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J Exp Biol. 2020 Jan 28;223(Pt 2). pii: jeb217109. doi: 10.1242/jeb.217109.

Energy compensation and received echo level dynamics in constant-frequency bats during active target approaches.

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Zoophysiology, Department of Bioscience, Aarhus University, 8000 Aarhus, Denmark
Department of Mechanical Engineering, 1075 Life Science Circle, Virginia Tech, Blacksburg, VA 24061, USA.
Shandong University-Virginia Tech International Laboratory, 27 Shanda South Road, Jinan, Shandong 250100, People's Republic of China.
Zoophysiology, Department of Bioscience, Aarhus University, 8000 Aarhus, Denmark.
Sea Mammal Research Unit, University of St Andrews, St Andrews KY16 9LZ, Scotland, UK.
Aarhus Institute of Advanced Studies, Aarhus University, 8000 Aarhus, Denmark.


Bats have been reported to adjust the energy of their outgoing vocalizations to target range (R) in a logarithmic fashion close to 20log10 R which has been interpreted as providing one-way compensation for increasing echo levels during target approaches. However, it remains unknown how species using high-frequency calls, which are strongly affected by absorption, adjust their vocal outputs during approaches to point targets. We hypothesized that such species should compensate less than the 20log10 R model predicts at longer distances and more at shorter distances as a consequence of the significant influence of absorption at longer ranges. Using a microphone array and an acoustic recording tag, we show that the output adjustments of two Hipposideros pratti and one Hipposideros armiger do not decrease logarithmically during approaches to different-sized targets. Consequently, received echo levels increase dramatically early in the approach phase with near-constant output levels, but level off late in the approach phase as a result of substantial output reductions. To improve echo-to-noise ratio, we suggest that bats using higher frequency vocalizations compensate less at longer ranges, where they are strongly affected by absorption. Close to the target, they decrease their output levels dramatically to mitigate reception of very high echo levels. This strategy maintains received echo levels between 6 and 40 dB re. 20 µPa2 s across different target sizes. The bats partially compensated for target size, but not in a one-to-one dB fashion, showing that these bats do not seek to stabilize perceived echo levels, but may instead use them to gauge target size.


Atmospheric absorption; CF bats; Echo level; Intensity compensation; Microphone array; Source level


Conflict of interest statement

Competing interestsThe authors declare no competing or financial interests.

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