Acoustic structure and variation in mountain and western gorilla close calls: a syntactic approach

in Behaviour
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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 structure and variation in mountain and western gorilla close calls: a syntactic approach

in Behaviour

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Figures

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    Spectrograms of representative gorilla close calls recorded in this study, their subdivision into units and categorization into unit types. Spectrograms a–f illustrate typical examples of syllabled calls: (a–e) double grunts, (f) single grunt. Spectrograms g–k illustrate non-syllabled calls: (g) grumble; (h, i) hums; (j, k) mixed calls. Calls were subdivided into units (indicated by black lines) based on the occurrence of periods of silence of less than 2 s duration (a–e, h, i) or abrupt changes in the distribution of energy (j, k). The units were categorized as atonal or tonal according to the presence/absence of harmonic frequency bands. Indicated above the lines are the unit type each unit was assigned to via cluster analysis: a1, atonal grunts; t1, short hums; t2, short tonal grunts; t3, long hums; t4, grumbles.

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    Operational definitions and schematic overview of analyses used in this study.

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    Spectrograms of the 5 unit types. Short hums (t1) were usually under 1-s short, tonal, high pitched units. Tonal grunts (t2) were short, low pitched grunts. Atonal grunts (a1) were on average short, noisy grunts that, however, could be up to 2 s long. Grumbles (t4) were up to 5 s long and low pitched. Long hums (t3) were long tonal, high pitched units.

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    Combinatorial associations between unit types in both gorilla groups. Lines indicate the average Dice index of combinatorial association as a measure of how frequently any two unit types were observed together in the same call. Average values were calculated from the individuals’ Dice indices for a given pair of unit types. The legend shows the thickness of the lines corresponding to the minimum and maximum average Dice index.

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    Sequential associations between unit types in the blackbacks and adult females of the mountain gorilla group. Lines indicate the average Dice index for sequential association as a measure of how frequently any two unit types followed one another in immediate succession. Average values were calculated from the individuals’ Dice indices for a given pair of unit types. The legend depicts the thickness of the lines corresponding to the minimum and maximum average Dice index. For reasons of clarity illustrations include only transitions with a Dice index > 0.05.

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    Results of Spearman correlations between acoustic parameters measured for (a) atonal and (b) tonal units. (a) Correlations between acoustic parameters measured for atonal units. (b) Correlations between acoustic parameters measured for tonal units.

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    Goodness of fit (using BIC) of different cluster solutions for atonal and tonal units as a function of the number of unit types (clusters). Differently coloured lines indicate different parameterizations of cluster solutions, varying in cluster shape. The best cluster solution is indicated by an arrow. (a) Atonal units; (b) tonal units.

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    Silhouette coefficients of the different cluster solutions for tonal and atonal units as a function of the number of unit types (clusters). A silhouette coefficient < 0.25 indicates strong intergradation, silhouette coefficients between 0.25 to 0.5 weak clustering.

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    Silhouette plots illustrating the most discrete cluster solutions with four unit types (clusters) for atonal units, with an overall silhouette coefficient of 0.15 (left) and with four unit types for tonal units, with an overall silhouette coefficient of 0.48 (right). The plot displays the silhouette widths for each unit as horizontal bars, ordered by clusters and by silhouette value within cluster. The silhouette width is a measure of how clearly a unit belongs to its assigned cluster. Compact clusters consist of units with high silhouette widths. N indicates the number of units allocated to each cluster. S(cn) is the silhouette coefficient of each cluster.

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    Best cluster solutions for 2 to 10 unit types (clusters) of tonal units. Indicated are the values of the principal component for acoustic parameters describing the fundamental frequency on the y-axis and the transformed duration on the x-axis. Different colours and symbols indicate different clusters. For each cluster the cluster mean is marked and ellipses indicate corresponding covariance. Note that an increase in the number of clusters usually largely lead to a split of a certain cluster rather than a more fundamental change in the arrangement of units into clusters.

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    Sequential associations between unit types in the adult females of both gorilla groups. Lines indicate the average Dice index of sequential association as a measure of how frequently any two unit types followed one another in immediate succession. Average values were calculated from the individuals’ Dice indices for a given pair of unit types. The legend depicts the thickness of the lines corresponding to the minimum and maximum average Dice index. Included are only unit types shared by all adult females.

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