Abstract
ABSTRACTThe accurate estimation of field metabolic rates (FMR) in wild animals is a key component of bioenergetic models, and is important for understanding the routine limitations for survival as well as individual responses to disturbances or environmental changes. Several methods have been used to estimate FMR, including accelerometer-derived activity budgets, isotope dilution techniques, and proxies from heart rate. Counting the number of breaths is another method used to assess FMR in cetaceans, which is attractive in its simplicity and the ability to measure respiration frequency from visual cues or data loggers. This method hinges on the assumption that over time a constant tidal volume (VT) and O2 exchange fraction (ΔO2) can be used to predict FMR. To test whether this method of estimating FMR is valid, we measured breath-by-breath tidal volumes and expired O2 levels of bottlenose dolphins, and computed the O2 consumption rate (V̇O2) before and after a pre-determined duration of exercise. The measured V̇O2 was compared with three methods to estimate FMR. Each method to estimate V̇O2 included variable VT and/or ΔO2. Two assumption-based methods overestimated V̇O2 by 216-501%. Once the temporal changes in cardio-respiratory physiology, such as variation in VT and ΔO2, were taken into account, pre-exercise resting V̇O2 was predicted to within 2%, and post-exercise V̇O2 was overestimated by 12%. Our data show that a better understanding of cardiorespiratory physiology significantly improves the ability to estimate metabolic rate from respiratory frequency, and further emphasizes the importance of eco-physiology for conservation management efforts.
Highlights
Marine mammals live a life of dual constraints, with food located underwater, and the oxygen (O2) required to fuel aerobic metabolism available only at the surface
VT (Fig. 1), and the volume of O2 exchanged per breath decreased, while end-expiratory and average O2 content increased (Fig. 2, P
We used breath-by-breath respirometry to determine if counting the number of breaths can be used to estimate metabolic rate in a small cetacean at rest, or during recovery from exercise
Summary
Marine mammals live a life of dual constraints, with food located underwater, and the oxygen (O2) required to fuel aerobic metabolism available only at the surface. Marine mammals have several traits that enhance time underwater such as: increased muscle and blood O2 stores, a shape that minimizes hydrodynamic drag, and a physiological response during diving that helps to manage gases and maximize the aerobic dive time (Butler and Jones, 1997; Ponganis et al, 2011; Scholander, 1940) While many of these traits have been described for a range of pinnipeds and smaller cetaceans (Fahlman et al, 2008a; Kooyman et al, 1971; Reed et al, 1994, 2000; Sparling and Fedak, 2004; Sparling et al, 2007; Williams et al, 1993; Yazdi et al, 1999), we still know very little how larger, free ranging cetaceans manage their energy budgets (Kooyman et al, 1975; Sumich, 2001; Wahrenbrock et al, 1974)
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