Life-history theory predicts that predator-induced defences should incur fitness costs. In larval amphibians, the most frequently reported cost is reduced energy intake due to lower foraging rates; however, recent reports suggest that this trade-off may be uncoupled through morphological and physiological adaptations. Metabolism is a measure of energy expenditure and plasticity in respiration may potentially offset other costs of predator-induction. The aim of this study was to measure the indirect effects of dragonfly larvae on the respiration rate of tadpoles of the Arabian toad (Bufo arabicus) over time and at different predator concentrations. We performed two experiments. In the first experiment (time-response), we exposed tadpoles either to the indirect presence of odonate larvae or predator-free conditions and measured respiration rates 3, 5, 8, 11, 13, 15, 19 and 26 days after the start of the experiment. In the second experiment (dose-response), we used three levels of predator chemicals, equivalent to 1 predator per 10, 100 or 1,000 l plus controls. The respiration rate of predator-exposed tadpoles varied initially, but was 56. 3% of controls after 26 days. In the dose-response experiment, the respiration rate of all predator-exposed tadpole groups was reduced by between 19. 1 and 27. 2% after 21 days. The study demonstrates a mechanism by which tadpoles may be able to adjust their physiology to partially offset the costs of lower energy intake due to predator avoidance.
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