Browsing by Author "Jacobs, David"

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    ETD: Target strength of bat prey in relation to pulse frequency and vegetation density in bat species, Rhinolophus fumigatus (Rüppell's horseshoe bat)
    (2023) Freeks, Micaela; Janion-Scheepers, Charlene; Jacobs, David
    The relationship between echolocation and morphology is evident in several insectivorous bat species where a negative correlation between peak echolocation frequency and body size is observed. However, there are various exceptions to the general allometric relationship observed between body size and echolocation pulse frequency of both low-duty cycle and high-duty cycle bats. One such example in high duty-cycle bats is Rhinolophus fumigatus, where east African populations echolocate at a frequency of 55.1 ± 1.5 kHz, lower than that predicted by its body size (12.7 ± 0.9 g). The foraging habitat hypothesis states that a deviation from the allometric relationship between pulse frequency and body size is related to the foraging habitat and foraging style of the species and predicts a negative relationship between peak echolocation frequency and wing loading. Lower echolocation frequency penetrates dense vegetation more effectively than higher frequency pulses, resulting in greater energy for the generation of audible target strengths from the insect prey. Furthermore, a small body size allows manoeuvrable flight which is required for foraging in dense vegetation. The combination of low echolocation frequency and small body size, which represents a deviation from allometry between frequency and body size may be an adaptation for detecting and capturing, respectively, insect prey in dense vegetation. The target strengths produced by Rhinolophus fumigatus_East, Rhinolophus fumigatus_West (a sister lineage with a larger body size (18.8 ± 1.5 g) and similar echolocation frequency (55.1 ± 1.5 kHz)), and Rhinolophus capensis (a species of similar body size (12 ± 1.7 g) but higher echolocation frequency (84.8 ± 3.6 kHz) were measured in three relative vegetation densities and compared to determine if the deviation of Rhinolophus fumigatus_East from the general allometric relationship can be explained by the foraging habitat hypothesis. Moths were ensonified with semi-synthesized echolocation calls of the three bats in sparse, moderate, and dense vegetation densities and the returning echoes measured using Avisoft SASLab Pro. Target strengths were then calculated after accounting for atmospheric attenuation and non-parametric tests were conducted as the data did not meet the requirements for parametric tests, even after normalisation techniques were applied. Within lineage and species analysis showed no significant difference in target strength between the three vegetation densities. Between lineage and species analysis showed a significant difference between Rhinolophus fumigatus_East and Rhinolophus capensis in all three vegetation densities, for both high (HDC) and low duty cycles (LDC). However, within the series of various tests (where each lineage and species pulses were played consecutively) a significant difference exists between both R. fumigatus lineages and with R. capensis for both HDC and LDC pulses. In the series of natural pulses, a significant differences was found to exists between R. fumigatus_West and R. capensis for HDC pulses, and R. fumigatus_East and R. capensis for LDC pulses. When combining all the HDC and LDC data, a significant difference was found between R. fumigatus_East and R. capensis, and R. fumigatus_West and R. capensis. The results of the study do not support the foraging habitat hypothesis, and this may be due to Rhinolophidae being clutter forage specialists. Their echolocation pulses are already suited for clutter foraging and any slight deviations are unlikely to confer any additional benefit in prey detection. Allopatric divergence may explain R. fumigatus_East's deviation in echolocation frequency where extended periods of geographic isolation lead to natural and sexual selection on signalling systems (the sensory drive hypothesis) which allowed speciation to occur. Alternatively, R. fumigatus_East's deviation may also be caused by phenotypic plasticity as well as genetic differences. Additionally, this may have important implication for intraspecific communication, where studies have shown the role that echolocation plays in communication in bats. Other morphological traits may be better predictors of echolocation frequency (i.e., nose-leaf width, pinna size, and cochlea size) and although other studies have produced varied results, this provides avenues for further research.
