Weakly electric fish produce and detect electric fields and use their electrosensory modality in a number of behaviours including navigation and communication. They can modulate their electric discharge in frequency and amplitude to produce electrocommunication signals in variable patterns during social interactions. In one model neuroethological species, Apteronotus leptorhynchus, the most commonly produced communication signal is the ‘small chirp’ – a brief 10-30ms modulation. Individuals tend to produce these signals at high rates during agonistic interactions. In this thesis I will explore the social value of chirps, and to a lesser extent other communication behaviours, in A. leptorhynchus using a variety of experimental designs involving different staged social contexts. I use time series analysis methods to explore the patterns of chirps produced and accompanying aggressive behaviours. I first characterize electrocommunication and chirping in pairs of free swimming fish and correlate signal production with aggressive displays. Bursts of echoed, or reciprocated, chirps tend to be produced in the intervals separating aggressive attacks. Behavioural analysis shows that fish respond to conspecific chirps with echoed chirps and decreased aggression in social contexts outside the range in which previous modelling and electrophysiological data predicted that chirps could be encoded effectively. I then characterize the chirping and aggressive responses to playbacks simulating intruders with different chirping styles to test whether alternative chirp patterns differentially influence conspecific behaviour. In response to simulated intruders producing chirps that echo the real fish’s chirps with a short latency, less aggressive fish tend to produce more of their chirps in bursts than more aggressive fish. For randomly chirping intruders, the response of fish depends on the rate of chirps delivered. Fish respond less aggressively, with fewer chirps, and echo the stimulus chirps at a higher rate when high rates of random chirps are delivered than when responding to simulated intruders with low rates of randomly delivered chirps. Further, across all playback scenarios, fish that produce chirps in response to the playbacks are more aggressive than those that do not chirp. Finally, to better understand the electrosensory inputs during these interactions, I characterize changes in the electric image received by a restrained fish during movements of a free-swimming conspecific and correlate these with chirp production. When one fish is restrained, bursts of chirps tend to be associated with approach behaviours. Communication signals often function to promote individual assessment of potential rivals during agonistic encounters and bursty, antiphonal chirp exchanges may facilitate these assessments and deter potentially costly physical escalations.

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Animals coordinate their behavior with each other during cooperative and agonistic social interactions. Such coordination often adopts the form of “turn-taking”, in which the interactive partners alternate the performance of a behavior. Apart from acoustic communication, how turn taking is coordinated, is not well known. Furthermore, the neural substrates that regulate persistence in engaging in social interactions are poorly studied. Here, we use Siamese fighting fish (Betta splendens), to study visually-driven turn-taking aggressive behavior. Using encounters with real conspecifics and with computer animations, we discover the visual cues from an opponent and the behavioral dynamics that generate turn taking. Through a brain-wide screen of neuronal activity during aggressive behavior, followed by targeted brain lesions, we then discover that the caudal portion of the dorsomedial telencephalon, an amygdala-like region, promotes continuous participation in aggressive interactions. Our work highlights how dynamic visual cues shape the rhythm of social interactions at multiple timescales and points to the pallial amygdala as a region controlling the drive to engage in such interactions.

A surge of recent work elucidating a role for learning and memory in avian nest-building behaviour has challenged the long-standing assumption that nest building develops under genetic control. Whereas that work has been addressed at describing the cognitive mechanisms underpinning nest-building behaviour, almost nothing is known about either the neurobiological processes controlling nest building or the selection pressures responsible for the diversity in avian nest-building behaviour. Here, I sought to identify both the neural substrates involved in nest-building behaviour and some of those selection pressures. First, I used expression of the immediate early gene product Fos, an indirect marker of neuronal activity, to identify brain regions activated during nest-building behaviour in the brains of nest-building and control zebra finches (Taeniogypia guttata). I found that neural circuits involved in motor control, social behaviour, and reward were activated during nest building. Furthermore, I found that subpopulations of neurons that signal using the nonapeptides vasotocin and mesotocin and the neurotransmitter dopamine located within some of these neural circuits were also activated during nest building, suggesting these cell-signalling molecules may be involved in controlling nest-building behaviour. Next, I found that variation in the amount of folding in the cerebellum, a brain structure thought to be involved in manipulative skills, increased with increasing nest structural complexity, suggesting that the cerebellum is also involved in nest building. Finally, using evolutionary statistical models, I found support for the hypothesis that nest-site competition off-ground and increased predation pressure on the ground in Old World babblers (Timaliidae) led to the co-evolution of building domed nests on the ground. Here, then, I provide the first evidence of potential neural substrates controlling and selection pressures contributing to variation in nest-building behaviour.

