Validating a novel capability of assessing pathways of animal water gain and loss.

Understanding variations in the routes by which wild animals gain and lose water is challenging, and common methods require longitudinal sampling, which can be prohibitive. However, a new approach uses Δ'¹⁷O_BW (Δ'¹⁷O of animal body water), calculated from measurements of δ'¹⁷O and δ'¹⁸O in a single sample, as a natural tracer of water flux. Δ'¹⁷O_BW is promising, but its relationship to organismal variables such as metabolic rate and water intake have not been validated. Here, we continuously measured oxygen influxes and effluxes of captive deer mice (Peromyscus maniculatus), and manipulated their water intake and metabolic rate. We used these oxygen flux data to predict Δ'¹⁷O_BW for the mice and compared these model predictions with Δ'¹⁷O_BW measured in blood plasma samples. As expected, Δ'¹⁷O_BW positively correlated with drinking water intake and negatively correlated with metabolic rate. All predicted Δ'¹⁷O_BW (based on measured oxygen fluxes) values differed from measured Δ'¹⁷O_BW values by <30 per meg (mean absolute difference: 11 ± 9 per meg), suggesting high accuracy for this modelling approach because studies currently report a range of 300 per meg for Δ'¹⁷O_BW among mammals, birds and fish.