As a bat closes onto its flying prey, it changes its echolocation calls from relatively long, intense and slowly repeated “search” pulses to a shorter, fainter and more rapidly repeated train of pulses called the terminal “buzz”.  We (Fullard, Jeff Dawson and Dave Jacobs (2003) Journal of Experimental Biology 206, 281-294)) discovered that the ears of moths do not hear the terminal buzz pulses as well as they do the search calls and that this decrease in audibility could be exploited by the bat to reduce its acoustic conspicuousness.  The Northern long-eared bat, Myotis septentrionalis is unusual in the large proportion of moths that it includes in its diet and previous studies have explained this as a result of its ability to glean moths from vegetation.  A recent study by my former students, John Ratcliffe and Jeff Dawson ((2003) Animal Behaviour 66, 847–856)) however, showed that M. septentrionalis also feeds while on the wing, although its search calls do not differ much from those of non-moth feeding bats such as the Little Brown Bat, Myotis lucifugus.  We played the echolocation attack sequence of M. septentrionalis into the ears of two sympatric species of notodontid moths while we monitored the auditory nerve responses to see if this bat also reduces its conspicuousness at terminal phase of its attack.  Whereas the previous study suggested that an attacking bat effectively “disappears” 100 – 200 msec before it contacts the moth, the exposure trials of the calls of M. septentrionalis shows that this bat expands this period of cloaking to almost 1 second before contact.  We believe that this time will give the bat enough of an advantage to overcome the moth’s ear-based defence and allow it to increase the number of moths in its diet.
 
 

        The defensive behaviours of animals are both enabled and limited by the sensory systems that evoke them.  The evolution of defensive senses is usually complicated by the fact that other behaviours are influenced by these systems and it is difficult to specify the selective forces acting upon them.  The ears of nocturnal moths are an exception to this since for most species their only function is to detect the echolocation calls of foraging bats and avoid predation.  This specificity of function allows an examination of the evolutionary changes that occur to sensory systems and the defensive behaviours they govern if the selective force that originally shaped them disappears.  The south Pacific islands of French Polynesia represent the only habitat in the world that contains both a bat-free habitat and endemic species of moths that have evolved in the absence of the only selective force acting on their ears.  With funding from the National Geographic Society we have shown that, while the ears of Tahitian endemic moths are still neurologically responsive, the defensive flight of acoustic startle response they govern has disappeared.  These results suggest that the relatively small cellular investment of the peripheral nervous system outlasts the comparatively complex central neural architecture that integrates sensory signals with adaptive behavioural output.  Our study in 2005 examined the different response abilities of the two auditory receptors in endemic versus adventive noctuid moths with the prediction that the cell which evokes the ASR will show greater regression in moths who no longer require this potentially dangerous behaviour.