Our understanding of the functioning of a species’ vocal repertoire can be greatly improved by investigating acoustic variation and using objective classification schemes based on acoustic structure. Here we used a syntactic approach to investigate the acoustic structure of the gorilla close distance vocalizations (‘close calls’), which remain as yet little understood. We examined 2130 calls of 10 mountain gorillas (Gorilla beringei beringei) from Bwindi Impenetrable National Park, Uganda, and 5 western lowland gorillas (Gorilla gorilla gorilla) from Bai Hokou, Central African Republic. We segmented calls into units using distinct acoustic features and employed model-based cluster analyses to define the repertoire of unit types. We then examined how unit types were combined into calls. Lastly, we compared unit type use between age–sex classes and the two study groups. We found that the gorilla close calls consist of 5 intergraded acoustic unit types which were flexibly but yet non-randomly concatenated into 159 combinations. Our results are in line with previous quantitative acoustic analyses demonstrating a high degree of acoustic variation in a variety of animal vocal repertoires, particularly close distance vocalizations. Our findings add on to (1) the recent argument that the common practice of describing vocal repertoires as either discrete or graded may be of little value as such distinctions may be driven by human perception and non-quantitative descriptions of vocal repertoires, and (2) recent studies indicating that flexibility in close range social calls can come about through combinatorial systems, which previously have been studied primarily in long distance vocalizations. Furthermore, our study highlights differences in the vocal repertoire of western and mountain gorillas, as expected given differences in environment and social behaviour. Our results offer opportunities for further in-depth studies investigating the function of the gorilla close calls, which will contribute to a more comprehensive understanding of ape vocal communication in general.
Acoustic variability and individual distinctiveness in the vocal repertoire of red-capped mangabeys (Cercocebus torquatus). —
J. Comp. Psychol.126:
Mountain gorilla tug-of-war: silverbacks have limited control over reproduction in multimale groups. —
Proc. Natl. Acad. Sci. USA102:
FraleyC.RafteryA.MurphyT.B.ScruccaL. (2012). MClust version 4 for R: normal mixture modelling for model-based clustering classification and density estimation. — Available online at http://cran.r-project.org/web/packages/mclust/index.html.
Respiratory units of motor production and song imitation in the zebra finch. —
Vocal relationships of wild mountain gorillas. — In:
Mountain gorillas. Three decades of research at Karisoke (
RobbinsM.M.SicotteP.StewartK.J. eds). Cambridge University PressLondon p.
Vocalizations and social relationships of wild gorillas: a preliminary analysis. — In:
Current perspectives in primate social dynamics (
TaubD.M.KingF.A. eds). Van Nostrand ReinholdNew York, NY p.
Encoding of situations in the vocal repertoire of piglets (Sus scrofa): a comparison of discrete and graded classifications. —
PloS ONE8: e71841.