To better understand the importance of social activity, ultrasonic calls made by lesser horseshoe bats (Rhinolophus hipposideros) in North Wales, U.K. were recorded during extended pre-set periods since 2010 inside and outside nursery roosts from May to July and in a hibernaculum in October. Ultrasonic calls with fundamental frequencies between 15–42 kHz were identified as seven categories of infant development calls and 15 categories of adult ultrasonic social calls according to the frequency and duration of the fundamental, the number of harmonics and the number of calls in a sequence. Activities were monitored remotely using infrared video cameras with simultaneous time expansion sound recorders. Distinctive polyharmonic isolation calls of newborn R. hipposideros enabled timing of births to be determined and infant echolocation marked the time when they started to fly. Trill advertisement calls in the hibernaculum indicated mating behaviour in October. Comparison of call-triggered recording times enabled variable levels of activity to be determined quickly. The longest sonogram times were in July (50.2 min/h) and in June (49.1 min/h). During these recordings, 95 and 90 adults returned to the roost after foraging and there were 16 and 42 babies, respectively. In May sound and video recordings outside a nursery roost revealed ultrasonic social calls during the emergence of 200 adult R. hipposideros when the light level was low at two lux, which showed that R. hipposideros were engaged in social activity, not light sampling. Surveys of ten nursery roosts showed that this social behaviour was typical of R. hipposideros in Wales.
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