Syntax and compositionality in animal communication

Abstract

Syntax has been found in animal communication but only humans appear to have generative, hierarchically structured syntax. How did syntax evolve? I discuss three theories of evolutionary transition from animal to human syntax: computational capacity, structural flexibility and event perception. The computation hypothesis is supported by artificial grammar experiments consistently showing that only humans can learn linear stimulus sequences with an underlying hierarchical structure, a possible by-product of computationally powerful large brains. The structural flexibility hypothesis is supported by evidence of meaning-bearing combinatorial and permutational signal sequences in animals, with sometimes compositional features, but no evidence for generativity or hierarchical structure. Again, animals may be constrained by computational limits in short-term memory but possibly also by limits in articulatory control and social cognition. The event categorization hypothesis, finally, posits that humans are cognitively predisposed to analyse natural events by assigning agency and assessing how agents impact on patients, a propensity that is reflected by the basic syntactic units in all languages. Whether animals perceive natural events in the same way is largely unknown, although event perception may provide the cognitive grounding for syntax evolution. This article is part of the theme issue 'What can animal communication teach us about human language?'

Keywords: grammar; language evolution; meaning; permutation; primate communication; semantics.

Figure 1.

Tree structure of a complex English expression conveying a sequence of three social events involving people interacting with a linguist, first as a patient (of a consultation), then as an agent (responding to consultation, showing lack of interest). A, adjective; AP, adjective phrase; C, conjunction; CP, conjunction phrase; D, determiner; I, inflection-bearing element; IP, inflectional phrase; N, (pro)noun; NP, noun phrase; S, sentence; V, verb; VP, verb phrase. Reproduced with permission from Townsend et al. [8] under the Creative Commons Attribution license. (Online version in colour.)

Figure 2.

Suffixation in Campbell's monkey alarm calls: (a) krak alarm calls (given to leopards) have a descending frequency transition (large dashed red arrow) and can take on an optional ‘oo’ suffix (dashed red oval line) to form krak-oo (b); (c) hok alarm calls (given to crowned eagles) have no frequency transition (small dashed red arrow) and can take on an optional ‘oo’ suffix (dashed red oval line) to form hok-oo (d). Adapted with permission from Ouattara et al. [30] under the Creative Commons Attribution license. (Online version in colour.)

Figure 3.

Spectrogram and information content of chimpanzee pant-hoot calls. The top panel refers to the location within the expression where different types of information were most strongly encoded. The bottom panel depicts a spectrogram of a typical chimpanzee pant-hoot, featuring all four call units. Reproduced with permission from Fedurek et al. [35] under the Creative Commons Attribution license. (Online version in colour.)

Figure 4.

Proportion of time the listener spent looking upwards depending on the experimental condition. The figure shows raw data (one line per individual), model estimates (black circles) and bootstrapped model estimates (coloured circles, 1000 bootstraps). Subjects looked more upwards when they were tested with sequences elicited by an aerial predator or by a predator in the canopy. Reproduced with permission from Berthet et al. [44]. (Online version in colour.)

Figure 5.

Assessments of meaning of calls given by bonobos when encountering foods of differently perceived qualities. Reproduced with permission from Clay and Zuberbühler [45] under the Creative Commons Attribution license.