Complex dynamics of ageing processes in snow skinks

Central to evolutionary biology is understanding how and why organisms age. Biological ageing of organisms (senescence) is associated with a decline in function and an increase in mortality which is underpinned by processes occurring at the cellular level, potentially through declines in telomere length. Telomeres are protective sequences of nucleotides on the ends of DNA, which prevent errors in recombination. Senescence has been documented in a number of natural populations, revealing that the link between telomeres and ageing is highly nuanced and that patterns of ageing vary between species, individuals and populations. However, this research has been heavily biased towards birds and mammals. To fully understand the complexity of ageing, it is crucial that we broaden the taxonomic scope of ageing studies, focussing on non-model systems such as plants, insects and other ectotherms. Reptiles, as vertebrate ectotherms, therefore, represent a highly significant group in the study of senescence. For valid conclusions on the patterns and mechanisms of senescence, high-quality, long-term longitudinal data from wild populations are required coupled with detailed experimental manipulations. The long-term study of the spotted snow skink (Niveoscincus ocellatus) in Tasmania provides an excellent resource not only to document how individuals age within a population, but also to identify the extrinsic and intrinsic factors that have the most impact on ageing. This thesis combines long-term wild population data with experimental manipulations and state-of-the-art molecular approaches to investigate the factors that influence patterns of senescence from the cellular to the population level. I first explored broad patterns of ageing and their links to life history and telomere dynamics using long-term data. At the organism level, life history traits were shown to have pervasive and context-dependent effects on telomere dynamics in N. ocellatus both within and between populations situated at the climatic extremes of the species distribution. I then examined specific mechanisms that might be responsible for those patterns of senescence within individuals using targeted experiments. At the cellular level, I found significant site differences in telomere length and oxidative stress between the two populations and in the change in telomere length with thermal treatment, but not the negative relationship expected between oxidative stress and telomere attrition. This revealed that the relationships between temperature, metabolism, oxidative stress and telomere dynamics are not straightforward and future work incorporating detailed understanding of the thermal reaction norms of these and other linked traits is needed to fully understand these processes. I examined the cellular effects of tissue regeneration to further unpick the relationships between metabolism, cell division, oxidative stress and telomere dynamics. The regeneration of tail tissue proved to involve a response to oxidative stress that comes at a cost to telomere repair, demonstrating a potential long-term cost of tail regeneration beyond the re-growth of tissue itself. At the population level, I showed that female spotted snow skinks experience actuarial senescence, but no reproductive senescence and the ageing patterns appear to be less influenced by the extrinsic environment than predicted. Combined, the findings of my thesis highlight the plasticity of senescence as well as the complexity of links between environment, life history, ageing and telomere biology in an ectothermic species, and demonstrate how an ectothermic system can inform evolutionary theories of ageing.