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Everyday Bat Vocalizations Contain Information About Emitter, Addressee, Context, and Behavior

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Everyday Bat Vocalizations Contain Information About Emitter, Addressee, Context, and Behavior

Yosef Prat et al. Sci Rep.

Abstract

Animal vocal communication is often diverse and structured. Yet, the information concealed in animal vocalizations remains elusive. Several studies have shown that animal calls convey information about their emitter and the context. Often, these studies focus on specific types of calls, as it is rarely possible to probe an entire vocal repertoire at once. In this study, we continuously monitored Egyptian fruit bats for months, recording audio and video around-the-clock. We analyzed almost 15,000 vocalizations, which accompanied the everyday interactions of the bats, and were all directed toward specific individuals, rather than broadcast. We found that bat vocalizations carry ample information about the identity of the emitter, the context of the call, the behavioral response to the call, and even the call's addressee. Our results underline the importance of studying the mundane, pairwise, directed, vocal interactions of animals.

Figures

Figure 1
Figure 1. Distinct types of Egyptian fruit bat vocalizations.
(A–E) Spectrograms of calls associated with different behaviors. Spectrograms show a time span of 1 sec. and a frequency band of 0–15 kHz (for A) and 0–60 kHz (for BE). (A) Mating calls - produced by males during mating. (B) Isolation calls – produced by pups when left alone in the roost or when detached from their mothers. (C) Seclusion/distress calls – emitted mostly by distressed adults which are separated from others. (D) Echolocation clicks – lingual click-based sonar signals. (E) Aggression calls – the most common call type; produced during the many everyday squabbles between pairs of bats. These vocalizations are composed of sequences of broad-band multi-harmonic syllables (usually 1-20 syllables). Two examples for each aggression (aggr.) context: (E1) Feeding aggr., (E2) Mating aggr., (E3) Perch aggr., and (E4) Sleep aggr. (F) Occurrence of aggression calls along the day, presented as average calls/10-min. per bat. White/gray background indicates light/dark time of day. The peaks at the beginning/end of the night are associated with the disintegration/reintegration of the day-time sleeping cluster. (G) Occurrence of aggression calls in the four analyzed contexts (assessed according to the proportions of these contexts among annotated recordings).
Figure 2
Figure 2. Emitter and addressee of the vocalizations.
(A) A confusion matrix for the prediction of the emitting individuals. The number in each box indicates the fraction of each true class (row) assigned to each predicted class (column), i.e., each row sums to 1. The total number of vocalizations in each class (N) is indicated to the right of the matrix. Yellow – highest value in the matrix, Blue – lowest value. (BA = 71%, chance = 14%, p < 0.01) (B) 2D Projection of the vocalizations of different emitters (only test data) through the trained models: each vocalization is represented as a 7-dimensional vector – the score it received for each model, then LDA is applied (for visualization only) and the two first linear discriminators are used (see Materials and Methods). Inset: only emitters F5-F7 in an alternative projection. (C) A confusion matrix for the prediction of the addressees of vocalizations produced by bat F6 (BA = 50%, chance = 33%, p < 0.01). (D) A confusion matrix for the prediction of the vocalizations addressed to a male vs. those addressed to a female (BA = 64%, chance = 50%, p < 0.01).
Figure 3
Figure 3. The context of the vocalizations.
(A) A confusion matrix for the prediction of the context (BA = 61%, chance = 25%, p < 0.01). The number in each box indicates the fraction of each true class (row) assigned to each predicted class (column). The total number of vocalizations in each class (N) is indicated to the right of the matrix. (B) Projection of the different contexts through the trained models (see Materials and Methods). Inset: only contexts perch aggr. and sleep aggr. in another projection. (C) A confusion matrix for the prediction of the context and emitter together (BA = 57%, chance = 4%, p < 0.01; exact numbers for each cell are given in Supplementary Table S4). (D and E) Projection of two emitters – F5, F6, in two contexts – feeding aggr. and perch aggr. (D and E) depict the same plot: red – F5, blue – F6, dark color – feeding aggr., light color – perch aggr. The black and grey lines in (D) portray the 0.3 isoline of the 2D histogram (with max. normalized to 1) of feeding aggr. and perch aggr., respectively. The red and blue lines in (E) portray the 0.3 isoline of the 2D histogram (with max. normalized to 1) of F5 and F6, respectively.
Figure 4
Figure 4. Predicting the behavioral outcome of the interaction.
A confusion matrix for the prediction of the outcome of the vocal interactions. Two possible outcomes were defined: Depart and Remain together (BA = 62%, chance = 50%, p < 0.01). The number in each box indicates the fraction of each true class (row) assigned to each predicted class (column). The total number of vocalizations in each class (N) is indicated to the right of the matrix.
Figure 5
Figure 5. Acoustic features conveying the information.
An example of the distribution of two pairs of acoustic features in the analyzed contexts. Colored lines mark the 0.5 isoline of the 2D histograms (with max. normalized to 1). (A) Mel-cepstral coefficients 1 and 2. (B) Mel-cepstral coefficients 6 and 8. In both cases, the distributions mostly overlap each other. The least overlapping regions are marked with dashed rectangles. (C–F) For each such region: the fraction of each context, out of the vocalizations which contains values inside the marked rectangle, is shown in the bar plot. Spectrograms of two syllables from the prevalent context in each region are presented, i.e. (C) mating aggr., (D) feeding aggr. (E) perch aggr. (F) sleep aggr. Red bars below the spectrograms mark the regions which contains values inside the marked rectangle. For all spectrograms: frequency axis is 0–40 kHz, time scale as indicated in (D). To avoid any possible biases in this example, the displayed distributions are taken only from one recording channel (the one with the largest yield, channel 3, cage 2), all of the spectrograms are from vocalizations produced by the same individual (F7), and in each pair of spectrograms - each was taken from a different day. In total 64 cepstral coefficients were used to describe vocalizations, allowing separation of classes among many more dimensions.

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