Primate vocal production and the riddle of language evolution

Abstract

Trying to uncover the roots of human speech and language has been the premier motivation to study the signalling behaviour of nonhuman primates for several decades. Focussing on the question of whether we find evidence for linguistic reference in the production of nonhuman primate vocalizations, I will first discuss how the criteria used to diagnose referential signalling have changed over time, and will then turn to the paradigmatic case of semantic communication in animals, the alarm calls of vervet monkeys, Chlorocebus pygerythrus. A recent in-depth analysis of the original material revealed that, while the alarm calls could be well distinguished, calls of similar structure were also used in within- and between-group aggression. This finding is difficult to reconcile with the idea that calls denote objects in the environment. Furthermore, nonhuman primates show only minimal signs of vocal production learning, one key prerequisite for conventionalized and symbolic communication. In addition, the structure of calls in different populations or closely related species is highly conserved. In conclusion, any continuity between nonhuman primate and human communication appears to be found at the level of the processing of signals. Why and how the ancestors of our own species one day began to talk to each other continues to be an enigma. Future research should focus on changes in the neural structure supporting volitional control over vocalizations, the gene networks associated with vocal production, and the developmental processes involved in the integration of production and perception of vocalizations.

Keywords: Alarm calls; Primate communication; Referential signalling; Speech evolution.

Fig. 1

Structure and discriminability of female vervet monkey, Chlorocebus pygerythrus, vocalizations given in alarm and aggressive contexts. (a) Scatter plot of the discriminant scores with corresponding spectrograms of female alarm calls given in response to leopards, eagles, and snakes. (b) Scatter plot of the discriminant scores with corresponding spectrograms of female alarm calls given in response to leopards, eagles, and snakes, as well as during within- and between-group aggression. DF discriminant function. Modified from Price et al.,

Fig. 2

Structure and discriminability of male vervet vocalizations given in alarm and aggressive contexts. (a) Scatter plot of the discriminant scores with corresponding spectrograms of East African male alarm calls given in response to leopards, eagles, and snakes. (b) Frequency distribution of the discriminant scores with corresponding spectrograms of South African male vocalizations given in response to leopards and during within- and between-group aggression. DF discriminant function. Modified from Price et al.,

Fig. 3

Scatterplot and spectrograms illustrating population differences in the acoustic structure of C. sabaeus, C. p.hilgerti, and C. p. pygerythrus barks. The scatterplot presents the distribution of the first and second LDA discriminant scores. Spectrograms illustrate a typical call exemplar for each call group, with typical calls defined as those that were most likely to be assigned by LDA to the correct caller/population. From Price et al.,