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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:569781 |
| Date | January 2011 |
| Creators | Lee, Thomas James |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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