Abstract
Many physiological adjustments occur in response to salt intake in several marine taxa, which manifest at different scales from changes in the concentration of individual molecules to physical traits of whole organisms. Little is known about the influence of salinity on the distribution, physiological performance, and ecology of passerines; specifically, the impact of drinking water salinity on the oxidative status of birds has been largely ignored. In this study, we evaluated whether experimental variations in the salt intake of a widely-distributed passerine (Zontotrichia capensis) could generate differences in basal (BMR) and maximum metabolic rates (Msum), as well as affect metabolic enzyme activity and oxidative status. We measured rates of energy expenditure of birds after 30-d acclimation to drink salt (SW) or tap (fresh) water (TW) and assessed changes in the activity of mitochondrial enzymes (cytochrome c oxidase and citrate synthase) in skeletal muscle, heart, and kidney. Finally, we evaluated the oxidative status of bird tissues by means of total antioxidant capacity (TAC) and superoxide dismutase activities and lipid oxidative damage (Malondialdehyde, MDA). The results revealed a significant increase in BMR but not Msum, which resulted in a reduction in factorial aerobic scope in SW- vs. TW-acclimated birds. These changes were paralleled with increased kidney and intestine masses and catabolic activities in tissues, especially in pectoralis muscle. We also found that TAC and MDA concentrations were ~120 and ~400% higher, respectively in the liver of animals acclimated to the SW- vs. TW-treatment. Our study is the first to document changes in the oxidative status in birds that persistently drink saltwater, and shows that they undergo several physiological adjustments that range that range in scale from biochemical capacities (e.g., TAC and MDA) to whole organism traits (e.g., metabolic rates). We propose that the physiological changes observed in Z. capensis acclimated to saltwater could be common phenomena in birds and likely explain selection of prey containing little salt and habitats associated with low salinity.
Highlights
The consumption of salt and its concomitant physiological cost represent a significant challenge to birds and may limit species range as well as colonization of new habitats (Nehls, 1996; Gutiérrez et al, 2011)
A posteriori analyses revealed that SW-acclimated birds had larger intestine (p = 0.003) and kidney (p = 0.0004) mass than tap (fresh) water (TW)-acclimated birds, but there was no effect of drinking water treatment on liver (p = 0.12), small intestine (p = 0.25), gizzard (p = 0.30), pectoralis muscle (p = 0.77), or heart (p = 0.81) mass
Our results suggest that the significant increase in cost of maintenance (BMR) associated with increased salt intake significantly affected energy budget in Z. capensis, and this is likely related to the osmoregulatory costs associated with excretion of excess electrolytes (Peña-Villalobos et al, 2013)
Summary
The consumption of salt and its concomitant physiological cost represent a significant challenge to birds and may limit species range as well as colonization of new habitats (Nehls, 1996; Gutiérrez et al, 2011). These costs are due to the exacerbated functioning of the machinery required for metabolizing, processing, and excreting excess salts, and are usually characterized by an increase in the costs of organismal maintenance. Peña-Villalobos et al (2013) experimentally found a 30% increase in BMR in a small passerine, the rufous collared sparrow (Zonotrichia capensis), that drank saltwater for 30 days in comparison to control animals that drank fresh (tap) water
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