We estimated tidal volumes of 768 inhaled breaths over 36 trials on 6 animals at Dolphin Quest (2013 and 2017). Estimated flow rates and volumes were within 10% of measured values. At rest, tidal volume was on average 8.0±1.5 L; tidal volumes increased to 13.4±1.0 L when swimming, and inhale duration decreased. After exercise, tidal volume decreased from 11.6 L to resting levels and inhale duration increased. This represents the first time that free-ranging tidal volumes of dolphins have been estimated and put in context with exercise physiology.
Over 22h of tag recordings of 7 wild dolphins, we detected 4699 breaths. 300 were before animals were released, of which 95 had the pneumotach over the blowhole. At rest, the measured tidal volume was 4.5±1.4 L across all individuals, or 19.1±8.2 ml/kg. Estimated volumes were no different from measured, and we found no difference in estimated VT between breaths when the pneumotach was on vs. off. Overall, estimated tidal volume varied from 3.6-5.6 L (IQR; 15-26 ml/kg) when dolphins were held in water.
Following release, we detected 4534 total breaths, 4474 (99%) of which we were able to estimate volume. Tidal volumes of free-swimming, wild bottlenose dolphins were 29±7 ml/kg, 7 ml/kg (33%) higher compared to when held. Tidal volumes varied up to 7.4 L between breaths. Capturing this variability in volume between breaths leads to a much-improved estimate compared to the state-of-the-art; the current standard of assuming a constant, fixed-value based on body size results in a 32% overestimate of ventilation (L/min).
With this method, we can speak to the natural variation in tidal volume through time, as well as specific changes in the context of sound exposure, quantifying biologically significant reactions. Through RATE, we have made the first near-continuous breath-by-breath estimates of tidal volume in wild cetaceans, with major implications on estimating short-term ventilation and gas exchange in natural and disturbed behaviors.