Neuroethology and ecomorphology seek to understand ecology and behaviour from the perspective of specialised adaptations of sensory systems, such as vision. Sharks display a large variety of visual specialisations reflecting the diversity of different ecological niches they occupy. Many shark species are long-lived and wide ranging and often select different habitats for reproduction, growth, and feeding. Habitat complexity, ambient lighting conditions and feeding strategies can therefore change throughout a shark’s lifetime or between populations. Few comprehensive investigations of visual function exist for sharks as studies typically focus on a narrow aspect of visual function or a particular life history stage. Consequently, there is limited data on within-species plasticity of visual function in response to acclimation to different visual environments or ontogenetic development. The aim of this thesis is to undertake a functional analysis of the shark visual system. An integrated approach is employed to investigate optical, anatomical and physiological specialisations, linking such specialisations to known habitat and/or behavioural traits, with particular emphasis on ontogenetic, inter-population and inter-specific variability. Fundamental capabilities of the visual system are examined, including optical quality, eye morphology, spectral range, irradiance sensitivity, spatial and temporal resolution, contrast discrimination, and temporal and spatial summation. The main study species is the sandbar shark (<i>Carcharhinis plumbeus</i>; Carcharhinidae), a cosmopolitan species of ecological and economic importance. <i>C. plumbeus</i> occupies a wide range of natural habitats from highly turbid coastal estuaries, to relatively clear waters off the outer continental shelves and near pristine clear waters over the slopes of oceanic islands. This provides an opportunity to explore the relationship between habitat variability and the adaptation of visual specialisations and subsequent behaviour. For inter-specific comparison, the visual systems of two other species of shark with contrasting ecological niches are also assessed: the shortspine spurdog (<i>Squalus mitsukurii</i>; Squalidae) and the tiger shark (<i>Galeocerdo cuvier</i>; Carcharhinidae). The study finds marked differences in visual specialisations of the three species studied. The eyes of <i>S. mitsukurii</i> are adapted to enhance retinal illumination within a dim light environment with a large eye, immobile pupil, reflective tapetum and a relatively high optical sensitivity (2.72 μm² steradians). Visual features include a short wavelength lenticular filter, a high spatial resolving power (7.2 cycles/degree) and a large binocular overlap in the dorsal visual field, suggesting adaptations may facilitate the visualisation of bioluminescent prey. In contrast, the eyes of <i>C. plumbeus</i> are optimised for vision under variable light conditions with a mobile pupil and an occlusible tapetum. The sandbar shark shows an optical sensitivity of 1.11 μm² steradians. Visual resolution is highest in the lateral visual field, reaching a peak spatial resolution of 8.9 cycles/degree. An ERG derived spectral response curve for this species indicates maximal response to blue light between 460-490 nm. Interestingly, the tiger shark is maximally sensitive to a brighter range of light intensities compared to sandbar sharks, implying that tiger sharks occupy a more photopic light environment. However, sandbar sharks have a visual system with higher temporal resolution, as evaluated by the ERG response, (54 Hz) than tiger sharks (38 Hz). These results may reflect a difference in the importance of motion perception between <i>C. plumbeus</i> and <i>G. cuvier</i>. Phenotypic variability in visual function is shown between different populations of <i>C. plumbeus</i> occupying habitats with different ambient light conditions. This study provides new evidence of plasticity of visual function in response to acclimation to different visual environments within the same species. Sandbar sharks show an adaptive plasticity in visual sensitivity and temporal resolution, which appears to enable both temporal and population-specific adaptations to local light environments. In addition, the eyes of <i>C. plumbeus</i> and <i>S. mitsukurii</i> continue to grow even in adulthood. Visual performance, with respect to spatial resolving power and optical sensitivity, improve with eye growth. For example, peak spatial resolution increases with eye growth from 4.3 to 8.9 cycles/degree in <i>C. plumbeus</i> and from 5.7 to 7.2 cycles/degree in <i>S. mitsukurii</i>. These studies suggest that the light environment strongly influences visual function in this ancient class of vertebrates. Anthropogenically induced changes in water clarity may, therefore, impact on visually-mediated behaviours such as prey detection, agonistic signals or vertical migration. Anatomical and physiological parameters obtained from these studies provide a platform from which to model visual behaviours such as 1). Prey detection capabilities, 2). The impacts of water clarity on the limits of visually-mediated behaviour, and 3). The visual strategies that would allow sharks to maximise visual function, such as spatial and temporal summation under low light conditions. In conclusion, neuroethological studies can be a useful means to enrich information obtained from life-history and tagging studies and, together, can inform us of the functional role of sharks in marine ecosystems.

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