Novel acoustic lure methodology facilitates detection of the cryptic ghost bat at a landscape scale

Abstract

Context The conservation of the threatened ghost bat (Macroderma gigas) is currently hampered by a lack of standardised survey methodology for detecting the species away from known roosts. The low amplitude of the species’ echolocation call, together with only sporadic production of social calls while foraging, means that acoustic detection is unreliable. However, the ghost bat is known to be physically responsive to full-spectrum playback of its own social calls when these are broadcast in close vicinity (~100 m) to known roosts. Aims Using a two-phase investigation, we aimed to establish whether playback could be used to detect ghost bats away from the roost, in open woodland habitat in the Northern Territory. Methods In phase one, a trial of paired (active treatment and control) call playback experiments was conducted at three distances (1 km, 2.5 km and 5 km) from four known roosts using the ghost bat’s ‘squabble’ social vocalisation. Call playback, distance, roost and moon phase were used as explanatory variables in an information-theoretic approach using generalised linear models. In phase two, a 65 010-ha survey area was broken into a grid of adjacent hexagons with centroids spaced 5 km apart; replicate surveys using call playback were conducted at each centroid. An occupancy model was used to determine detection probability for the method in the survey area. Key results Ghost bats were successfully detected. In phase one, one model had substantial support and this contained only one variable: that of the presence or absence of the ‘squabble’ vocalisation. In phase two, ‘hotspots’ of ghost bat activity were detected in a region for which records were scant and predominantly historical. The occupancy model identified that 72% of sites were occupied and established a detection probability of 0.505, which provided a cumulative probability of 0.75. Conclusions Our results show that call playback is a reliable method of detecting the cryptic and threatened ghost bat at a landscape scale in the Northern Territory. Implications We suggest that call playback could be used as a standardised survey technique across the range of the ghost bat.