Browsing by Author "Holderied, Marc W"

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    The influence of foraging habitat on acoustic signal source levels in two bat species, Neoromicia capensis (Vespertilionidae) and Tadarida aegyptiaca (Molossidae)
    (2018) Mutavhatsindi, Itani Victor; Jacobs, David S; Holderied, Marc W
    The source level of echolocation pulses is an essential parameter because it has an impact on the range at which bats perceive their environment and, most importantly, at which they detect prey. Echolocation pulse source level is under the control of the echolocator and its operational range is likely to vary with body size and foraging habitat because these determine the operational range needed by the bat. This study thus attempted to answer the following questions; (1) is the source levels of animal acoustic signals different in different situations? (2) Does body size and foraging habitat affect the source levels of animal acoustic signals? There are only a few studies that report on the source levels of echolocation pulses of free-ranging bats because of the difficulty of measuring the distance of the bat from the recording microphone. This distance is essential in calculating source levels of echolocation pulses. I used multiple microphone array system to investigate the echolocation sound signals, three-dimensional (3-D) acoustic flight paths and source levels of echolocation pulses at 10 cm standard reference distance from the mouth of a free-flying frequency-modulated (FM) bat, Neoromicia capensis and quasi-constant frequency (QCF) bat, Tadarida aegyptiaca. The two bat species differ in body size and foraging habitat. I found as predicted, that T. aegyptiaca, the larger of the two species and an open-air aerial hawker, emitted echolocation pulses of higher source levels and therefore had greater maximum detection distances than the smaller, clutter-edge aerial hawker, N. capensis. Tadarida aegyptiaca emitted echolocation pulses with an average of 146.9±4.6 (range 137.7 - 154.8) dB peSPL during emergence from the roost and an average of 143.0±4.9 (range 136.4 - 153.3) dB peSPL during foraging, extending the known range of free-flying bats. Neoromicia capensis emitted echolocation pulses with an average of 129.3±4.0 (range 119.3 - 138.7) dB peSPL during emergence from the roost and average of 132.8±5.0 (range 117.8 - 142.9) dB peSPL during foraging. As anticipated, I found N. capensis to be emitting echolocation pulses of higher source levels during foraging than when it emerged from the roost. However, there was no significant difference in the source levels of T. aegyptiaca between emergence from the roost and foraging habitat. The estimated maximum detection distances for the three insect size categories i.e. small, medium and large was greater for T. aegyptiaca than N. capensis. My data therefore suggests that bats might adjust their echolocation pulse source levels to suit their foraging habitat and situation.
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    Variation of echolocation pulse source levels and detection distances for bat assemblages across an environmental gradient: “a test of the acoustic adaptation hypothesis”
    (2022) Wechuli, David Basara; Jacobs, David Steve; Holderied, Marc W
    The use of multiple microphone arrays to measure echolocation pulse source levels of free-flying bats does not allow one to determine the species of the bat being recorded. However, the echolocation pulses can be assigned to species based on pulse parameters used in conjunction with a reference library of pulses, the distribution records of bat species and the identification of captured individuals sampled in the area of recording (Chapter 2). Echolocation pulses were recorded as bats emerged from their own roosts, using the multiple microphone array system. Several parameters were measured from pulses within each echolocation sequence to identify a representative pulse type for each species. These initial species assignations were confirmed through multivariate analyses so that source level of echolocation pulses could be assigned to species. Source levels used by bats impacts on the distance at which bats perceive their targets like prey in their habitats. Habitat and prevailing climatic conditions present different challenges for echolocation systems, and so the quality and content of information derived from echolocation pulse reflects these environmental challenges. Hence, echolocation pulses within or between species may vary from one habitat to the next due to variable selection pressure, resulting in local adaptation as formalised in the Acoustic Adaptation Hypothesis, which proposes that acoustic properties of the environment influence sound propagation and ultimately the evolution of echolocation pulses. To test the Acoustic Adaptation Hypothesis, I used multiple microphone arrays to measure the source levels of echolocation pulses of fourteen bat species in several bat assemblages across seven sites in different biomes in South Africa. Source levels generated from echolocation pulses, together with frequency and weather parameters were used to calculate detection distances (Chapter 3). In Chapter 4, detection distances were calculated using long-term climate data of 40 years, which is the same data used to assess whether predictive models could explain detection distances. In both chapters, the resultant detection distances were used to test the predictions of the Acoustic Adaptation Hypothesis. Results show that bats in the same assemblage used different echolocation pulse source levels and frequencies resulting to different detection distances, which differ among bat assemblages occupying different sites. Detection distance is species-specific and remained similar within species between assemblages, hence species is a better predictor of detection distances than site as indicated by Miniopterus natalensis across sites in biomes (Chapter 3). Results in Chapter 4 show that bats belonging to the same assemblage used different echolocation pulse source levels and frequencies resulting to different detection distances, which differ among bat assemblages occupying different sites under the prevailing climatic conditions. Detection distances between sites were different only in some sites, suggesting that the AAH was partially supported. Detection distance is species-specific and remained similar within species between assemblages, hence species is a better predictor of detection distances than climatic conditions as indicated by Miniopterus natalensis across sites. Detection distances for bat assemblages were correlated with temperature and longitude, whereas for Miniopterus natalensis, they were correlated with longitude, providing partial support for the Acoustic Adaptation Hypothesis. Detection distances were however not correlated with relative humidity, atmospheric pressure and latitude. Because temperature may change at different longitudes owing to diverse geographical features that affect atmospheric circulation, it suggests that temperature is the most important climatic variables that impacts echolocation and any human induced climate change that results in changes in temperature are likely to impact the survival of bats.