Wake respirometry may quantify stress and energetics of free-living animals

Quantifying activity-specific energy expenditure in free-living animals is a major challenge as current methods require calibration in the lab and animal capture. We propose "wake respirometry", a new method of quantifying fine-scale changes in CO2 production in unrestrained animals, using a non-dispersive infrared CO2 sensor positioned downwind of the animal i.e. in its wake. We parameterise the dispersion of CO2 in a wake using known CO2 concentrations, flow rates and wind speeds. Tests with three bird species in a wind tunnel demonstrated that the system can resolve breath-by-breath changes in CO2 concentration, with clear exhalation signatures increasing in period and integral with body size. Changes in physiological state were detectable following handling, flight and exposure to a perceived threat. We discuss the potential of wake respirometry to quantify stress and respiratory patterns in wild animals and estimate activity-specific metabolic rates through the full integration of CO2 production across the wake.