Towards a new taxonomy of primate vocal production learning

Abstract

The extent to which vocal learning can be found in nonhuman primates is key to reconstructing the evolution of speech. Regarding the adjustment of vocal output in relation to auditory experience (vocal production learning in the narrow sense), effects on the ontogenetic trajectory of vocal development as well as adjustment to group-specific call features have been found. Yet, a comparison of the vocalizations of different primate genera revealed striking similarities in the structure of calls and repertoires in different species of the same genus, indicating that the structure of nonhuman primate vocalizations is highly conserved. Thus, modifications in relation to experience only appear to be possible within relatively tight species-specific constraints. By contrast, comprehension learning may be extremely rapid and open-ended. In conjunction, these findings corroborate the idea of an ancestral independence of vocal production and auditory comprehension learning. To overcome the futile debate about whether or not vocal production learning can be found in nonhuman primates, we suggest putting the focus on the different mechanisms that may mediate the adjustment of vocal output in response to experience; these mechanisms may include auditory facilitation and learning from success. This article is part of the theme issue 'What can animal communication teach us about human language?'

Keywords: Chlorocebus; Papio; alarm calls; learning; speech evolution; vocal production.

Figure 1.

Spectrograms of rhesus macaque coo calls. Recordings were obtained from two subjects at the age of one to two weeks, three to four weeks, and four to five months, respectively.

Figure 2.

Spectrograms of baboon loud calls from Guinea baboons (Papio papio), olive baboons (Papio anubis) and chacma baboons (Papio ursinus). (a) Females and (b) males. (c) Distribution of different baboon species on the African continent. Baboon drawings by Steven Nash. (Online version in colour.)

Figure 3.

Spectrograms of West African green monkey and East African vervet monkey alarm calls, in response to different predator/threat types. (a) Female green monkey calls; (b) male green monkey calls; (c) female vervet monkey calls; and (d) male vervet monkey calls.

Figure 4.

Acoustic differences of Chlorocebus alarm calls in relation to context and species. Boxplots and individual values for female and male vervet monkey (C.p.) and green monkey (C.s.) alarm calls (blue: aer, aerial; orange: leo, leopard; green: snk, snake). (a) Element duration, (b) mean of the central frequency (DFA2), (c) mean dominant frequency (DF1) and (d) mean frequency range. Boxplots indicate median and interquartile range. Whiskers show values within 1.5 times of the interquartile range. Dots represent individual values. Reprinted with permission from [53].

Figure 5.

Heat maps reflecting the acoustic similarity of West African green monkey (GM) and East African vervet monkey alarm calls (V). (a) Males and (b) females. aer, aerial alarms; snk, snake alarms; leo, leopard alarms.