Echolocating bats exhibit differential amplitude compensation for noise interference at a sub-call level

Area of Science:

• Animal Communication
• Bioacoustics
• Neuroethology

Background:

• The Lombard effect, an increase in vocal amplitude with ambient noise, is crucial for vertebrate communication.
• While documented in many vertebrates, its influence on sub-call structures like lexical stress in animals remains largely unexplored.
• Bats utilize complex echolocation calls with distinct constant-frequency (CF) and frequency-modulated (FM) components for navigation and foraging.

Purpose of the Study:

• To investigate the Lombard effect in the CF and FM components of echolocation calls in two species of Hipposideros bats.
• To determine if bats exhibit differential amplitude modulation in response to varying noise conditions.
• To explore the functional significance of these adjustments, particularly in relation to masking release.

Main Methods:

• Recording echolocation calls from two Hipposideros bat species under varying ambient noise conditions.
• Analyzing the amplitude modulation of the CF and FM components of the echolocation calls.
• Comparing amplitude changes in response to spectrally overlapping and non-overlapping noise.

Main Results:

• Ambient noise induced a significant Lombard effect in both CF and FM components of bat echolocation calls.
• Differential amplitude compensation was observed, specifically in spectrally overlapping noise conditions, suggesting a role in masking release.
• Bats displayed a robust Lombard effect even in spectrally non-overlapping noise, challenging previous assumptions.

Conclusions:

• Echolocating bats demonstrate sophisticated vocal plasticity, employing a differential Lombard effect to enhance communication in noisy environments.
• The findings suggest bats may use amplitude modulation to mitigate acoustic masking, particularly in spectrally overlapping noise.
• Hipposideros bats serve as a valuable mammalian model for studying the neural mechanisms of vocal production control and adaptation to acoustic challenges.