Surviving cave bats: auditory and behavioural defences in the Australian noctuid moth, Speiredonia spectans

The Australian noctuid moth, Speiredonia spectans shares its subterranean day roosts ( caves and abandoned mines) with insectivorous bats, some of which prey upon it. The capacity of this moth to survive is assumed to arise from its ability to listen for the bats' echolocation calls and take evasive action; however, the auditory characteristics of this moth or any tropically distributed Australian moth have never been examined. We investigated the ears of S. spectans and determined that they are among the most sensitive ever described for a noctuid moth. Using playbacks of cave-recorded bats, we determined that S. spectans is able to detect most of the calls of two co-habiting bats, Rhinolophus megaphyllus and Miniopterus australis, whose echolocation calls are dominated by frequencies ranging from 60 to 79 kHz. Video-recorded observations of this roost site show that S. spectans adjusts its flight activity to avoid bats but this defence may delay the normal emergence of the moths and leave some 'pinned down' in the roosts for the entire night. At a different day roost, we observed the auditory responses of one moth to the exceptionally high echolocation frequencies (150-160 kHz) of the bat Hipposideros ater and determined that S. spectans is unable to detect most of its calls. We suggest that this auditory constraint, in addition to the greater flight manoeuvrability of H. ater, renders S. spectans vulnerable to predation by this bat to the point of excluding the moth from day roosts where the bat occurs.