Spectral and temporal characteristics of echolocation calls in pregnant and lactating big brown bats

Abstract

While they are pregnant and rearing pups, bats continue to leave their roosts to forage for food. Many bats use echolocation vocalizations as part of this process. Other mammalian species including primates experience changes in vocal characteristics during pregnancy and lactation. As echolocation is a vital tool for spatial navigation and prey detection in most bats, investigating echolocation characteristics during pregnancy through lactation may provide new insight into how reproduction, pregnancy and pup rearing influence vocalizations. We measured changes in mass and recorded echolocation calls of pregnant (n = 21) and non-pregnant (n = 2) female wild-caught big brown bats (Eptesicus fuscus) released by hand into roost emergence-like flight. Recording began ~15 days prepartum and ended when the last bat reached 34 days postpartum, when pups were expected to be weaned. Analyses were completed using MATLAB and R, primarily with repeated measures ANOVAs focused on echolocation calls present in the ~562 ms before and ~562 ms after take-off. Based on vocal changes experienced by humans during pregnancy and post-birth, correlations found between bat echolocation call characteristics and the effects of differences in mass on bat echolocation, we predicted that female bats in late-stage pregnancy would emit calls of shorter duration, longer pulse interval, narrower bandwidth, and lower centroid frequency compared to calls emitted by the same bat post-parturition and compared to non-pregnant bats, while source level remained unchanged. We found that pulse interval and source level did not change while pregnant/lactating or control bats were in flight, and that increases in call duration and decreases in centroid frequency and bandwidth in flight began in pregnancy and continued through the lactation period while remaining unchanged for the control bats.