Collective close calling mediates group cohesion in foraging meerkats via spatially determined differences in call rates

During group movements, many socially living and group-foraging animals produce contact calls. Contact calls typically function to coordinate and maintain cohesion among group members by providing receivers with information on the callers' location or movement-related motivation. Previous work suggests that meerkats, Suricata suricatta, also produce short-range contact calls, so-called ‘close calls’, while foraging to maintain group cohesion. Yet, the underlying mechanism of how meerkats coordinate cohesion via close calling is unclear. Using a combination of field observations and playback experiments we here show that foraging meerkats adjusted the call rates of their continuously produced close calls depending on their spatial position to group members. Specifically, meerkats called at higher rates when foraging at a closer distance to and when surrounded by conspecifics; however, the number of calling individuals or their call rates did not affect a subject's close call rate. Overall, close call playbacks elicited a call response in receivers and attracted them to the sound source. Our results suggest that differences in individual close call rates are determined by a meerkat's proximity to other group members, being assessed through their vocal interactions. We discuss how local differences in individual call rates may extrapolate to the group level, where emerging ‘vocal hotspots’ indicate areas of high individual density, in turn attracting and potentially guiding group members' movements. Hence, the described pattern illustrates a so far undocumented call mechanism where local differences in the call rates of continuously produced close calls can generate a group level pattern that mediates the cohesion of progressively moving animal groups

Meaningful call combinations and compositional processing in the Southern Pied Babbler

Language’s expressive power is largely attributable to its compositionality: meaningful words are combined into larger/higher-order structures with derived meaning. Despite its importance, little is known regarding the evolutionary origins and emergence of this syntactic ability. Whilst previous research has demonstrated a rudimentary capability to combine meaningful calls in primates, due to a scarcity of comparative data, it is unclear whether analogue forms might also exist outside of primates. Here we address this ambiguity and provide evidence for rudimentary compositionality in the discrete vocal system of a social passerine, the pied babbler (Turdoides bicolor). Natural observations and predator presentations revealed babblers produce acoustically distinct alert calls in response to close, low-urgency threats, and recruitment calls when recruiting group members during locomotion. Upon encountering terrestrial predators both vocalisations are combined into a ‘mobbing-sequence’, potentially to recruit group members in a dangerous situation. To investigate whether babblers process the sequence in a compositional way, we conducted systematic experiments, playing back the individual calls in isolation, as well as naturally occurring and artificial sequences. Babblers reacted most strongly to mobbing-sequence playbacks, showing a greater attentiveness and a quicker approach to the loudspeaker, compared to individual calls or control sequences. We conclude the sequence constitutes a compositional structure, communicating information on both the context and the requested action. Our work supports previous research suggesting combinatoriality as a viable mechanism to increase communicative output, and indicates that the ability to combine and process meaningful vocal structures, a basic syntax, may be more widespread than previously thought

Chestnut-crowned babbler calls are composed of meaningless shared building blocks

A core component of human language is its combinatorial sound system: meaningful signals are built from different combinations of meaningless sounds. Investigating whether non-human communication systems are also combinatorial is hampered by difficulties in identifying the extent to which vocalizations are constructed from shared, meaningless building blocks. Here we present a novel approach to circumvent this difficulty and show that a pair of functionally distinct chestnut-crowned babbler (Pomatostomus ruficeps) vocalizations can be decomposed into perceptibly distinct, meaningless entities that are shared across the two calls. Specifically, by focusing on the acoustic distinctiveness of sound elements using a habituation-discrimination paradigm on wild-caught babblers under standardized aviary conditions, we show that two multi-element calls are composed of perceptibly distinct sounds that are reused in different arrangements across the two calls. Furthermore, and critically, we show that none of the five constituent elements elicits functionally relevant responses in receivers, indicating that the constituent sounds do not carry the meaning of the call; so are contextually meaningless. Our work, which allows combinatorial systems in animals to be more easily identified, suggests that animals can produce functionally distinct calls that are built in a way superficially reminiscent of the way that humans produce morphemes and words. The results reported lend credence to the recent idea that language’s combinatorial system may have been preceded by a superficial stage where signalers neither needed to be cognitively aware of the combinatorial strategy in place, nor of its building blocks

