Zoology - Theses
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ItemColour pattern evolution in Australian dragon lizards (Agamidae)( 2011)Many animals communicate using elaborate visual signals such as conspicuous colours and patterns, which are influenced by numerous selective forces. However, the relative influences of natural and sexual selection on types of colour and pattern exhibited and the number and complexity of diverse colour pattern elements remain unclear for most taxa. In this study, I applied phylogenetic comparative approaches to examine the evolution of colour patterns in 69 Australian agamid lizard species, a morphologically and ecologically diverse group that relies primarily on visual communication. I examined colour patterns on nine body regions, which include dorsal, lateral, and ventral areas. I tested for associations with indices reflecting different selective pressures from both visual predators and conspecifics. Indices of natural selection included habitat openness, life style, substrate types, and ecological generalism, and my indices of sexual selection were the degree of sexual dichromatism and body and head size dimorphism (SSD). I found that both carotenoid/pteridine-based yellow-red colours and melanin-based black colour are likely to be used as sexual signals in male agamid lizards, as indicated by their association with two indices of sexual selection, the degree of sexual dichromatism and sexual size dimorphism. The use of both yellow-red and black sexual signals suggests that the costs associated with different colour production mechanisms do not limit the expression of sexual signals. Furthermore, the prevalence of different types of colour on different body regions potentially reflects different signalling strategies in this group. I then examined the evolution of colour pattern complexity in Australian agamid lizards. I developed a novel approach to quantify colour pattern complexity based on the Shannon-Wiener species diversity index. Sex differences in colour pattern complexity and the degree of complexity in males were associated with SSD and sexual dichromatism, respectively. These results suggest that the evolution of colour pattern complexity is driven primarily by sexual selection. Greater colour pattern complexity of males than females suggests that sexual selection has led to signal innovation in males, involving the use of additional and/or novel signal types, rather than elaboration of existing signals, which does not influence complexity. By contrast, I found no strong associations between any ecological factors and colour patterns or with the degree of colour pattern complexity, which suggests that the influence of natural selection on colour pattern evolution in this group is difficult to identify within a broad phylogenetic comparative study.