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, 4 (8), e6746

Versatility and Stereotypy of Free-Tailed Bat Songs

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Versatility and Stereotypy of Free-Tailed Bat Songs

Kirsten M Bohn et al. PLoS One.

Abstract

In mammals, complex songs are uncommon and few studies have examined song composition or the order of elements in songs, particularly with respect to regional and individual variation. In this study we examine how syllables and phrases are ordered and combined, ie "syntax", of the song of Tadarida brasiliensis, the Brazilian free-tailed bat. Specifically, we test whether phrase and song composition differ among individuals and between two regions, we determine variability across renditions within individuals, and test whether phrases are randomly ordered and combined. We report three major findings. First, song phrases were highly stereotyped across two regions, so much so that some songs from the two colonies were almost indistinguishable. All males produced songs with the same four types of syllables and the same three types of phrases. Second, we found that although song construction was similar across regions, the number of syllables within phrases, and the number and order of phrases in songs varied greatly within and among individuals. Last, we determined that phrase order, although diverse, deviated from random models. We found broad scale phrase-order rules and certain higher order combinations that were highly preferred. We conclude that free-tailed bat songs are composed of highly stereotyped phrases hierarchically organized by a common set of syntactical rules. However, within global species-specific patterns, songs male free-tailed bats dynamically vary syllable number, phrase order, and phrase repetitions across song renditions.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Structure of T. brasiliensis song.
One complete song showing the three types of phrases: chirp, trill, and buzz (A). Expanded section of a chirp phrase showing one motif which is composed of two types of syllables: type A and type B (B). Expanded section of a trill (C). Expanded section of a buzz (D). This song is a chirp-buzz-trill-buzz song type.
Figure 2
Figure 2. Spectrograms of songs from bats from the two regions.
Two bats are from Austin (A and B) and two from College Station (C and D). Each chirp phrase is enclosed by a dashed rectangle, each trill is enclosed by an oval, and each buzz is enclosed by a solid rectangle. Time waveforms (Figure S1) and audio files (Audio S1– S4) of each of these songs are also provided. All songs are variants of the chirp-trill-buzz song type.
Figure 3
Figure 3. The distribution of song lengths at the two locations.
The frequency of songs with 2 through 7 or more phrases at Austin (white bars, N = 200) and College Station (filled bars, N = 91). The distribution of song lengths is highly similar at the two locations.
Figure 4
Figure 4. Song variants and song types.
The number of song variants (with repetitions, A) and song types (without repetitions, B) as a function of the number of songs recorded for each bat. For each graph a line is shown for the case when every song recorded is unique (y = x) and when every song recorded is the same (y = 1). Only bats with a minimum of four recorded songs were included (N = 26 bats).
Figure 5
Figure 5. Observed and expected transition frequencies.
Observed frequencies (bars) and expected frequencies (circles) of transitions from the start of song (A), chirps (B), trills (C) and buzzes (D) to each phrase or to the end of the song (“end”). For example, the first bar in A represents the observed frequency of beginning-chirp transitions. Expected frequencies were calculated in proportion to the relative abundance of phrases. Transitions were taken from all songs with greater than one phrase (N = 291 songs, 1,767 total transitions).
Figure 6
Figure 6. Model of song based on transition frequencies.
Arrows represent transitions from one phrase to the next. Plus (+) symbols represent transitions that deviated from expected more than 10% (+), 20 % (++) or 50% (+++). Arrow thickness increases with transition frequencies based on values in Figure 5. No arrows were drawn for frequencies less than 0.05. See Figure 1 and Figure 2 for examples of each phrase.
Figure 7
Figure 7. Observed and expected three-phrase combination frequencies.
Observed frequencies (bars) and expected frequencies (circles) from the first-order model of three phrase combinations without trill or buzz repeats. “c” = chirp, “t” = trill and “b” = buzz. Combinations to the left of the dashed line occurred more frequently than expected (labeled with +) while combinations to the right of the dashed line on the right occurred less frequently than expected (labeled with –). Only songs with greater than three phrases were included (N = 183 songs, 302 total transitions).

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