Experimental evidence for phonemic contrasts in a nonhuman vocal system

The ability to generate new meaning by rearranging combinations of meaningless sounds is a fundamental component of language. Although animal vocalizations often comprise combinations of meaningless acoustic elements, evidence that rearranging such combinations generates functionally distinct meaning is lacking. Here, we provide evidence for this basic ability in calls of the chestnut-crowned babbler (Pomatostomus ruficeps), a highly cooperative bird of the Australian arid zone. Using acoustic analyses, natural observations, and a series of controlled playback experiments, we demonstrate that this species uses the same acoustic elements (A and B) in different arrangements (AB or BAB) to create two functionally distinct vocalizations. Specifically, the addition or omission of a contextually meaningless acoustic element at a single position generates a phoneme-like contrast that is sufficient to distinguish the meaning between the two calls. Our results indicate that the capacity to rearrange meaningless sounds in order to create new signals occurs outside of humans. We suggest that phonemic contrasts represent a rudimentary form of phoneme structure and a potential early step towards the generative phonemic system of human language

Compositionality in animals and humans

Funding: University of Zurich Research Priority Program (grant number URPP, Evolution in Action; URPP U-702-06). Received by SWT and BB. Swiss National Science Foundation (grant number SWT: PP003_163860; SE: PP003_163860; P1ZHP3_151648; KZ grant: 31003A_166458). Received by SWT, SE and KZ. European Research Council under the European Union’s 7th Framework Programme (grant number FP7/2007-2013/ ERC grant agreement no (615988)). Received by SS.A key step in understanding the evolution of human language involves unravelling the origins of language’s syntactic structure. One approach seeks to reduce the core of syntax in humans to a single principle of recursive combination, merge, for which there is no evidence in other species. We argue for an alternative approach. We review evidence that beneath the staggering complexity of human syntax, there is an extensive layer of nonproductive, nonhierarchical syntax that can be fruitfully compared to animal call combinations. This is the essential groundwork that must be explored and integrated before we can elucidate, with sufficient precision, what exactly made it possible for human language to explode its syntactic capacity, transitioning from simple nonproductive combinations to the unrivalled complexity that we now have.Publisher PDFPeer reviewe

Syntax-like structures in maternal contact calls of Chestnut-Crowned Babblers (Pomatostomus ruficeps)

The combination of meaning-bearing units (e.g., words) into higher-order structures (e.g., compound words and phrases) is integral to human language. Despite this central role of syntax in language, little is known about its evolutionary progression. Comparative data using animal communication systems offer potential insights, but only a handful of species have been identified to combine meaningful calls together into larger signals. We investigated a candidate for syntax-like structure in the highly social chestnut-crowned babbler (Pomatostomus ruficeps). Using a combination of behavioral observations, acoustic analyses, and playback experiments, we test whether the form and function of maternal contact calls is modified by combining the core “piping” elements of such calls with at least one other call element or call. Results from the acoustic analyses (236 analysed calls from 10 individuals) suggested that piping call elements can be flexibly initiated with either “peow” elements from middle-distance contact calls or adult “begging” calls to form “peow-pipe” and “beg-pipe” calls. Behavioral responses to playbacks (20 trials to 7 groups) of natural peow-pipe and beg-pipe calls were comparable to those of artificially generated versions of each call using peow elements and begging calls from other contexts. Furthermore, responses to playbacks (34 trials to 7 groups) of the three forms of maternal contact calls (piping alone, peow-pipe, beg-pipe) differed. Together these data suggest that meaning encoded in piping calls is modified by combining such calls with begging calls or peow elements used in other contexts and so provide rare empirical evidence for syntactic-like structuring in a nonhuman animal

Vocal Combinations in the Southern Pied Babbler (Turdoides bicolor) and the Chestnut-Crowned Babbler (Pomatostomus ruficeps): Implications for the Evolution of Human Language

