Distress Calls of a Fast-Flying Bat (Molossus molossus) Provoke Inspection Flights but Not Cooperative Mobbing

Abstract

Many birds and mammals produce distress calls when captured. Bats often approach speakers playing conspecific distress calls, which has led to the hypothesis that bat distress calls promote cooperative mobbing. An alternative explanation is that approaching bats are selfishly assessing predation risk. Previous playback studies on bat distress calls involved species with highly maneuverable flight, capable of making close passes and tight circles around speakers, which can look like mobbing. We broadcast distress calls recorded from the velvety free-tailed bat, Molossus molossus, a fast-flying aerial-hawker with relatively poor maneuverability. Based on their flight behavior, we predicted that, in response to distress call playbacks, M. molossus would make individual passing inspection flights but would not approach in groups or approach within a meter of the distress call source. By recording responses via ultrasonic recording and infrared video, we found that M. molossus, and to a lesser extent Saccopteryx bilineata, made more flight passes during distress call playbacks compared to noise. However, only the more maneuverable S. bilineata made close approaches to the speaker, and we found no evidence of mobbing in groups. Instead, our findings are consistent with the hypothesis that single bats approached distress calls simply to investigate the situation. These results suggest that approaches by bats to distress calls should not suffice as clear evidence for mobbing.

Fig 1. Variation of distress calls of Molossus molossus.

Spectrograms (FFT 1024, Blackman, auto padding, dynamic range of 90 dB) of distress calls playbacks with averaged power spectrum (aside) and waveform (below). Distress calls structures included multiharmonic shallowly modulated calls (A), calls with sideband modulations (B, C), and some calls with nonlinear phenomena (D). Sideband modulations of the distress call shown in C are depicted in E (FFT 2048).

Fig 2. Distress calls and pink noise stimuli.

Spectrograms (500 kHz sampling rate, 16 bit resolution, 1024 FFT, Blackman window) of a sequence of Molossus molossus distress calls (A) and pink noise controls (B) with averaged power spectrum of the third pink noise stimulus. Each 2-min playback sequence (C) begins with a 1-min silent period followed by a 1-min playback period with either distress calls (A boxes) or pink noise bursts (B boxes) repeated three times and spaced apart by 10 s of silence.

Fig 3. Effect of playback treatment on bat activity.

The mean responsiveness (number of passes during playback period–silent period) is shown for M. molossus (blue squares) and S. bilineata (red circles) in response to either M. molossus distress calls or pink noise. Errors bars show standard error of the mean.

Fig 4. Distributions of Molossus molossus responsiveness.

Frequency histograms of responsiveness during playback sequences that had passes of M. molossus (n = 116). Increases in activity are to the right of the solid line.

Fig 5. Distributions of Saccopteryx bilineata responsiveness.

Frequency histograms of responsiveness during playback sequences that had passes of S. bilineata (n = 59). Increases in activity are to the right of the solid line.

Fig 6. Molossus molossus activity and responsiveness to distress call playback near roosts over time.

Mean number of passes (and standard errors) during the 2-min playback sequences recorded on four evenings from 1838 to 2020 h. Data are pooled in 8-min time bins (2–6 counts per bin). Counts do not include observations of zero passes. Blue line shows the total number of M. molossus passes during both distress call and noise playbacks. Red line shows the responsiveness (number of passes during the 1-min playback period minus 1-min silent period) during distress call playbacks.