ETD: Target strength of bat prey in relation to pulse frequency and vegetation density in bat species, Rhinolophus fumigatus (Rüppell's horseshoe bat)

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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.