Triple Oxygen Isotope Measurements (Δ'17O) of Body Water Reflect Water Intake, Metabolism, and δ18O of Ingested Water in Passerines.

Pablo Sabat,Seth D Newsome,Stephanie Pinochet,Karin Maldonado,Juan Carlos Sanchez-Hernandez,Alexander R Gerson,John P Whiteman,Roberto Nespolo,Zachary D Sharp

doi:10.3389/fphys.2021.710026

Abstract

Understanding physiological traits and ecological conditions that influence a species reliance on metabolic water is critical to creating accurate physiological models that can assess their ability to adapt to environmental perturbations (e.g., drought) that impact water availability. However, relatively few studies have examined variation in the sources of water animals use to maintain water balance, and even fewer have focused on the role of metabolic water. A key reason is methodological limitations. Here, we applied a new method that measures the triple oxygen isotopic composition of a single blood sample to estimate the contribution of metabolic water to the body water pool of three passerine species. This approach relies on Δ'17O, defined as the residual from the tight linear correlation that naturally exists between δ17O and δ18O values. Importantly, Δ'17O is relatively insensitive to key fractionation processes, such as Rayleigh distillation in the water cycle that have hindered previous isotope-based assessments of animal water balance. We evaluated the effects of changes in metabolic rate and water intake on Δ'17O values of captive rufous-collared sparrows (Zonotrichia capensis) and two invertivorous passerine species in the genus Cinclodes from the field. As predicted, colder acclimation temperatures induced increases in metabolic rate, decreases in water intake, and increases in the contribution of metabolic water to the body water pool of Z. capensis, causing a consistent change in Δ'17O. Measurement of Δ'17O also provides an estimate of the δ18O composition of ingested pre-formed (drinking/food) water. Estimated δ18O values of drinking/food water for captive Z. capensis were ~ −11‰, which is consistent with that of tap water in Santiago, Chile. In contrast, δ18O values of drinking/food water ingested by wild-caught Cinclodes were similar to that of seawater, which is consistent with their reliance on marine resources. Our results confirm the utility of this method for quantifying the relative contribution of metabolic versus pre-formed drinking/food water to the body water pool in birds.

Highlights

Understanding the physiological mechanisms that species use to maintain water balance is becoming more relevant as increases in temperature and drought frequency represent significant ecological shifts that are affecting the behavior, distribution, and abundance of animals (McCarty, 2001; Albright et al, 2010; Şekercioğlu et al, 2012; IPCC, 2013; Remeš and Harmáčková, 2018)
We evaluated the effect of thermal acclimation on resting metabolic rates (RMRs), total evaporative water loss (TEWL), and water intake using a generalized linear mixed model (GLMM) with body mass as a covariate, acclimation temperature (15°C and 30°C) as fixed factors, and individual identity as a random factor to control for repeated measures
Sparrows acclimated at 15-day exposure to cold (15°C) exhibited higher RMR (93.2 ± 15.2 ml O2 h−1) and lower daily water intake (0.19 ± 0.05 ml H2O h−1) in comparison with when they were acclimated at 30°C (RMR: 70.8 ± 12.2 ml O2 h−1 and daily water intake: 0.26 ± 0.08 ml H2O h−1); there was no difference in TEWL between temperature treatments (Table 1)

Summary

Introduction

Understanding the physiological mechanisms that species use to maintain water balance is becoming more relevant as increases in temperature and drought frequency represent significant ecological shifts that are affecting the behavior, distribution, and abundance of animals (McCarty, 2001; Albright et al, 2010; Şekercioğlu et al, 2012; IPCC, 2013; Remeš and Harmáčková, 2018). Because of their diurnal habits and high mass-specific metabolic rates, birds are susceptible to increases in temperature and aridity (Riddell et al, 2021), so better understanding the environmental factors that influence their water balance is an important topic of research. Using a combination of physiological data, mechanistically informed models and climatic data predicted that the proportion of the ranges of the distribution of avian species with risk of lethal dehydration during heat waves will dramatically increase under future climate scenarios (Albright et al, 2017).

Methods

Results

Conclusion