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The study of defensive colouration in animals, specifically aposematism whereby prey advertise their unpalatability with a bright warning signal, has served for over a century as an important case study in evolutionary adaptation. Though well studied, to date there are a number of questions and paradoxes which remain unresolved. This thesis presents a series of theoretical models aiming to re-appraise our understanding of warning signal evolution and optimality based on new theory, concepts and published empirical data. The first chapter of the thesis shows, contrary to existing theory, that the initial evolution of warning signals may have been facilitated by novel food wariness in predators and proceeds to explore a series of ecological scenarios that can help evaluate the generality of this argument. The second chapter develops and applies models from the first chapter to a spatially distributed metapopulation, finding that population structure, clustering, isolation and migration levels are key factors influencing the viable spread of aposematism as an anti-predator trait. The simulations show that the most favourable conditions for the spread of a novel mutant in a metapopulation are where aposematism first reaches fixation in one or more sub-habitats followed by low levels of outward migration. Chapter 3 focuses on determining the optimal defence strategies of prey given that aposematism is already established within a prey population, and specifically examines how conspicuousness may have become a reliable indicator of prey toxicity as suggested from recent empirical studies. Previous models show that positive correlations can arise when defence and aposematic display compete for a common resource. Chapter 3 presents new results to suggest that positive correlations between conspicuousness and toxicity can arise when toxicity and display deplete a common resource, irrespective of whether bright colouration is used as a warning signal or not, given that it provides some additional fecundity advantage e.g. from thermoregulation or sexual signalling. Finally, the second part of the chapter looks more closely at when prey should adopt aposematism over pure crypsis and finds that resource availability can influence this decision. Chapter 4 presents the first theoretical model to examine the optimality of combined warning signals with crypsis and provides a series of predictions for what ecological conditions may influence the combination of those key traits. A range of prey distributions were tested in a simulated two dimensional habitat. The results show that where prey are distributed in close spatial proximity to predators or where they are commonly viewed from a short distance, that the optimal strategy is to adopt pure aposematism with no combined display. For all other tested prey distributions some combination of warning display, cryptic colouration and defence proved optimal. Further testing showed that the combination of cryptic displays with conspicuous warning signals could extend the phenotype space over which a positive correlation is observed between defence level and warning signal strength, suggesting that combined displays may have important implications for models of signal honesty in aposematic prey. Finally chapter 5 investigates mimicry between prey species. This chapter presents the first model to explicitly consider the coevolution of mimetic appearance alongside evolution in defence levels in prey and in turn makes exciting new predictions about the dynamics of mimetic evolution. Three novel predictions are that 1. MUllerian eo-mimics may gain an additional advantage from mimicry in that they can reduce their toxin investment in their post-mimetic state. 2. That increased toxicity of the model, rather than shifting warning patterns may be a common outcome of Batesian mimicry. 3. That the post-mimetic evolution and optimization of toxin levels in both species can change the dynamics of the relationship from mutualistic to parasitic. These results could influence a paradigm shift in the understanding of mimetic systems and shape future theoretical and empirical studies on mimetic evolution.

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Chronically implanted semi-micro electrodes in cortical and subcortical areas of the squirrel monkey brain provided stable recordings of the potentials evoked by flash stimuli of frequencies in the range of 1-4 Hz, for periods in excess of one year. During habituation by repeated presentation of the flash stimulus, changes occurred in the amplitudes of the average evoked potentials (AEPs), most of which could be ascribed to changes in the level of arousal of the animals as assessed by the appearance of synchrony in the EEG. During conditioning and instrumental learning, when a fruit juice reinforcement became associated with the flash stimulus, further changes in the amplitudes and forms of the evoked potentials occurred which were ascribed to changes in the experimental situation and to the acquisition of the stimulus-reinforcement and the stimulus-response-reinforcement associations. Two subjects successfully completed a discrimination learning task in which discrimination of two flash rates was facilitated by a juice reinforcement for a lick response to the CS+ flash rate and an air-puff reinforcement for not withholding a response to the CS- flash rate. Changes in amplitude and forms of the AEPs were again related to situational novelty and to the acquisition of learning. No differences were observed in the AEPs derived from trials in which correct or incorrect responses were given. Presentation to one subject of flash rates intermediate to the CS+ and CS- rates resulted in behavioural generalisation. It was noted that the form of the AEP was dependent on whether the behavioural response was appropriate to the CS+ or the CS-. It was possible that this apparent 'readout from memory' was an artefact caused by sampling bias. Spectral analysis of the EEG following a juice reward failed to reveal EEG activity characteristic of the 'post reward synchrony' previously reported in rats.

