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In high school I was sure I wanted to be a neuroscientist (yes, I’ve always been a nerd), but halfway through undergrad at the University of Iowa I realised that there’s an entire body of science where you get to study organisms in their environments. I convinced a plant ecologist (Steve Hendrix) to let me do an honours project on avian nesting ecology in his lab, and so it began. After I finished my BSc, I spent 6 months at a penguin colony in Argentina learning how to catch and band feisty 5 kg birds and survive as a vegetarian in one of the most carnivorous countries in the world. But university life beckoned…so I moved to Vancouver to do a MSc in behavioural ecology, learn how to properly use the word “eh?” and attempt snowboarding. My MSc project with Ron Ydenberg was on the breeding and migratory strategies of western sandpipers, a little wader that (luckily for me) breeds in the Yukon Delta of western Alaska. This meant I got to spend an incredible summer in Alaska as well as several weeks each year at coastal migratory stopovers in British Columbia.    

Towards the end of my MSc, I had 2 epiphanies. First, I was working with a 25 g bird that (amazingly) migrates 10,000 kms every year, which got me thinking about the physiology that underlies behaviour. Second, I designed an apparatus to test flight maneuverability in wild sandpipers, and realised that I really enjoy experimental biology. So, I knew I wanted a PhD project that allowed me to learn more about these two types of science. Working with the Physiological Ecology group at The University of Queensland has given me the opportunity to learn more about these bodies of research.

 

 

 

Temperature is one of the most important environmental factors affecting growth, development and performance, due to its pervasive effects at all levels of biological function, from cellular to whole-animal. Though thermal change is ubiquitous in nature, many empirical studies have failed to investigate responses of organisms to ecologically-realistic variation in temperature. In fact, we understand little about how short-term, or diel, thermal variation affects the behaviour, physiology or morphology of organisms. In this thesis, I investigate the phenotypic consequences of diel variation in temperature on developing amphibians and insects. My work represents the first attempt to characterise and predict amphibian phenotypes in dielly-fluctuating temperatures and includes the first empirical tests of existing acclimation models in realistic environmental conditions. Development is a critical period during the lifetime of organisms, as the environment can have dramatic and irreversible effects on phenotype within a short temporal window. Many organisms experience wide thermal variation over the course of their development, particularly those that inhabit tidal or ephemeral water bodies and which are unable to physiologically maintain a thermal homeostasis (i.e. ectothermic). The striped marsh frog (Limnodynastes peronii) and fall field cricket (Gryllus pennsylvanicus) represent excellent model systems to test predictions about thermal plasticity. Both species undergo some portion of their development in thermally-variable environments, and previous work has established that L. peronii tadpoles are able to acclimate to seasonal, or long-term, changes in temperature.

 

 

Initially, I determined that L. peronii larvae raised in fluctuating or stable thermal environments differed in larval growth and development, and performance both at metamorphosis and later, after a 10-week period of food deprivation (Chapter 2). I had predicted that wide thermal variation (in this case, 18-32°C versus 24°C) would be costly to developing amphibians, resulting in smaller body sizes and poorer performance. However, this study suggested that thermal variability may in fact have positive consequences for amphibians, and that these consequences differ between larval and metamorphic stages.

Though simple comparisons (as in Chapter 2) provide valuable baseline patterns, a primary goal of ecology is to elucidate patterns that are generalisable across taxonomic scales. Thus, in Chapter 3 I establish whether empirical models of amphibian growth and development based on a range of stable temperatures (18, 22, 26, 30 and 34°C) can be used to make accurate predictions about growth and development in variable temperatures (18-28°C and 18-34°C) between embryogenesis and metamorphosis. Though growth rates differed inconsistently from my predictions, larvae from the widely-variable environment metamorphosed at the smallest body sizes ever observed for this species. I found that development was generally faster than expected in variable environments, which may represent an adaptive response in natural conditions. In shallow pools, thermal variation acts as a signal of desiccation, and previous work has shown that amphibian larvae that can accelerate metamorphosis under such conditions have higher probabilities of survival. Furthermore, empirical models may not accurately predict phenotypes if larvae can adjust their physiological performance (i.e. acclimate) in response to thermal variability (Chapters 5-6).

 

In Chapter 4, I examined the direct (i.e. morphological) and indirect (i.e. physiological) consequences of developmental temperature or thermal variability on the locomotor performance of metamorphic frogs, Limnodynastes peronii. I found that among stable temperatures, performance could be linked directly to temperature itself, but was also indirectly linked to temperature via metamorphic body size, with larger frogs jumping farther. In contrast, thermal variability was only indirectly linked to metamorphic performance, via positive effects on metamorphic body size. Relationships among the environment, morphology and performance differed depending on whether stable temperatures or the magnitude of temperature variation was considered, signifying the importance of including realistic thermal variability in studies of developmental plasticity. Furthermore, relationships differed among developmental stages, demonstrating that ontogeny plays an important role in performance grandients. 

 

In Chapter 5, I predicted that L. peronii larvae would respond to diel thermal variability by extending the range of temperatures over which they could perform. I expected that larval performance curves would be broader for individuals raised in widely-variable environments (14-34°C) compared with stable environments (24°C). Contrary to my expectations, I found that L. peronii larvae showed no acclimatory response in the reactivity of two metabolic enzymes (lactate dehydrogenase and cytochrome C oxidase), heart rate or oxygen consumption. Larvae raised in stable environments performed just as well as those raised in variable environments across a wide range of temperatures (10-38°C). However, most thermal traits showed a high degree of thermal sensitivity. Because time spent at high temperatures is presumed to be metabolically costly to ectotherms, and metabolism thus may play an important role in constraining the body size of individual larvae that undergo development in widely-fluctuating temperatures (as shown in Chapter 3).

                                        

In Chapter 6, I extend this study to ask whether acclimatory responses are dampened by stochastic or unreliable signals of environmental change, as predicted by current acclimation models. I raised immature G. pennsylvanicus in a constant temperature (25°C) or one of three thermal regimes mimicking a seasonal decline in temperature (25°C to 12°C), with varying levels of stochasticity. I then measured several performance traits considered relevant to fitness, including growth rate, running velocity, feeding rate, metabolic rate, and cold tolerance. Contrary to my predictions, crickets did not appear to acclimate many traits, and responses were unaffected by stochasticity of diel temperatures.

 

My thesis work demonstrates that characterising phenotypic plasticity in stable environments tells us little about the development of phenotypes in realistic, ecologically-relevant conditions. I have shown that existing empirical and theoretical models are incapable of describing general responses—enhancement and continued empirical testing of such models represents an important future direction for thermal ecological research. In addition to such large-scale approaches, we must continue to examine thermal plasticity on the sub-cellular level if we are to truly understand how temperature affects the growth, development and performance of ectotherms.

 

In the short-term, I’m finishing my PhD in early September and having a baby a few weeks later. Next year (when I’m sleeping regularly again), I plan to continue along an academic career track by expanding my collaborations with overseas researchers and working towards obtaining a postdoctoral fellowship to continue research. Following on from my PhD, I’d like to examine how environmental variability affects the ecology and evolution of ectotherms, ideally in a model system with important conservation or health value.