Collaborative Research: Experimental Tests of the Adaptive Significance of Ectotherm Thermoregulation

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The body temperature of an animal has profound influence on all aspects of its biology. Extreme temperatures can be damaging physiologically or even lethal, but even intermediate body temperatures determine rates of physiological activities and of reproductive output. Endothermal animals (mainly birds and mammals) control their body temperatures by adjusting physiology (for example, metabolic heat production, perspiration); but ectothermal animals (e.g., nematodes, insects, fishes, reptiles) are generally unable to do this. Nevertheless, ectotherms can instead adjust their behavior (for example, amount of time spent in sun versus shade) to gain remarkable control over body temperature: many carefully regulate their body temperature at narrow levels ("thermal preferences") that vary from species to species. Such thermal preferences of species often correlate with temperatures that maximize physiological performance (sprint speed, digestive efficiency, sensory acuity), and accordingly a classical hypothesis proposes that thermal preferences of ectotherms will have evolved to match temperatures that maximize rates of population growth ("fitness"). This adaptive hypothesis is widely accepted and is fundamental to physiological ecology, but yet has never been tested directly.This proposal provides a rigorous exploration of the biological consequences of thermal preferences of ectotherms. Specifically, it develops an integrated set of experimental, theoretical, and comparative studies focusing on two species [fruitfly (Drosophila melanogaster), soil nematode (Caenorhabditis elegans)] that serve as general models for economically and ecologically important animal groups (insects and nematodes). The project exploits powerful new methods (developed by neurobiologists) of experimentally manipulating thermal preferences. Thermoregulatory precision of both animals will be manipulated both surgically and via mutation, and thermal preferences themselves will be shifted via artificial selection (selective breeding). Then the resultant impacts on fitness will be measured. If the classical hypothesis holds, then (for example) individuals with shifted thermoregulatory set-points will have reduced fitness in a thermal gradient relative to their fitness at a fixed temperature, whereas control individuals will show similar fitness in both environments. The project also completes a novel theoretical model of thermal preferences. The preliminary model shows that optimal set-points in fluctuating environments are actually lower than the temperature maximizing fitness in a constant environment. Comparative data on insects and lizards will be used to challenge the model's predictions.These studies have considerable applied relevance. Establishing whether and how thermal preferences actually maximize rates of population growth will have crucial implications not only for those studying the effects of the thermal environment on population growth of arthropod and nematode pests or disease vectors, but also for applied entomologists needing to determine cost-effective estimates of optimal temperature for mass rearing of bio-control agents.