience an intention to be understood.
Recent research on call meaning has focused on three main questions: (i) what is the referential content or meaning of individual calls? (ii) How are the meanings of individual calls combined? (iii) Is the meaning of calls sometimes enriched by “pragmatic” principles of competition, notably an “informativity principle” whereby a more informative call is preferred whenever possible over a less informative one?
Campbell’s monkeys afford a rich case study (Schlenker et al. 2016). Males use a call krak to raise leopard alerts and hok for raptor alerts, as well as suffixed krak-oo for alerts of all sorts and hok-oo for non-ground alerts. The challenge is to assign meanings to krak, hok, and -oo. Further complexity is added by Campbell’s call use in another habitat, Tiwai Island, Sierra Leone, where leopards have not been seen for decades. Although both monkey populations produce the same call types, there are relevant differences in call use. On Tiwai, krak raises unspecific alerts (as does krak-oo), rather than specific leopard alerts, possibly a case of limited “dialectal” variation: krak has a leopard meaning in Tai and a general meaning on Tiwai. On an alternative theory, the “informativity principle” plays a key role: hok has a meaning of aerial alert, and -oo weakens the resulting alert, hence hok-oo is used for nonserious aerial alerts. Hok is further enriched by competition with the (more informative) call hok-oo, which explains why hok comes to be used for serious aerial alerts – hence raptors. Krak has a meaning of (general) alert at both sites, hence krak-oo is used for nonserious alerts. By the “informativity principle,” the meaning of krak is further enriched by competition with krak-oo (“there is a nonserious alert”) and hok (“there is an aerial alert”). In the end, krak can only be used for serious (not krak-oo) ground (not hok) disturbances – hence the leopard uses in Tai. On Tiwai, the “informativity principle” would yield a useless meaning due to the absence of serious ground predators, hence krak retains its basic meaning.
The debate on the meaning of Campbell’s calls is still open, but several species might provide independent evidence for the “informativity principle.” Often calls are used in highly heterogeneous contexts and seem to have a general alert meaning; but in the event of a specific threat for which there is a precise call the more informative call trumps the general one.
Concerning call combination, a key question is whether call sequences are “compositional,” which is the case if their meaning can be systematically derived from that of their parts. As mentioned, in black-and-white Colobus monkeys, putty-nosed monkeys, and titi monkeys, some sequences seem to be interpreted “wholesale” and thus to present challenges to compositionality. Three general research directions are currently explored (Schlenker et al. 2016): (i) some call sequences might indeed be irreducible to the meaning of their component parts (possibly in Colobus and putty-nosed monkeys); (ii) sometimes a compositional analysis can be maintained if call meanings are taken to be very weak, and to be enriched by some competition principles; (iii) sometimes the complexity of the call sequence might reflect the caller’s changing perceptions as the events unfold.
Intentionality in Animal Communication
In great apes, there is evidence that signalers are aware of the social consequences of some of their signals, but there is no strong evidence that they also actively inform others or take their mental states into account during call production. Signal production may be based on simple social categories, such as dominance or affiliation, rather than the shared history with individual recipients and their momentary mental states, such as knowledge or ignorance. Although primates can perceive others as goal-directed agents, they may not consider their mental states, such as beliefs or knowledge, when addressing them. Related to this, primates are not generally motivated to use communication in a cooperative way to inform others about facts or events relevant to them. Noteworthy exceptions are studies on predator encounters where signalers, especially adult males, appear to be concerned about the wellbeing of others (Zuberbühler 2015). Other forms of seemingly altruistic signaling, such as when encountering food, can be explained by callers trying to avoid negative consequences if failing to advertise the event.
Conclusion
Many social animals, including humans, possess a repertoire of species-specific vocalizations, with different calls produced to more or less specific events. However, humans have evolved an additional layer of vocal control, characterized by highly coordinated movements of the jaws, lips, and tongue in union with controlled laryngeal sound production. Nevertheless, recent work on nonhuman primate calls has shown some acoustic flexibility within individual calls and also at the level of call sequences.
The default model is that animal signals serve evolved biological and social functions, e.g., to dissuade predators, to recruit others, or to induce competition for copulation calls, but these contingencies are readily absorbed and actively interpreted by listeners, a sort of by-product semantics. Several studies have further shown that receivers attend to subtle acoustic variation within some call types in ways that suggest that they are meaningful to them. Progress has also been made by applying linguistic theory to the study of primate call sequences, in particular to assess the respective roles of call meaning, principles of call combination, and potential rules of competition among calls.
Another major theme in animal communication is whether nonhuman individuals produce signals with an intention to inform others. Here, the evidence is relatively weak, with only a handful studies suggesting that callers monitor whether others have perceived or understood their signals or show concern to provide information that is relevant for their recipients. Studies on ape gestural communication, however, suggest that basic mental state attribution is within the cognitive realm of primates and can play a role in communication.
To conclude, although differences are vast, human vocal communication, including spoken language, is the result of evolutionary continuity with relevant precursors seen in nonhuman primates in almost every relevant capacity.
Cross-References
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Zuberbühler, K., Chemla, E., Schlenker, P. (2021). Stereotyped Vocalizations. In: Shackelford, T.K., Weekes-Shackelford, V.A. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-19650-3_3330
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