Urban noise and grey-headed flying-fox vocalisations: evidence of the silentium effect

We investigated whether urban noise affected communication behavior of grey-headed flying-foxes (Pteropus poliocephalus), a highly vocal, communally roosting megabat species that is increasingly inhabiting urban areas. We tested the hypothesis that no difference existed between the acoustic structure of calls of flying foxes in rural roosts with lower environmental noise and urban roosts with higher environmental noise, because the flying-foxes are conforming to the ultimate level hypothesis and calling to one another at maximum power levels. We recorded and compared sound pressure levels and dominant frequencies of colony sounds in roosts along a gradient from highly rural to highly urban as well as signal structure of individual flying-fox calls. No differences existed between the colony soundscapes (soundscapes with flying-foxes vocalizing) of the five roosts regarding amplitude or the dominant frequency of flying-fox vocalisations. No differences existed between highly rural roosts and highly urban roosts regarding the dominant frequency or syllable rate of individual grey-headed flying-fox courtship brays. However, flying-foxes temporarily ceased vocalizing when low aircraft fly-overs caused environmental noise levels to exceed colony noise levels by over 100%. Cessation of vocalizing with anthropogenic noise, termed here the silentium effect, has been noted in cetacean and anuran species. Our findings reveal that current typical urban noise levels do not appear to alter the acoustic structure of grey-headed flying-fox bray vocalisations, indicating that this species’ communication behaviour (i.e. extremely loud vocalising in close proximity to one another) may account for its tolerance of current anthropogenic noise pollution levels in urban habitats. However, high noise levels from airline traffic may affect communication behavior.