Greater horseshoe bat (Rhinolophus ferrumequinum) ultrasound calls outside a nursery roost indicate social interaction not light sampling

Abstract

Detailed observations of greater horseshoe bat (Rhinolophus ferrumequinum) excursions around the exit hole and surrounding archway, of a nursery roost in West Wales, were made with simultaneous video and ultrasound recordings. The number of flight excursions in July and October around the exit hole (0.42, 0.76, fe/hr/bat; respectively) and the archway (2.17, 0.51, fe/hr/bat; respectively) were not proportional to differences in light levels during emergence (- 17.74%, -94.80 %; respectively). Flight excursions during the night in July around the exit hole (5.67 fe/hr/bat) and the archway (9.89 fe/hr/bat) exceeded those during emergence even though the difference in light level was negligible (-0.05%). Typical echolocation calls at 83-84 kHz were recorded together with social calls that had the fundamental frequency in the range 15-39 kHz and harmonics 2-7. The social call rates during the exit in July (0.11 calls/hr/bat) and October (0.14 calls/hr/bat) were not proportional to the difference in light levels and the social call rate was higher in the dark (6.97 calls/hr/bat). In July and October complex individual recognition calls and advertisement calls made during excursions outside the nursery roost demonstrated social interaction. The findings from this study demonstrate that R. ferrumequinum flight excursions outside a nursery roost have a social function and are not simply light sampling.