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Spatial location influences vocal interactions in bullfrog choruses
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Image of FIG. 1.
FIG. 1.

Diagrams of the two chorusing sites. On each diagram, the two black square/white cross symbols outside of the pond outline indicate the position of the two acoustic sensors. (A) Map of pond 1 showing estimated locations and numbers of actively calling bullfrogs (designated by open circles with numbers) on the night of 6/12/05. Each circle is placed at the median of the vector intersection points derived from the computational model. Only one bullfrog was present at each indicated location. Locations are numbered in the order in which they were identified in the analysis. (B) Map of activity at pond 1 on the night of 7/5/05. Five vocalizing bullfrogs were now present at location 2. (C) Map of pond 2. The locations of bullfrogs were similar on all three nights (7/5/06, 7/6/06, and 7/9/06) at this site; the variability in numbers of animals at each location is indicated. On all three nights, location 4 contained only one bullfrog.

Image of FIG. 2.
FIG. 2.

Estimates of source locations for two bullfrogs belonging to the same cluster to illustrate the effective accuracy of localization. (A) Spectrograms to 400 Hz of three note advertisement calls produced by two individual male bullfrogs (frog A and frog B), calling sequentially over a 14 s time interval. For calculation of these and other spectrograms by ADOBE AUDITION, sounds were downsampled to 1500 Hz (Blackmann-Harris window, 512 frequency bands). The first harmonic frequencies of the notes in each bullfrog’s advertisement call differ (190 Hz for frog A and 170 Hz for frog B). (B) Localization estimates derived from calls in A. The black square/white cross symbols outside the pond’s outline show the locations of the two sensors. The active microphones (1, 3 and 5, 7) in each sensor are indicated. The symbols inside the pond outline show the points of vector intersection computed from the advertisement call notes of each individual male. Estimates for frog A are indicated by the black triangles, and estimates for frog B are indicated by the gray circles. The localization program provided 20 intersection points for the three notes emitted by frog A, and 28 intersection points for the three notes emitted by frog B. Although all of these points are plotted, not all are visible because many of their locations overlap. Variability is indicated by black and gray crossed lines for standard deviations in the east-west and north-south dimensions calculated from all location estimates for frog A and frog B, respectively. The intersection points of each cross designate the mean locations calculated from the intersection points for the two frogs. Frog A and frog B are separated by a mean diagonal distance of 1.68 m. The maximum dispersion between all intersection points for these two frogs, excluding the points outside the pond, is 3.5 m. Dispersion is due largely to effects of reverberation on each localization estimate.

Image of FIG. 3.
FIG. 3.

Spectrogram examples of the four types of vocal interactions (bouts) identified in the recordings. Advertisement calls were low-pass filtered to show only the low frequency harmonics. Arrows indicate the first notes of each bullfrog’s individual call. The relative darkness of the spectrograms provides an indication of the relative amplitude of each individual’s call notes. (A) Note alternation, in which one bullfrog begins calling after the completion of the first note of another bullfrog’s call and the two continue with their notes alternating in time. (B) Note overlap, in which more than one-half of the notes in the first bullfrog’s call is overlapped by the call notes of another bullfrog. In this example, the second bullfrog does not begin calling until the completion of the first note in the leading bullfrog’s call, but subsequent notes of both bullfrogs overlap considerably. (C) Call alternation, in which one bullfrog begins calling within 2 s after the completion of another bullfrog’s call. (D) Call overlap, in which one bullfrog begins calling after the completion of more than one-half of the notes of another bullfrog’s call. In this example, only the last note of the first bullfrog’s call overlaps with the first note of the second bullfrog’s call.

Image of FIG. 4.
FIG. 4.

Calling rates (normalized for the total numbers of calling events per night) at the two different chorusing sites. (A) Rate ( axis) of individual calls (black bars) and multiple frog interactions (bouts; gray bars) across all five nights ( axis) of chorus activity. The rate of multiple frog bouts was significantly higher in July than in June. (B) Rate of calling events at pond 1 on 6/12/05 (black bars) and 7/5/05 (gray bars). call; alternation; overlap; alternation; overlap. (C) Rate of calling events at pond 2 on 7/5/06 (black bars), 7/6/06 (gray bars), and 7/9/06 (dark gray bars). Note overlap was the most common type of multiple frog interaction on all three nights, but there were many instances of all types of calls and variability between the three nights.

