Signal lengthening is not a strategy for animals to mitigate noise interference

We obtained echolocation calls of the great leaf-nosed bats in silence and a noise condition of 50 dB SPL. We analysed a total of 20,6667 echolocation calls from 23 bats, with a median of 9240 calls per bat (range, 1366–15828 calls). Echolocation calls of the bat were recorded with an array of 16 ultrasonic microphones (NEUmic, Ultra Sound Advice, London, UK) on the floor of a recording room. The echolocation calls recorded by the microphones were high-passed filtered at 20 kHz by an analog filter (VBF40, Kemo Ltd, Dartford, UK) and digitized by a sound-recording system (PXIe 8840, with a data acquisition card PXIe 6358, National Instruments, Austin, USA) at a sampling rate of 250 kHz.