Task learning drives adaptive vocal adjustments in echolocating bats

Vocal production learning (VPL), a core ability underlying human speech, is rare among mammals. Apart from humans, two of the four identified VPL mammalian lineages (cetaceans and bats) are echolocators, exhibiting remarkable vocal flexibility for echolocation. It has been posited that enhanced vocal flexibility can facilitate the evolution of VPL, which operates at longer time scales. However, empirical data linking vocal flexibility to VPL is still lacking. Using a psychophysical reinforcement learning paradigm, we trained Pratt's roundleaf bats, Hipposideros pratti, to detect pure tones and virtual echoes, both with and without background noise. We report that bats learned to adapt their call amplitude to auditory tasks across various time scales, ranging from instantaneous to months. Surprisingly, bats modified the noise-induced call amplitude adaptation, a reflexive audio-vocal behavior known as the Lombard effect, in a task-specific manner. Moreover, over months of task learning and relearning, bats showed gradual, task-specific call amplitude adjustments that reflected their task performance. We posit that perceptual demands for echolocation efficiency over longer time scales may have facilitated the evolution of the rare VPL ability in echolocating mammals.