Phenotypic plasticity of gas exchange pattern and water loss in Scarabeus spretus (Coleoptera: Scarabaeidae): deconstructing the basis for metabolic rate variation
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Investigation of gas exchange patterns and modulation of metabolism provide insight into metabolic control systems and evolution in diverse terrestrial environments. Variation in metabolic rate in response to environmental conditions has been explained largely in the context of two contrasting hypotheses, namely metabolic depression in response to stressful or resource- (e.g. water) limited conditions, or elevation of metabolism at low temperatures to sustain life in extreme conditions. To deconstruct the basis for metabolic rate changes in response to temperature variation, here we undertake a full factorial study investigating the longer- and short-term effects of temperature exposure on gas exchange patterns. We examined responses of traits of gas exchange [standard metabolic rate (SMR); discontinuous gas exchange (DGE) cycle frequency; cuticular, respiratory and total water loss rate (WLR)] to elucidate the magnitude and form of plastic responses in the dung beetle, Scarabaeus spretus. Results showed that short- and longer-term temperature variation generally have significant effects on SMR and WLR. Overall, acclimation to increased temperature led to a decline in SMR (from 0.071±0.004mlCO2h–1 in 15°C-acclimated beetles to 0.039±0.004mlCO2h–1 in 25°C-acclimated beetles measured at 20°C) modulated by reduced DGE frequency (15°C acclimation: 0.554±0.027mHz, 20°C acclimation: 0.257±0.030mHz, 25°C acclimation: 0.208±0.027mHz recorded at 20°C), reduced cuticular WLRs (from 1.058±0.537mgh–1 in 15°C-acclimated beetles to 0.900±0.400mgh–1 in 25°C-acclimated beetles measured at 20°C) and reduced total WLR (from 4.2±0.5mgh–1 in 15°C-acclimated beetles to 3.1±0.5mgh–1 in 25°C-acclimated beetles measured at 25°C). Respiratory WLR was reduced from 2.25±0.40mgh–1 in 15°C-acclimated beetles to 1.60±0.40mgh–1 in 25°C-acclimated beetles measured at 25°C, suggesting conservation of water during DGE bursts. Overall, this suggests water conservation is a priority for S. spretus exposed to longer-term temperature variation, rather than elevation of SMR in response to low temperature acclimation, as might be expected from a beetle living in a relatively warm, low rainfall summer region. These results are significant for understanding the evolution of gas exchange patterns and trade-offs between metabolic rate and water balance in insects and other terrestrial arthropods.