School of BioSciences - Theses
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ItemThe temperature and water dynamics of reptiles in changing climates( 2017)The dynamics of heat and water flux through the biosphere are fundamentally interconnected and together determine environmental and biological processes across vast scales. However, despite the physical ties between heat and water dynamics, the biological implications of temperature change are most often considered independently of precipitation change. Thermal stress is predicted to drive global extinctions of ectothermic species as high as a 20% species loss. Yet these predictions disregard the potential for simultaneous changes in precipitation and aridity to cause additional stress for ectotherms. With few exceptions, water loss rates of ectothermic species are positively correlated with temperature and can be substantial, meaning hydric stress is expected to increase with thermal stress. My goal was to determine how species might respond to hydric stress brought on by changing climates. Using a combination of empirical and modelling approaches, I investigated the influence of water budgets on the physiology and behaviour of ectothermic species. I begin by describing a mechanistic model of evaporative water loss rates, an important part of an ectotherm’s water budget, which can be parametrized with species specific physiological data. I validated the model’s predictions of evaporative water loss rates against a database of experimentally collected data for 39 species of squamate reptiles. I also compiled existing data which can be used to fit water budget models for an additional 103 reptile species. Next, I applied an integrated heat and water budget model to predict the influence of several physiological and behavioural hydroregulatory mechanisms on the water budgets of two closely related yet ecologically disparate Australian skinks, Liopholis striata and Egernia cunninghami. My results suggest that behavioural mechanisms can be equally important to regulating water balance as physiological mechanisms. Moreover, I found substantial variation in the amount of water conserved via each hydroregulatory mechanism between the two species, suggesting that a species’ vulnerability to climate change will also show strong variation across habitat and lifestyle. The water budget model’s predictions hint to the potentially strong influence of an often disregarded avenue of water loss on reptilian water budgets: the wet surface of the eye. I measured ocular water loss rates for the arid zone Australian skink, Liopholis inornata and found that ocular evaporation can instantaneously surpass both cutaneous and respiratory evaporation, reaching up to 65.8% of total evaporative water loss. I also found evidence that two species of Australian arid zone skink regulate their ocular water loss by manipulating eye closure while basking in response to aridity, and estimated that this hydroregulatory behaviour reduced their overall evaporative water loss rates by almost 20%. The water budget model also predicts that activity patterns and retreat selection can strongly influence reptilian water budgets. I looked for experimental evidence of three hydroregulatory behaviours - retreat site selection, retreat patterns, and diel preference – being used by arid zone skinks Liopholis striata and Liopholis inornata, under different environmental conditions and hydration states. Both these species were previously thought to be primarily nocturnal, a relatively rare lifestyle for lizards other than geckos. I found that both species are neither primarily diurnal nor primarily nocturnal, and would be better described as crepuscular. Moreover, I found that both species reacted behaviourally to changes in humidity or hydration state. The behavioural responses were not consistent across species, but were always in directions consistent with the regulation of water budgets. Moreover, both species were always highly selective of hides that provide the most humid conditions. Ultimately, the application of the water budget model predicted that reptilian water budgets should be very influential to the behaviours, activity patterns and distributions of reptile species, and my experimental investigations have supported this prediction. However, we still know comparatively little about reptilian hydroregulation. Before we can predict the effects of changing climates on reptilian species, we must understand how animals regulate both their heat and water budgets, as both these aspects of their physiology have been shown to influence activity, and thereby ultimately influence the present and future distribution of reptiles.