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    Geographic variation in the phenotype of an African horseshoe bat species, Rhinolophus damarensis, (Chiroptera: Rhinolophidae)
    (2018) Maluleke, Tinyiko; Jacobs, David; Winker, Henning
    Studies involving geographic variation in the phenotypes of bats help scientists to explain why these mammals are the most species rich mammalian order second only to rodents, with well more than 1 300 species occurring worldwide. Such species richness or high diversity is the manifestation of the generation of biodiversity through the splitting of lineages within bat species. A lineage of bat species can diversify into several lineages which then differentiate from each other in allopatry. Thus, the spatial separation of a lineage into several lineages could be attributed to geographical, ecological and environmental factors across the distributional range of the species. Similarly, vicariant events may also play a role in separating lineages within species. The Damara horseshoe bat species, Rhinolophus damarensis, is widely distributed but restricted to the western half of southern Africa, where it occurs across several major biomes. Formerly regarded as the subspecies, R. darlingi damarensis, it was elevated to full species status on the basis of genetic and phenotypic differences between it and R. darlingi darlingi. Rhinolophus damarensis is itself made up of two ecologically separated genetic lineages. A total of 106 individuals of R. damarensis were sampled from seven localities across its distributional range, with a view to determining and documenting the extent of geographic variation in body size, echolocation parameters, wing parameters, cranial shape and postcranial morphology of individuals from populations of R. damarensis across the distributional range of the species. Firstly, an investigation into geographic variation in resting echolocation frequency (RF) of the horseshoe bat species, R. damarensis was carried out in the western half of southern Africa (Chapter 2). Three hypotheses were tested. The first one, James’Rule (JR), states that individuals occurring in hot humid environments generally have smaller body sizes than conspecifics that occur in cooler, dryer environments, and the largest are expected to occur in cool, dry areas. On this basis and because of the known relationship between body size and RF, it was predicted that there should be a correlation between body size and climatic factors and between body size and RF. The second hypothesis was Isolation by Environment (IbE) mediated through sensory drive, which proposes that diversification of lineage may be driven by environmentally-mediated differences in sensory systems. Under this hypothesis, it was predicted that call frequency variation should be correlated with climatic variables. The third hypothesis was that Isolation by Distance (IbD) can influence phenotypic trait variation by restricting gene flow between populations. Under the Isolation by Distance (IbD) Hypothesis, it was predicted that call frequency variation should be partitioned in accordance with geographic distance between populations. To investigate the probability of the JR, IbE and IbD, the Akaike’s information criterion AICc candidate models were evaluated with different combinations of environmental (annual mean temperature and relative humidity), spatial (latitude and region) and biological (forearm as a proxy for body size) predictor variables to determine their influence on resting frequency (RF) across the distributional range of R. damarensis. Linear mixed effects models (LMEs) were employed to analyse the relationship between the response variable (RF) and the environmental, spatial and biological predictor variables. The influence of prey detection range and atmospheric attenuation was also investigated. The results showed no evidence for JR or for random genetic drift. Body size was neither correlated with RF nor environmental variables, suggesting that variation in RF was not the result of concomitant variation in body size as proposed by JR. Similarly, the Mantel test showed no IbD effect and there was therefore no evidence that genetic drift was responsible for the variation in RFs. In contrast, the LMEs showed that there was support for IbE in the form of an association between RF and region (in the form of the variable “Reg”) which was based on the two geographically separated genetic lineages. Furthermore, RF variation was also associated with the climatic variable AMT. The taxonomic status of R. damarensis was investigated using ecological traits and phenotypic characters including skulls, wings and echolocation (Chapter 3) and three dimensional (3D) scanned skulls and mandibles (Chapter 4). The main objective (Chapter 3 and Chapter 4) was to test whether previously reported genetic divergence between the two R. damarensis lineages was associated with phenotypic divergence. Morphometric and echolocation measurements were taken from hand held individual bats in the field, and skull measurements were taken from field collected voucher specimens as well as museum specimens. Discriminant Function Analyses (DFA) revealed that there was geographic variation among populations and lineages of R. damarensis. Multivariate Linear Regressions (MLV) and Linear models (LM) on the basal parts of bacula revealed significant differences between the southern and northern lineages of R. damarenis. The bacula of the two lineages of R. damarensis appear to have different shapes. Diversification through shape analyses (Chapter 4) was investigated using three dimensional (3D) geometric morphometric analyses based on X-ray microcomputed tomography (µCT) scanning of dried skulls and mandibles of R. damarensis. Procrustes Anova results of both mandibles and skulls indicated that there were no significant differences between sexes but that the shape of skulls and mandibles varied across different localities (Chapter 4). Canonical Variate Analysis (CVA) suggested that geographic variation in R. damarensis mandibles was based on the shape and thickness of the alveolar bone. Geographic variation in the skulls was based on changes in the rostrum, anterior medial swelling and brain case. Some populations had slightly deeper rostra, slightly larger anterior medial swellings and smaller braincases, whilst others had slightly shallower rostra, slightly smaller anterior medial swellings and larger braincases. The northern lineage was found to be separated from the southern lineage based on the changes in skull and mandible shape. Therefore, separation of lineages within R. damarensis (Chapter 4) could be associated with the foraging and feeding behaviour of the species under different ecological conditions due to ecological opportunity. Overall, differences in the RF were found to be associated with Isolation by Environment mediated through sensory drive and this has led to the formation of two regional (northern and southern) groupings in RF (Chapter 2). The two lineages were supported by both the phenotypic divergence (Chapter 3) and shape variation within R. damarensis skulls and mandibles (Chapter 4). Thus, phenotypic differences corresponded to genetic differences between the two lineages and provide support for IbE.
