Factors Associated With Geographic Variation And Jamming Avoidance Response In Bat Echolocation Calls

Most social behaviours are dependent on intraspecific communication and the interaction between species and environment. Geographic variation in animal vocalizations provides a best opportunity to clarify the myriad factors shaping the evolution and divergence of communication signals and test fundamental hypotheses about the evolution of behaviour. Geographic variation in intraspecific acoustic signals has been observed in a range of animal groups, including invertebrates, birds and mammals. However, bats, as a biological indicator in the ecosystem, both the cause and the meaning of geographic variation in echolocation signals remains poorly understood despite some factors have been documented to explain variation in the ultrasonic frequency of bat echolocation within species among geographic isolates or islands. Therefore, study on geographic variation in echolocation calls of bats are very valuable for clarifying the mechanisms of speciation and the maintenance of biodiversity. Additionly, noise pollution may generate potential maladaptive responses of bats'echolocation, sexual selection and communication behaviours, and thus lead to direct bearings on breeding success and ultimately population growth rate. As a results, the study would provide scientific grounds for bats species conservation.The pattern of geographic variation and its impact factors in echolocation calls of the three bats species Hipposideros larvatus, Rhinolophus pusillus and Myotis daivdii were investigated by filed sampling, behavioral research and lab analysis on these aspects: morphology, ecology, culture and genetics from 2007 to 2010. In addition, jamming avoidance response (JAR) in Rhinolophus ferrumequinum with narrow-band echolocation pluses was studied originally at the Department of Integrative Biology and Physiology, University of California, Los Angeles, USA. The details are as followed.1. Variation patterns were examined in the resting frequency (RF) of echolocation calls emitted by the intermediate leaf-nosed bat, Hipposideros larvatus, on a broad geographical scale. Data mining technology and Kruskal–Wallis test both showed substantial variation with a longitudinal pattern in RF in H. larvatus among colonies, and this variation was associated with geographical distance not body size. In addition, we found that a high degree of variability between individuals was hidden under the geographical variation. The results support an effect of random cultural drift, and challenge the prey detection hypothesis. Moreover, acoustic differences among local island colonies may be indicative of a vocal dialect. We found that each colony of H. larvatus seems to maintain a'private bandwidth', which could be used for colony identity and individual communication thus helping individuals and colonies to get a number of fitness benefits. 2. We investigated variation patterns in the constant frequency of echolocation calls emitted at rest and when not flying ("resting frequency"RF) of the least horseshoe bat, Rhinolophus pusillus, on a broad geographical scale and in response to local environmental variables. Significant differences in RF were observed among populations throughout the species range in Mainland China, but this variation was not associated with geographical distance. Sex dimorphism in the RF of R. pusillus may imply that females and males at each site keep their frequencies within a narrow range for sex recognition. The Mantel test and the stepwise multiple regression model showed that bats adjusted resting frequency to local weather conditions and humidity, respectively, which may imply partitioning diet by prey size or the influence of rainfall noise. These results indicated that bats adjust echolocation call frequency to adapt to enviromental selection. Therefore, environmental selection might shape the diversity of echolocation call structure of R. pusillus in geographically separated populations. Simultaneously, we suggested that conservation efforts should focus on change in local climate change and environmental noise.3. Myotis davidii is a Chinese endemic species with wide geographic distribution. Kruskal–Wallis and Dunn's multiple-comparison test both showed substantial variation in call structure in M. davidii among colonies, and this variation was only associated with genetic structure not geographical distance and climate conditions. However, there were significant correlation between genetic structure and geographical distance, and genetic structure and climate conditions. Therefore, we suspected that the effect of climate conditions on call difference might be translated through genetic structure by a cascade reaction, as a bridge function. In addition, M. davidii, with high aspect ratios and low wing tip indices, and previous study both showed that David's Myotis migrates frequently within regions and a small quantity of long-distance colonization by individuals among regions. Thus, genetic drift and social selection might be a key factor for the diversification of call structure because echolocation calls can be influenced by conspecifics. Our results highlight the importance of migration in shaping the geographical patterns of echolocation call design, and imply that defining protected management units for conservation efforts should consider both genetic marker and ecologically adaptive traits.4. Pink noise significantly altered the echolocation behavior at rest by shifting the call frequencies, altering call amplitude and call stracture, such as the variation in dCF and frequency modulation (FM) component. Stronger noise stumuli yields a bigger shift in echolocation pulses, but the shift was only associated with noise amplitude instead of different echo attenuation. Interestingly, the strongest frequency shifts were not observed when interfering stimuli were centered on the resting frequency (RF1) but instead on the fundamental frequency (RF0). The result showed that horseshoe bats listen to RF0 in their own calls (rather than in the echoes) to maintain call frequencies at a stable level. In general, pink noise presented during Doppler-shift compensation (DSC) yielded similar effects on call frequency and noise amplitude than for calls emitted at rest. The most significant difference to calls emitted at rest was, however, that different echo attenuations yielded a very pronounced effect on DSC performance, showing an increase in DSC depth with increasing echo intensity. Therefore, Our results suggest, that not the signal-to-noise ratio, but instead the absolute amplitude of the returning echo is necessary and sufficient for the quality of DSC performance. We show for the first time that bats with CF calls have a specific jamming avoidance response (JAR) depending on the frequency band in which the interfering acoustical stimuli were presented. These findings may provide new insights into the complexity of echolocation behaviours and will aid in understanding evolutionary aspects of echolocation and DSC behavior.5. In brief, echolocation calls, like any other phenotype, evolve through the combined effects of drift ans selection. Expanding these scenarios, variability of acoustic signals can facilitate population divergence, reproductive isolation and, ultimately speciation. The constrain of noise will promate the adaptive evolution of echolocation strategies. These results are vary valuable for understanding adaptive evolution of echolocation behaviours and promoting species conservation in bats.