Image of FIG. 5.
FIG. 5.

Within-cluster and between-cluster calling rates on four nights at the two recording sites. Data show combined calling rates for all four types of acoustic interactions. Asterisks denote statistical significance of comparisons of between-cluster and within-cluster rates on a given chorusing night. statistically significant. There was a significantly higher number of between-cluster than within-cluster interactions at pond 1 on 7/5/05. At pond 2, there were significantly more within-cluster interactions on two of the nights (7/5/06 and 7/9/06), but no difference on the third night (7/6/06).

Image of FIG. 6.
FIG. 6.

Calling rates for the four different kinds of bouts for the four July chorus nights at the two chorus sites. (A) Rate of call alternation events. Call alternation was significantly more likely to occur between clusters compared to within clusters on all four nights. (B) Rate of call overlap events. The pattern of call overlap differed significantly between clusters compared to within clusters in all choruses. Call overlap was more likely to occur between clusters. (C) Rate of note alternation events. Note alternation occurred exclusively within clusters at pond 1 (7/5/05), so no statistical test could be performed on these data. At the other three choruses, note alternation was significantly more likely to occur within clusters than between clusters. (D) Rate of note overlap events. Note overlap was observed only within clusters and never between clusters at pond 1. At the other choruses, note overlap occurred significantly more often within clusters than between clusters.

Image of FIG. 7.
FIG. 7.

Patterns of sequential calling in choruses, derived from first-order analysis in UNCERT. The numbers in italics on each pond diagram are the probability of that location (cluster) responding immediately after the most active location (cluster). Arrows represent the direction of interaction. The arrow that goes back onto itself shows the probability of the most active cluster (location) vocalizing immediately after itself. (A) Pond 1 on 6/12/05. The most active location on this night is location 6. The probability of the bullfrog at location 6 vocalizing immediately after itself is 0.7216. The bullfrog in location 6 was most likely to vocalize after the bullfrog in location 3 (0.5556), its farthest neighbor. Locations 1 and 2 were not included in the analysis, because the animals at these locations called only once during the recording session. (B) Pond 1 on 7/5/05. The chorus organization differed on this night than on the earlier night shown in (A). The animal in location 6 was absent from the chorus, and the most active location was now location 2. Numbers in italics show the probabilities that any bullfrog at location 2 followed the calls of any individuals at the other locations. The sequential probabilities of calling were similar to the three farthest neighbors (locations 5, 3, and 1) and lowest to the nearest neighbor at location 4. The bullfrogs in location 2 followed themselves with a probability of 0.3409. (C) Pond 2 on 7/9/06. The most active location at this night was location 1. Bullfrogs in this location followed themselves with a probability of 0.5347. The highest sequential probabilities of calling were to the farthest neighbors at locations 3 and 5.

Image of FIG. 8.
FIG. 8.

Demonstration of how overlap of call notes results in increased AM due to interference between signals. (A) Spectrograms to 700 Hz and envelopes of the seven notes in an advertisement call of an individual bullfrog, frog 1, calling alone. (B) Spectrograms and envelopes of the five notes in an advertisement call of another bullfrog, frog 2, also vocalizing alone. (C) Spectrograms and envelopes of call notes from frog 1 and frog 2, which were artificially superimposed by aligning and mixing the calls to overlap their individual notes. (D) Spectrogram and envelopes of call notes from two other bullfrogs, frog 3 and frog 4, which the frogs themselves produced in an overlapping pattern. This is an example of actual note overlap occurring naturally. The overlapping advertisement call notes in (C) and (D) show more complex envelopes than the nonoverlapped call notes in (A) and (B). Most of the envelopes for the overlapped notes, whether artificial (C) or real (D), show roughly 10–30 cycles of AM (gray ovals) on top of the smoother envelope for the notes by themselves.


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Events measured in simulated and empirical data.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Spatial location influences vocal interactions in bullfrog choruses