Estimating energetics in cetaceans from respiratory frequency: why we need to understand physiology

Abstract

The 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 O2exchange 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 O2levels of bottlenose dolphins, and computed the O2consumption rate (V̇O2 ) before and after a pre-determined duration of exercise. The measuredV̇O2 was compared with three methods to estimate FMR. Each method to estimateV̇O2 included variable VT and/or ΔO2 Two assumption-based methods overestimatedV̇O2 by 216-501%. Once the temporal changes in cardio-respiratory physiology, such as variation in VT and ΔO2, were taken into account, pre-exercise restingV̇O2 was predicted to within 2%, and post-exerciseV̇O2 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.

Keywords: Eco-physiology; Energy budget; Exercise; Field metabolic rate; Marine mammal; Oxygen consumption rate; Oxygen debt; Recovery.

Fig. 1.

Changes in tidal volume with breath number. (A) Absolute tidal volume (VT, litres), or (B) tidal volume expressed as percentage total lung capacity (%TLC_est) decrease with breath number during the post-exercise recovery period in bottlenose dolphins. Grey line shows exponential relationship (Eqn 1A or Eqn 1B). %TLC_est=VT·TLC_est⁻¹·100, where TLC_est=0.135·M_b^0.92 (Fahlman et al., 2011; Kooyman, 1973).

Fig. 2.

Changes in lung O₂ content with breath number. (A) Average expired O₂ content (O_2exp, %) and (B) average estimated lung O₂ concentration (%) upon expiration, before (Pre) and immediately following (Post) exercise. Data are pooled for all dolphins. Grey line shows exponential relationship (Eqn 1C or Eqn 1D).