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    Population substructuring in Schreibers' long-fingered bat (Miniopterus schreibersii) in South Africa
    (2001) Miller-Butterworth, Cassandra Michaela; Jacobs, David; Harley, Eric
    Schreibers' long-fingered bat, Miniopterus schreibersi migrates seasonally between winter (hibernacula) and summer (maternity) colonies in South Africa. Previous behavioural studies suggested that roost fidelity is well developed in this species, and that juvenile dispersal may be limited, possibly in both sexes. If males and/or females are strongly philopatric, this may lead to restricted gene flow among colonies, resulting in genetically distinct breeding subpopulations. The population structure of M. schreibersii in South Africa was investigated using microsatellites and mitochondrial DNA (mtDNA), with the aim of determining the degree of genetic differentiation among colonies, and the extent and direction of bat movement among the colonies. A genomic library was constructed for M. schreibersii, and was screened for (CA)0 and (GA)0 microsatellite repeats. Five novel, highly polymorphic loci were identified. These five loci, and an existing mammalian microsatellite locus, were amplified in. 301 individuals, sampled from ten colonies throughout South Africa. Significant genetic heterogeneity exists within the M. schreibersii population, such that the population can be subdivided into three partially discrete breeding subpopulations. Little genetic differentiation exists between colonies within
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    Variation in wing area and prey detection volume of Rhinolophus Capensis in response to different climates
    (2021) Duncan, Aurora; Jacobs, David
    Wing shape and echolocation are two novel adaptations in the Chiroptera and are strongly influenced by environmental conditions. Wing shape is influenced by environmental clutter. Shorter, broader wings allow for more maneuverable flight, and are advantageous for bats living in highly cluttered environments. Longer, narrower wings help bats to increase flight speed, and are best suited for bats living in more open environments. It is likely that wing shape is also influenced by temperature, given the potential for wings to act as thermoregulatory appendages. Wings provide a thermal gradient across their surfaces, dissipating excess heat from the body. However, the importance in thermoregulation in determining wing size is unknown. If thermoregulation is a strong selective pressure, bats in hotter, more arid regions should have larger wings. Environmental conditions also influence echolocation pulse design. Echolocation pulses must successfully reach a target and generate an audible echo despite atmospheric attenuation. High-duty cycle (HDC) pulses, calls with longer durations than the interval between them, are particularly useful in environments with high amount of environmental clutter. HDC echolocators use an acoustic fovea and Doppler shift compensation to detect the fluttering wings of insect prey in dense vegetation. However, the flexibility of these pulses is limited by the bat's acoustic fovea. Wing shape and echolocation combined form an adaptive complex, providing bats with a highly specialized system of foraging. Climate change poses an enormous risk to a bat's foraging success, because rising ambient temperatures are likely to change the selective pressures on wing size (due to the potential thermoregulatory benefits) as well as prey detection volumes of the bat's echolocation (because sound propagation is influenced by temperature). As an adaptive complex any selection on either wings or echolocation is likely to influence changes in the other, with consequences for the foraging efficiency of bats. The potential impact of climate change on the foraging efficiency of bats can be gauged by the bats' adaptive responses to different climatic conditions over their geographic range. I examined these two traits in different localities across the geographic range of the Cape horseshoe bat, R. capensis to determine if wing and echolocation parameters are adapted to current climatic conditions. I measured wing area and echolocation parameters at sites within the distribution of R. capensis that were representative of the different climates across its range. I measured wing areas using digital image analysis software, and I measured echolocation parameters using a microphone array system. Temperature was a predictor in the top fitting linear mixed effects models for both wing area and prey detection volume. For differences in wing area, body mass was the only significant explanatory variable. However, body mass may itself be influenced by environmental conditions. NDVI, latitude, and average winter minimum temperature significantly related to differences in prey detection volume. My results indicate geographic variation in both wing area and prey detection volume, an indication that these traits are adapted to local climate conditions. Geographic variation in wing area is a consequence of body mass, which may or may not be a function of climate. However, geographic variation in prey detection volume is directly influenced by the environment. Therefore, increases in ambient temperature due to human-induced climate change are likely to have an effect on the foraging efficiency of R. capensis.