Language’s expressive power is one of its key characterising features. This generative capacity is achieved through language’s double articulatory nature: meaningless sounds (phonemes) are combined to create meaningful words (phonology/combinatoriality), and words are assembled into higher-order meaningful phrases (syntax/compositionality). Comparative work on non-human animals investigating the evolutionary origin of combinatorial abilities has so far focused on singing species or on primates. Although these studies have shed light on the combinatorial capacities outside of humans, evidence for basic phoneme-like or semantically compositional structures in non-human communication systems is rare. By taking a comparative approach, investigating the prevalence and diversity of combinatoriality within the discrete call system of two highly social passerine birds, this dissertation aimed to unveil selective drivers promoting combinatorial capacities, and provides analogue examples to, and potential precursors of, language’s combinatorial layers. Work on chestnut-crowned babblers (Pomatostomus ruficeps) demonstrates the reuse of two meaningless sounds (A & B) in different arrangements to generate the functionally distinct A

Element repetition rates encode functionally distinct information in pied babbler ‘clucks’ and ‘purrs’

Human language is a recombinant system that achieves its productivity through the combination of a limited set of sounds. Research investigating the evolutionary origin of this generative capacity has generally focused on the capacity of non-human animals to combine different types of discrete sounds to encode new meaning, with less emphasis on meaning-differentiating mechanisms achieved through potentially simpler temporal modifications within a sequence of repeated sounds. Here we show that pied babblers (Turdoides bicolor) generate two functionally distinct vocalisations composed of the same sound type, which can only be distinguished by the number of repeated elements. Specifically, babblers produce extended ‘purrs’, composed of, on average, around 17 element repetitions when drawing young offspring to a food source, and truncated ‘clucks’ composed of a fixed number of 2-3 elements when collectively mediating imminent changes in foraging site. We propose that meaning-differentiating temporal structuring might be a much more widespread combinatorial mechanism than currently recognised, and is likely of particular value for species with limited vocal repertoires in order to increase their communicative output

An information and coding theoretical approach to combinatorial communication

A fundamental characteristic of human language is its combinatorial nature, which facilitates the communication of infinite meanings (i.e., words) built from finite sounds (i.e., phonemes). Investigating the selective drivers of this combinatorial feature represents a major challenge in the field of language evolution and has prospered into a rich interdisciplinary field. Here, we discuss the emergence of combinatorial structures in (human and non-human) communication systems from an information and coding theoretical perspective. We describe how “noise” (i.e., factors constraining communication processes) can hamper and distort signal transmission and perception, and how such noise-induced impairments can be circumvented through a combinatorial coding scheme that adds redundancy to a signalling system, in turn increasing its robustness and enhancing signal detection and discrimination. In doing so we argue that basic combinatoriality has emerged due to universal constraints imposed on signalling systems, and that human language-like productive combinatoriality builds upon this phenomenon

Internal acoustic structuring in pied babbler recruitment cries specifies the form of recruitment

Language is inherently combinatorial, and parallels of this combinatorial capacity are found in nonhuman systems, with animals combining sounds and calls into larger meaningful structures. However, further analogue examples are central in unveiling the diversity, distribution, and evolutionary drivers of combinatoriality. Here, we provide evidence for internal “meaning-refining” acoustic variation within a larger stereotyped signal in pied babblers (Turdoides bicolor). Using acoustic analyses, we demonstrate that males produce 2 long, raucous, “cry-like” structures, both starting with a wind-up segment grading into repetitions of A/single-note or AB/double-note motifs. Behavioral observations indicated that, consistent with similarities in their larger stereotyped structure, both variants function overall in recruiting group members during locomotion, but the internal A or AB substructure specifies the “precise” form of recruitment, from approaching the caller’s announced location to following it over longer distances. Playing back cries from a stationary loudspeaker further supported that the 2 variants elicit different responses, with more individuals approaching the loudspeaker in response to single-note compared with double-note cries. Additionally, despite similarities in overall distance travelled, group movement was only directional for single-note, but undefined for double-note cries. We suggest that the overall structure of the 2 cry variants conveys the same general meaning, with embedded variation refining this meaning. These results further illustrate the variability of generative mechanisms outside of human language and lend support to the hypothesis that combinatorial structuring may have emerged in species with limited or fixed vocal repertoires in order to enhance communicative output