Collaborative Research: Triple oxygen isotopes as a new method to study water inputs and metabolism in wild animals

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All animals require water and energy to stay alive. Water is obtained from a variety of external sources (e.g., rivers, lakes, food), and internally as a byproduct of the chemical reactions that occur during the breakdown of food. Energy is obtained from food and is processed at a rate that limits everything an animal does: foraging and hunting, dispersing, competing with other individuals, reproducing, and other activities. Thus, assessing the contributions of different water sources to animal intake, and assessing animal rates of energy use, are critically important for understanding all aspects of animal function. However, measuring water inputs and energy use of wild animals is prohibitively difficult. Methods to date require capturing and handling animals at least two different times, or sampling every potential water source in a given habitat. These tasks are frequently impossible in the field, severely limiting the study of these important variables. To overcome this challenge, this project will develop the use of Δ17O measurements from a single sample of animal body water (Δ17OBW) to quantify the fractional contributions from external and internal sources to the body water pool of an animal. In addition, the fractional contribution of internal water will provide an index of the rate of energy use, because this internal water is created at a rate that directly reflects the rate of energy processing. The understanding of Δ17OBW from this project will create new opportunities to incorporate animal function into a broad variety of studies.The value of Δ17O reflects deviations in the relationship between δ17O and δ18O. For most animals, two sources provide 80–99% of the input into their body water: 1) drinking/food water that is ultimately sourced from precipitation and 2) metabolic water. These two inputs have relatively constant and strikingly different Δ17O values: drinking/food water Δ17O ≈ 0.030‰ and metabolic water Δ17O ≈ -0.450‰. Importantly, the Δ17O values of these sources exhibit little variation because they are determined by mass-independent fractionation processes that are not affected by changes in δ17O and δ18O values. This project will study animals in captivity to evaluate the influence of changes in exogenous water intake and metabolic rate on Δ17OBW. In addition, longitudinal changes in the Δ17OBW values of wild animals will be tracked during natural fluctuations in temperature and precipitation that will influence their water balance and metabolism. These Δ17OBW results will be compared to traditional approaches involving δ18O modeling and the doubly-labeled water technique. Finally, the analytical methods of Δ17O measurements will be optimized for biological samples, including organic tissues, which will reduce the costs and required expertise of this approach and promote its adoption by the ecophysiological and ecological research communities. The new understanding of Δ17OBW can be used to study animal responses to the shift towards a hotter, drier climate that is occurring in many regions of the planet, and to assess qualitative variation in field metabolic rate, a critical but notoriously difficult-to-measure variable.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.