The hermaphrodite, colonial ascidian <I>Diplosoma listerianum </I>(Chordata: Urochordata) mates by releasing sperm that disperse to neighbours, where they fertilise eggs that have been retained internally rather than spawned. The species is able to utilise highly dilute sperm: comparison with published information on a sea urchin, which released both eggs and sperm for external fertilisation, showed that <I>D. listerianum</I> maintained comparable levels of fertilisation at sperm concentrations two or three orders of magnitude lower than the echinoderm. Laboratory clones of <I>D. listerianum</I> displayed surprisingly high levels of sexual incompatibility. Fecundities of numerous pairwise crosses varied widely and suggested a continuous scale of computability. Although correlations of computability between reciprocal crosses appeared positive, considerable noise was present in the data and some crosses showed strongly asymmetrical compatibility. Patterns of sperm precedence with a five-day mating internal showed clear initial bias towards the first of two acting males. The proportion of second-male paternity (P<SUB>2</SUB>) subsequently increased with time. Estimated P<SUB>2</SUB> for entire progeny arrays was consistently greater than 0.5, but varied widely. When mating interval was reduced, mate order effects appeared to be moderated. In competition with an alternative sperm source, acting males fathered more progeny if previously mated to a particular female than if no mating history existed, an advantage probably derived from fertilisations by stored sperm. When virgin acting female colonies were given mixtures of sperm at widely divergent concentrations, offspring were shared between the two sperm sources in approximately the ratio of each mixture. However, there existed a small but statistically significant deviation from the fair raffle model, in that sperm at the lower concentration consistently achieved a greater than expected share of paternity. Environmentally-determined fixed female preferences could be responsible for this negative frequency dependence ('rare male effect').

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Free-flying groups of <I>P. auritus </I>were taken into captivity and fed on noctuid moths supplemented with mealworms. Roosting behaviour of lactating and non-reproductive bats was studied using infra-red sensitive video recording. Direct contact between mother and young declined from 90% of records on day 1 of lactation to 15% by day 50. There was a significant decline in grooming of the young, carrying behaviour and visits to the roost at night during lactation. Lactating females displayed significantly lower levels of total grooming activity (i.e. self grooming added to grooming of the young) compared to self grooming in non-reproductive females. The use of elimination rates of <SUP>65</SUP>Zn to measure individual food intakes, was validated in white mice <I>(Mus. sp.)</I>. Whole body elimination of <SUP>65</SUP>Zn followed a biexponential decline in counts over days 1 to 48 after labelling. Rate of turnover in the second phase of elimination, incorporating the variation in zinc body pool size, was the best predictor of food intake, and accounted for 60% of the variability over days 37 to 48. The <SUP>65</SUP>Zn elimination method, validated in mice, was used to estimate relative food intake in groups of bats. The rate of elimination of <SUP>65</SUP>Zn over the second phase of the biexponential decline was significantly greater for lactating females than non-reproductive females. This was most likely due to an increased food consumption by these bats. Measures of food consumption were obtained by subtracting weights of uneaten food from weights of food supplied to groups of bats comprising different numbers of lactating and non-reproductive bats. The average dry food consumption for non-reproductive and lactating bats was 1.52 and 2.05 g. bat<SUP>-1</SUP>.day<SUP>-1</SUP> respectively.