Close-call vocalizations of mountain gorillas

This analysis focuses on a unique dataset of close-call vocalizations produced by 10 adult mountain gorillas living in a single group of 16 individuals in Bwindi Impenetrable National Park, Uganda. The dataset comprises 2189 close-call sequences, each composed of one to six acoustically distinct vocal units, for a total of 4294 units. The close-calls were recorded over 312 days between October 2007 and October 2008 using a combination of focal-follows and opportunistic recordings of individuals in proximity to the focal gorilla.

The dataset used in this analysis consists of close-call vocalizations recorded from 10 adult mountain gorillas (five female: five male, mean age in years = 21, s.d. = 8, range = 11–31) living in a single group of 16 individuals in Bwindi Impenetrable National Park, Uganda. The mountain gorillas are a primarily herbivorous species of great ape that live in stable single- or multi-male groups ranging from 2 to 34 individuals in size. The close-calls comprise nine acoustically distinct types (e.g. grunts, grumbles, hums), which are often combined in different ways and primarily produced in feeding and resting contexts. The dataset was collected over a period of 312 days between October 2007 and October 2008, using a combination of focal-follows and opportunistic recordings of individuals in proximity to the focal gorilla.

The vocalizations were recorded at 48 kHz sampling rate with a Marantz PMD670 digital recorder and Sennheiser ME66 shotgun microphone. Calls were selected for analysis based on their signal-to-noise ratio determined through inspection of the spectrograms in Avisoft SASLab Pro Version 5.1.23. The final dataset consisted of 2189 close-call sequences (composed of one to six ‘vocal units' for a total of 4294 units). The duration of the units was measured from spectrograms with a 20 Hz frequency and 1 ms temporal resolution using Avisoft SASlab Pro.

<WRAP centerbox> </WRAP>

Figure 1: 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. Reproduced with permission from Hedwig et al. [2].

Table 1: Summary of key numbers for the dataset of 'close-call' sequences produced by wild mountain gorillas (Gorilla beringei beringei).

Table 1:

Table 2:

Table 1 and 2:Table showing the number of vocal elements in 'close-call' sequences produced by wild mountain gorillas and the number of vocal elements by individual to determine whether a negative relationship existed between the number of vocal units in a sequence and the duration of its constituent units, in accordance with Menzerath's law.

The data processing steps involved categorizing the close-calls according to a cluster analysis detailed in Hedwig et al. [2]. Acoustic parameters were semi-automatically extracted from the units using ‘LMA’ software developed by K. Hammerschmidt. The reliability of manually extracted measurements was ascertained through comparing the results of manual with automatic measurements on a subset of tonal units (Spearman correlation; ρ > 0.92, N = 55, p < 0.001). The sequence was defined as a series of units separated by less than two seconds of silence between each unit and the next. The individual units were identified by either a period of silence (min. 10 ms, max. 2000 ms, 95% of intervals less than 550 ms) or sudden spectral shifts between them. Sequence lengths for which we only had a single example were excluded from analysis (although the outcome of our analyses remained similar if these were included).

The dataset analysis yielded various findings related to the acoustic characteristics of the close-call vocalizations. The distribution of the occurrence of each of the five different types of vocal units across sequence lengths is illustrated in electronic supplementary material, figure S3 [1]. Additional details regarding acoustic analyses and categorization of close-call call types can be found in Hedwig et al. [2].

The analysis of the close-call vocalizations in mountain gorillas contributes to the understanding of the species' vocal communication and can provide insights into their social behavior. The results highlight the importance of documenting and analyzing acoustic characteristics of vocalizations in animal communication research. Further research could investigate the contextual and social factors that influence the production of the different types of vocal units in mountain gorillas.

[1] Watson, S. K., Heesen, R., Hedwig, D., Robbins, M. M., & Townsend, S. W. (2020). An exploration of Menzerath's law in wild mountain gorilla vocal sequences. Biology Letters, 16(10), 20200380.

[2] Hedwig D, Hammerschmidt K, Mundry R, Robbins MM, Boesch C. 2014Acoustic structure and variation in mountain and western gorilla close calls: a syntactic approach. Behaviour 151, 1091-1120.