Abstract
How bats adapt their sonar behavior to accommodate the noisiness of a crowded day roost is a mystery. Some bats change their pulse acoustics to enhance the distinction between theirs and another bat's echoes, but additional mechanisms are needed to explain the bat sonar system's exceptional resilience to jamming by conspecifics. Variable pulse repetition rate strategies offer one potential solution to this dynamic problem, but precisely how changes in pulse rate could improve sonar performance in social settings is unclear. Here we show that bats decrease their emission rates as population density increases, following a pattern that reflects a cumulative mutual suppression of each other's pulse emissions. Playback of artificially-generated echolocation pulses similarly slowed emission rates, demonstrating that suppression was mediated by hearing the pulses of other bats. Slower emission rates did not support an antiphonal emission strategy but did reduce the relative proportion of emitted pulses that overlapped with another bat's emissions, reducing the relative rate of mutual interference. The prevalence of acoustic interferences occurring amongst bats was empirically determined to be a linear function of population density and mean emission rates. Consequently as group size increased, small reductions in emission rates spread across the group partially mitigated the increase in interference rate. Drawing on lessons learned from communications networking theory we show how modest decreases in pulse emission rates can significantly increase the net information throughput of the shared acoustic space, thereby improving sonar efficiency for all individuals in a group. We propose that an automated acoustic suppression of pulse emissions triggered by bats hearing each other's emissions dynamically optimizes sonar efficiency for the entire group.
Highlights
Environmental noise degrades the transmission of all animal communication sounds (Ryan and Brenowitz, 1985; Ryan, 1986; Brumm and Slabbekoom, 2005; Jones, 2008), but echolocation by bats is sensitive because bats need to clearly hear their own faint echoes to hunt and navigate (Neuweiler, 2000; Schnitzler and Kalko, 2001)
This observation could be a product of reduced pulse emission rates, since the number of overlaps per second was strongly correlated with mean pulse emission rates per epoch for both real and simulated bats (R = 0.83, p < 0.0001 and R = 0.75, p < 0.0001, respectively)
Both data sets were well fit by the function rτn [r2 = 0.71, F(1, 140) = 344.9, P < 0.001], indicating that interferences had occurred randomly and their propensity was predictably based on mean emission rates and population density and that the bats were not timing their pulse emissions to avoid overlaps with one another
Summary
Environmental noise degrades the transmission of all animal communication sounds (Ryan and Brenowitz, 1985; Ryan, 1986; Brumm and Slabbekoom, 2005; Jones, 2008), but echolocation by bats is sensitive because bats need to clearly hear their own faint echoes to hunt and navigate (Neuweiler, 2000; Schnitzler and Kalko, 2001). Some bats display a jamming avoidance behavior in which they change their outgoing call pitch in order to minimize overlap in bandwidth (Ratcliffe et al, 2004; Ulanovsky et al, 2004; Gillam et al, 2007; Bates et al, 2008; Tressler and Smotherman, 2009; Necknig and Zahn, 2011), and some increase pulse amplitude in the presence of background noise (Simmons et al, 1978; Tressler and Smotherman, 2009; Tressler et al, 2011) These relatively minor changes in pulse acoustics have so far only been documented in pairs of bats and are considered unlikely to be effective for much larger groups of bats because their vocal parameters are tightly constrained by highly specialized laryngeal and respiratory mechanics (Metzner and Schuller, 2007), a finely tuned auditory system (Popper and Fay, 1995), and would force bats to alter pulse characteristics away from optimal parameters for foraging and navigation (Schnitzler and Kalko, 2001). In light of these limitations other more comprehensive answers are needed to explain how bats echolocate in groups
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