Browsing by Author "Jacobs, David Steve"

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    Listening carefully: increased perceptual acuity for species discrimination in multispecies signalling assemblages
    (Elsevier Ltd., 2015-03-01) Bastian, Anna; Jacobs, David Steve
    Communication is a fundamental component of evolutionary change because of its role in mate choice and sexual selection. Acoustic signals are a vital element of animal communication and sympatric species may use private frequency bands to facilitate intraspecific communication and identification of conspecifics (acoustic communication hypothesis, ACH). If so, animals should show increasing rates of misclassification with increasing overlap in frequency between their own calls and those used by sympatric heterospecifics. We tested this on the echolocation of the horseshoe bat, Rhinolophus capensis, using a classical habituation-dishabituation experiment in which we exposed R. capensis from two phonetic populations to echolocation calls of sympatric and allopatric horseshoe bat species (Rhinolophus clivosus and Rhinolophus damarensis) and different phonetic populations of R. capensis. As predicted by the ACH, R. capensis from both test populations were able to discriminate between their own calls and calls of the respective sympatric horseshoe bat species. However, only bats from one test population were able to discriminate between calls of allopatric heterospecifics and their own population when both were using the same frequency. The local acoustic signalling assemblages (ensemble of signals from sympatric conspecifics and heterospecifics) of the two populations differed in complexity as a result of contact with other phonetic populations and sympatric heterospecifics. We therefore propose that a hierarchy of discrimination ability has evolved within the same species. Frequency alone may be sufficient to assess species membership in relatively simple acoustic assemblages but the ability to use additional acoustic cues may have evolved in more complex acoustic assemblages to circumvent misidentifications as a result of the use of overlapping signals. When the acoustic signal design is under strong constraints as a result of dual functions and the available acoustic space is limited because of co-occurring species, species discrimination is mediated through improved sensory acuity in the receiver.
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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.