Development of echolocation and communication vocalizations in the big brown bat, Eptesicus fuscus

Big brown bats form large maternity colonies of up to 200 mothers and their pups. If pups are separated from their mothers, they can locate each other using vocalizations. The goal of this study was to systematically characterize the development of echolocation and communication calls from birth through adulthood to determine whether they develop from a common precursor at the same or different rates, or whether both types are present initially. Three females and their six pups were isolated from our captive breeding colony. We recorded vocal activity from postnatal day 1 to 35, both when the pups were isolated and when they were reunited with their mothers. At birth, pups exclusively emitted isolation calls, with a fundamental frequency range < 20 kHz, and duration > 30 ms. By the middle of week 1, different types of vocalizations began to emerge. Starting in week 2, pups in the presence of their mothers emitted sounds that resembled adult communication vocalizations, with a lower frequency range and longer durations than isolation calls or echolocation signals. During weeks 2 and 3, these vocalizations were extremely heterogeneous, suggesting that the pups went through a babbling stage before establishing a repertoire of stereotyped adult vocalizations around week 4. By week 4, vocalizations emitted when pups were alone were identical to adult echolocation signals. Echolocation and communication signals both appear to develop from the isolation call, diverging during week 2 and continuing to develop at different rates for several weeks until the adult vocal repertoire is established.