Does similarity in call structure or foraging ecology explain interspecific information transfer in wild Myotis bats?

Abstract Animals can gain important information by attending to the signals and cues of other animals in their environment, with acoustic information playing a major role in many taxa. Echolocation call sequences of bats contain information about the identity and behaviour of the sender which is perceptible to close-by receivers. Increasing evidence supports the communicative function of echolocation within species, yet data about its role for interspecific information transfer is scarce. Here, we asked which information bats extract from heterospecific echolocation calls during foraging. In three linked playback experiments, we tested in the flight room and field if foraging Myotis bats approached the foraging call sequences of conspecifics and four heterospecifics that were similar in acoustic call structure only (acoustic similarity hypothesis), in foraging ecology only (foraging similarity hypothesis), both, or none. Compared to the natural prey capture rate of 1.3 buzzes per minute of bat activity, our playbacks of foraging sequences with 23–40 buzzes/min simulated foraging patches with significantly higher profitability. In the flight room, M. capaccinii only approached call sequences of conspecifics and of the heterospecific M. daubentonii with similar acoustics and foraging ecology. In the field, M. capaccinii and M. daubentonii only showed a weak positive response to those two species. Our results confirm information transfer across species boundaries and highlight the importance of context on the studied behaviour, but cannot resolve whether information transfer in trawling Myotis is based on acoustic similarity only or on a combination of similarity in acoustics and foraging ecology. Significance statement Animals transfer information, both voluntarily and inadvertently, and within and across species boundaries. In echolocating bats, acoustic call structure and foraging ecology are linked, making echolocation calls a rich source of information about species identity, ecology and activity of the sender, which receivers might exploit to find profitable foraging grounds. We tested in three lab and field experiments if information transfer occurs between bat species and if bats obtain information about ecology from echolocation calls. Myotis capaccinii/daubentonii bats approached call playbacks, but only those from con- and heterospecifics with similar call structure and foraging ecology, confirming interspecific information transfer. Reactions differed between lab and field, emphasising situation-dependent differences in animal behaviour, the importance of field research, and the need for further studies on the underlying mechanism of information transfer and the relative contributions of acoustic and ecological similarity. Electronic supplementary material The online version of this article (10.1007/s00265-017-2398-x) contains supplementary material, which is available to authorized users.


Interspecific information transfer in wild Myotis bats
Behav. Ecol. Sociobiol.

Fig. S1: Field-measured call parameters for the bat species used as playback stimuli.
Data are literature values from field studies (see Table 1 for references) and are displayed as in these studies, either as mean ± std (solid lines) or mean ± sem (dotted line). Order of studies within species is as in Table 1 (from Ref.
[1] to [5]). Species that we classified as acoustically similar or dissimilar to M. capaccinii (red) are shown in orange and blue, respectively, with a description of their general call shape (FM: frequency-modulated, QCF: quasi-constant-frequency). See Fig. 1 for exemplary call spectrograms of all species.
Hügel et al. separately for the X-, Y-and Z-coordinates. Note that the signed differences are equally distributed around Zero, showing that the smoothing does not introduce a systematic bias and that the differences are rather due to random noise.

C + D)
Absolute difference between the measured and the smoothed position (C), and the absolute error normalized to the distance to the array (D), separately for the X-, Y-and Zcoordinates and the total difference (labelled t) in 3D-space.
Differences of the Y-coordinate (direction away from the array) are larger than those of the X-and Z-coordinates, which is to be expected for 3D-triangulation methods. Note that median differences are still only around 5 cm for the Y-coordinate; and the upper quartile is still less than 10 cm.
Box plots present median, quartiles and whiskers at up to 1.5 times the interquartile range beyond the quartiles. N = 818 bat positions.

Fig. S4: Assessment of the playback stimuli as indicators of profitable foraging patches.
Grey histograms and black box plots present data of 443 1-minute long pre-playback phases recorded in the field that contained at least one bat pass of Myotis capaccinii/daubentonii.
Red box plots are the data for our 1-minute long playback stimuli for Myotis capaccinii/daubentonii. Box plots present median, quartiles and whiskers at up to 1.5 times the interquartile range beyond the quartiles.
A) The number of presented echolocation sequences (12/min) was roughly twice as high as the number of bat passes observed in the field (median: 7/min, quartiles: 4-10).
B) The number of presented feeding buzzes (12/min) was about 12 times higher than the number of feeding buzzes observed in the field (median: 1/min, quartiles: 0-2).
C) The presented capture rate of 1 feeding buzz per bat pass was about 20 times higher than the observed capture rate in the field (median: 0.05 feeding buzzes / bat pass, quartiles: 0-0.28).

D)
The duration of presented echolocation sequences (median: 2.56 s, quartiles: 2.30-2.70) was equal to the duration recorded in the field (median: 2.26 s, quartiles: 1.32-3.97), although the variation in the field was larger than the presented variation. See Table S1 for the presented duration of all species.
E) The presented rate of feeding buzzes per minute of bat activity (median: 23.4 buzzes/min, quartiles: 22.18-26.12) is about 18 times higher than the rate observed in the field (1.3 buzzes/min, quartiles: 0-7.36).
Hügel et al. Bat distance to the speaker (A), trajectory curvature (B), change in flight direction (C), the PC1-PC3 scores of the previous trajectory parameters and relative flight height (see Fig. 5D,E) (D) and call interval (E) as a function of playback species and time relative to the playback, with t=0