2141 INTRODUCTION The aerobatic spirals, loops and dives moths initiate when their simple ears detect the echolocation cries of a nearby bat are not the only defences they employ against their acoustic predators. Tiger moths (family: Arctiidae) answer the echolocation attack of bats with ultrasonic clicks broadcast from bilateral metathoracic structures called tymbals (for a review, see Miller and Surlykke, 2001). Active debate over the functions of these sounds has produced three main hypotheses: startle, jamming and warning. Although some evidence exists for both startle (Bates and Fenton, 1990; Miller, 1991) and jamming (Miller, 1991; Masters and Raver, 1996; Tougaard et al., 1998; Tougaard et al., 2004), acoustic aposematism appears to be the major function of tiger moths’ acoustic reply to bat attack (Bates and Fenton, 1990; Hristov and Conner, 2005a; Ratcliffe and Nydam, 2008). A recent study (Hristov and Conner, 2005a) pitted naïve big brown bats (Eptesicus fuscus) against four naturally occurring species of tiger moths that varied in a pair of characters: presence or lack of a chemical defence, and ability or inability to produce sound. The learning profiles of capture success over seven nights showed that the bats failed to learn to avoid chemically protected moths unless those moths also provided an acoustic warning. Furthermore, moths that produced ultrasound in response to bat attack, but were not chemically protected, were captured and eaten by the bats. These results clearly support the warning model and demonstrate that the combination of chemical defence and acoustic warning is necessary to allow bats to associate moth-produced ultrasound with unpalatability. We have extended this work by demonstrating that these moth warning sounds function in acoustic mimicry complexes (Barber and Conner, 2007). After experience with a noxious sound- producing model tiger moth species, naïve red (Lasiurus borealis) and big brown bats avoid a second sound-producing species of tiger moth offered to them, regardless of whether it is chemically protected or not, demonstrating both Müllerian and Batesian mimicry in this acoustical system. In other words, the bats generalize the meaning of these prey-generated sounds to a second tiger moth species producing a different call. A subset of the red bats in these experiments discovered the palatability of the Batesian mimic and began eating these moths. These same red bats displayed the ability to discriminate between the palatable acoustic mimic and the unpalatable acoustic model, when the model was reintroduced. The metric used in the behavioural study discussed above (Barber and Conner, 2007) was the percentage of moths captured each foraging night. In order to elucidate the spatial and temporal specifics of decision making by the bats in these experiments we filmed each interaction with two synchronized high-speed video cameras that were calibrated with Direct Linear Transformation techniques to extract three-dimensional (3-D) kinematics from the trials (Abdel- Aziz and Karara, 1971; Chen et al., 1994; Hedrick, 2008). In addition, the acoustics of each bat–moth interaction were also recorded with an ultrasonic microphone. Here, we report that this detailed analysis of behaviour reveals that naïve red and big brown bats display a fine-scaled level of prey discrimination, despite generalizing the aposematic meaning of arctiid moth warning signals across multiple species. The Journal of Experimental Biology 212, 2141-2148 Published by The Company of Biologists 2009 doi:10.1242/jeb.029991 Naïve bats discriminate arctiid moth warning sounds but generalize their aposematic meaning Jesse R. Barber 1, *, Brad A. Chadwell 1 , Nick Garrett 1 , Barbara Schmidt-French 2 and William E. Conner 1 1 Department of Biology, Wake Forest University, Winston-Salem, NC 27109, USA and 2 Bat Conservation International, Austin, TX 78746, USA *Author for correspondence at present address: Colorado State University, Department of Fish, Wildlife and Conservation Biology, Fort Collins, CO 80523, USA (e-mail: barber.jesse@gmail.com) Accepted 31 March 2009 SUMMARY Naïve red (Lasiurus borealis Müller) and big brown (Eptesicus fuscus Beauvois) bats quickly learn to avoid noxious sound- producing tiger moths. After this experience with a model tiger moth, bats generalize the meaning of these prey-generated sounds to a second tiger moth species producing a different call. Here we describe the three-dimensional kinematic and bioacoustic details of this behaviour, first, as naïve bats learn to deal with an unpalatable model tiger moth and subsequently, as they avoid acoustic mimics. The tiger moths’ first clicks influenced the bats’ echolocation behaviour and the percentage of interactions that included terminal buzzes was associated with capture and investigatory behaviour. When the mimic was introduced, the bats decreased both their minimum distance to the tiger moth and the time at which they broke off their attack compared with their exposure to the model on the night before. These kinematic signatures closely match the bats’ behaviour on their first night of experience with the model. Minimum distances and time of pursuit cessation increased again by the last night of the mimic’s presentation. These kinematic and bioacoustic results show that although naïve bats generalize the meaning of aposematic tiger moth calls, they discriminate the prey-generated signals as different and investigate. Extrapolating to experienced bats, these results suggest that acoustic predators probably exert potent and fine-scaled selective forces on acoustic mimicry complexes. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/212/14/2141/DC1 Key words: kinematics, bioacoustics, bats, moths, Arctiidae, acoustic mimicry. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY