Sensory Ecology of Bats around Bodies of Water

Abstract

In this thesis I investigated how bats interpret the echo scene of their environment and more precisely of smooth, acoustic mirrors like e.g. bodies of water. I found that the echoacoustic recognition of a distinct habitat can be encoded with very simple cues and that it is hardwired, robust and innate. I showed that these echoacoustic cues are both sufficient and necessary for a bat in order to instigate drinking behaviour. Other sensory cues or previous experience with a locality alone are not enough. And finally I demonstrated that the same echoacoustic cues lead to an erroneous decision if they are found in a different spatial position and therefore might pose a conservation issue. For all my experiments I managed to confirm my hypotheses both in laboratory and field settings. Whenever possible, I further tried to generalize my results in using a comparative approach with different species.In chapter 1 I presented the results of laboratory experiments in our field station in Bulgaria and demonstrated how bats recognize bodies of water with echolocation. I showed that the cues for recognizing such an extended habitat structure can be relatively simple: any smooth, horizontal surface, acting like an acoustic mirror, is recognized as a water surface. This recognition pattern seems to be phylogenetically widespread as I was able to show it for 15 different European species. In an additional unpublished experiment we found the same behaviour for Neotropical bats as well. Very astonishing was the persistence of the bats’ drinking attempts, sometimes reaching over 100 attempts within ten minutes of flight time. I further explored and confirmed the robustness of this stereotypical behaviour in an additional experiment with a non-realistic physical situation for the smooth, horizontal surface. After repeating the main experiment under different conditions, I was able to suggest how bats value their different sensory inputs. Although bats apparently incorporate other sensory information, echolocation seems to be the dominating sense and even outweigh other contradicting information. To conclude this chapter, I showed that water recognition in bats is not learned but innate, which presents the first case of innate habitat recognition in a mammal.In chapter 2 I conducted experiments to illustrate the role of spatial memory and echo cues around water recognition. I created an experimental setup to see if bats would drink from an area which they before have experienced to be water, even if the relevant echo cues are missing. I demonstrated both in a laboratory setting in Bulgaria and a field experiment in the Jehuda desert of Israel that spatial memory alone is not sufficient to elicit drinking behaviour. The precise echoacoustic cues of a smooth, horizontal surface are necessary and the recognition hypothesis from chapter 1 could be confirmed in the field. Furthermore, these cues can even evoke a drinking response in a novel location where bats never experienced water before. Finally, these experiments also allowed me to prove that olfactory cues alone are not sufficient to stimulate bats to drink, adding to the multisensory hypothesis of chapter 1.In chapter 3 I demonstrated that acoustic mirrors can be sensory traps for bats. The same individual bats that would attempt to drink from a horizontal, smooth metal plate collided with a vertical, smooth metal plate, when passing at an acute angle. They likely perceived them as open flight paths. I grouped the behavioural response into three categories: bats that were on a collision course but managed to avoid the plate through evasive manoeuvres, bats that collided despite clear evasive manoeuvres and bats that collided without showing any reaction. Through 3D flight path analysis and echolocation recordings, I identified several factors that would contribute to such an erroneous decision. The amount of echolocation calls, the angle of approach and most importantly the time spent in a certain area close to the plate are the factors resulting in a collision when comparing the three reaction groups,. With increasing values all of them would increase the amount of information the bat receives and thereby reducing the risk of misinterpreting the echo scene. Supported by own field data and anecdotal reports of injured and dead bats around glass fronts of buildings, I argue that the detrimental effects of these sensory traps might be bigger than is known so far. To address this conservation issue I propose an increased monitoring effort to evaluate its real extent.In chapter 4 we showed in a collaborative work that sympatrically occurring trawling species exhibit a fine-scaled use of foraging niche around bodies of water. Although Myotis daubentonii and Myotis dasycneme have a strong overlap in diet and habitat utilization, small differences seem to allow them to occupy microhabitats within their general foraging guild around water. The analyses of functional morphology traits revealed small variances in wing parameters like wing loading and wingtip shape although sex had a stronger influence than species affiliation. Bite force, which could act as a proxy for prey choice and handling capabilities, showed more pronounced species differences, albeit expected as they were related to body size. A combination of classic, morphological and molecular diet analyses highlighted again the generally large overlap in diet but also pointed to small, consistent modifications in their prey choice. Overall the high similarity in morphological traits is as expected for trawling bats, confirms their niche affiliation but has only limited potential to explain the species’ sympatry. The dietary data, however, identified small differences like an apparent emphasis of Myotis dasycneme on Chironomids, increased preying on Chironomid pupae or the generally greater variety of consumed prey in Myotis daubentonii. In conclusion we highlight the importance of combining various methods to achieve a comprehensive understanding of a species’s foraging ecology.