Abstract
Predictions indicate that birds worldwide will be affected by global warming and extreme climatic events which is especially relevant for passerines because the diurnal habits, small body size, and high mass-adjusted metabolic rates of this group make it particularly susceptible to increases in temperature and aridity. Some bird species respond to conditions that stress osmoregulation by increasing their rates of energy expenditure, nevertheless, the effect of dehydration on metabolic rates in birds has produced contrasting results. It also remains unknown whether hydration state may cause shifts in tissue-specific metabolic rates or modify tissue oxidative status. We used the rufous-collared sparrow (Zonotrichia capensis), to experimentally test the effect of dehydration on metabolic enzymes in erythrocytes, tissue oxidative status, basal metabolic rate (BMR), and total evaporative water loss. We found a significant increase in mass-adjusted BMR in water restricted (WR) birds compared to control birds (CT). Activity of cytochrome-c-oxidase (COX) in red blood cells (RBCs) was also significantly higher in the WR group relative to the CT group and this activity was positively correlated with mass-adjusted BMR. We found a moderate effect of water restriction on membrane damage of skeletal muscle. In a second set of individuals subjected to the same experimental conditions, lean mass and total water were tightly correlated and decreased by 10 and 12%, respectively, in birds in the WR group relative to the CT group. Decreases in total water and lean mass leads to an increase in mass-adjusted BMR in WR Z. capensis, suggesting that birds may simultaneously increase protein catabolism and production of metabolic water through oxidation. The significant positive relationship between BMR and COX in RBCs is a finding that requires additional research to determine whether erythrocyte metabolism is affected by dehydration per se and or it more generally reflects rates of energy expenditure in birds.
Highlights
Increased frequency in droughts and heat waves are predicted to have a strong influence on plant and animal phenology, behavior, and physiology (Walther et al, 2002; Albright et al, 2017)
We found a significant effect of experimental time on body mass [RM ANOVA, F (3,48) = 47.48; p < 0.01], with a steeper decline in body mass over time in the water restricted (WR) group (Figure 1) such that by day 7 the two treatments significantly differed from one another [F (1,16) = 25.82; p < 0.01, Figure 1]
Sparrows in WR group had ∼12% lower whole-organism total evaporative water loss (TEWL) in comparison to birds in the control birds (CT) group [F(1,16) = 5.42, p = 0.033], there was no difference in whole-organism basal metabolic rate (BMR) between experimental treatments [F (1,16) = 1.85, p = 0.192; Table 2]
Summary
Increased frequency in droughts and heat waves are predicted to have a strong influence on plant and animal phenology, behavior, and physiology (Walther et al, 2002; Albright et al, 2017). Predictions indicate all birds worldwide will be affected by global warming and climatic extreme events, species that live in subtropical and temperate deserts will be disproportionately impacted by climate change because the magnitude and perhaps rate of change in these regions will likely be larger and faster than in other biomes (McKechnie and Wolf, 2010; Albright et al, 2017; Iknayan and Beissinger, 2018) This is especially relevant for passerines because the general behavioral (e.g., diurnal), evolutionary (e.g., small body size), and physiological (e.g., high mass-specific metabolic rates, dissipate heat via panting) traits of this group make it susceptible to increases in temperature and aridity (Yom-Tov, 2001; Gardner et al, 2011; Albright et al, 2017). The effect of dehydration on metabolic rates in birds has produced contrasting results (Cade et al, 1965; Gerson and Guglielmo, 2011)
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