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In animal communication, individuals within a species often vary widely in their behavioral responses to species-typical signals. These variations in behavior may be due to differences in the sensory processing of communication signals. Sensory processing of behaviorally relevant stimuli is likely to be influenced by reproductive hormones. Here I report investigations on the influence of sex and reproductive condition on auditory processing in the green treefrog, Hyla cinerea. I conducted electrophysiological experiments that tested how sex and reproductive condition influence the neural representation of sounds in the auditory midbrain, the torus semicircularis. I found differences between and within the sexes that were both frequency-dependent (low vs. high frequency) and stimulus-dependent (tones vs. calls). For sex differences at auditory threshold, females were less sensitive to frequencies outside the spectral range of the male advertisement call and were not different from males inside the range. Sex differences were also stimulus-dependent with females more sensitive to the advertisement call than males. For stimuli consistent with close-range communication, I tested whether or not sex differences in response strengths to advertisement call and noise stimuli depended on the reproductive state of the female. I found that in response to low frequency stimuli postmated females had significantly reduced response strengths compared to males and unmated females. Additionally, I tested whether circulating reproductive hormones influenced auditory processing by manipulating androgen levels and assessing neural thresholds and response strengths to auditory stimuli. Elevated androgen levels in females resulted in increased thresholds and reduced response strengths but only in response to stimuli that are consistent with species-typical communication. Together the evidence from these studies suggest that sex and reproductive hormones influence auditory processing in a way that shapes the filtering properties of the auditory system for the detection of communication signals.

Current literature suggests that little, if any, research has been conducted in South Africa to determine the impact of alien invasive trout on indigenous amphibian biodiversity. The primary aim of the research project was to establish whether waterfalls are seasonally important in protecting the indigenous Natal Cascade Frog Hadromophryne natalensis tadpole populations from predation by alien rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta at two sampling areas located at Injesuthi and Monk‟s Cowl Nature Reserve within the uKhahlamba Drakensberg Park World Heritage Site (UDPWHS). The dissertation argues in favour of a biotic rationale, namely trout predation, as being the primary cause for the decline of H. natalensis tadpoles below the waterfalls and systematically negates the influence of geo-physical (abiotic) environmental factors on tadpole abundance. Habitat isolation and fragmentation is identified as a latent threat to the continued persistence of H. natalensis populations in the UDPWHS. / Environmental Sciences / M. Sc. (Environmental Science)

Current literature suggests that little, if any, research has been conducted in South Africa to determine the impact of alien invasive trout on indigenous amphibian biodiversity. The primary aim of the research project was to establish whether waterfalls are seasonally important in protecting the indigenous Natal Cascade Frog Hadromophryne natalensis tadpole populations from predation by alien rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta at two sampling areas located at Injesuthi and Monk‟s Cowl Nature Reserve within the uKhahlamba Drakensberg Park World Heritage Site (UDPWHS). The dissertation argues in favour of a biotic rationale, namely trout predation, as being the primary cause for the decline of H. natalensis tadpoles below the waterfalls and systematically negates the influence of geo-physical (abiotic) environmental factors on tadpole abundance. Habitat isolation and fragmentation is identified as a latent threat to the continued persistence of H. natalensis populations in the UDPWHS. / Environmental Sciences / M. Sc. (Environmental Science)