Proxies of energy expenditure for marine mammals: an experimental test of "the time trap"

Abstract

Direct measures of energy expenditure are difficult to obtain in marine mammals, and accelerometry may be a useful proxy. Recently its utility has been questioned as some analyses derived their measure of activity level by calculating the sum of accelerometry-based values and then comparing this summation to summed (total) energy expenditure (the so-called "time trap"). To test this hypothesis, we measured oxygen consumption of captive fur seals and sea lions wearing accelerometers during submerged swimming and calculated total and rate of energy expenditure. We compared these values with two potential proxies of energy expenditure derived from accelerometry data: flipper strokes and dynamic body acceleration (DBA). Total number of strokes, total DBA, and submergence time all predicted total oxygen consumption [Formula: see text] ml kg^-1). However, both total DBA and total number of strokes were correlated with submergence time. Neither stroke rate nor mean DBA could predict the rate of oxygen consumption ([Formula: see text] ml min^-1 kg^-1). The relationship of total DBA and total strokes with total oxygen consumption is apparently a result of introducing a constant (time) into both sides of the relationship. This experimental evidence supports the conclusion that proxies derived from accelerometers cannot estimate the energy expenditure of marine mammals.

Figure 1

Pearson’s correlation coefficients for relationships between combinations of different running means and thresholds for mean ODBA (light colours) and VeDBA (dark colours) with ${s\dot{\mathrm{V}}O}_2$ (ml min⁻¹ kg⁻¹; A–C) and for total ODBA (light colours) and VeDBA (dark colours) with sVO₂ (ml kg⁻¹; D–F). Data presented separately for (A and D) large females diving (N = 4 animals; n = 130 trials); (B and E) male fur seals and sea lions swimming transitionally (N = 5 animals; n = 86); C and F female and juvenile fur seals and sea lions swimming transitionally (N = 4 animals; n = 47 trials).

Figure 2

Relationship between total oxygen consumption (${\mathrm{sVO}}_2$ml kg⁻¹; top panel) and swim duration (A), number of strokes (B) and VeDBA AUC (C) and relationship between diving metabolic rate (${s\dot{\mathrm{V}}O}_2$ ml min⁻¹ kg⁻¹; bottom panel) and swim duration (D), stroke rate (E) and average VeDBA (F). The relationship between swim duration and of total number of strokes (G) and VeDBA AUC (H) are displayed for comparative purposes. Open circles are small females and subadults (N = 4 animals; n = 47 trials), closed grey circles are males (N = 5 animals; n = 86) and closed black circles are large females (N = 4 animals; n = 130 trials). For comparisons with other papers the average VeDBA used in F has a running mean of 2 seconds and no threshold^,. *Represents an outlier that was removed when fitting the regression.

Figure 3

ODBA (g) calculated with a running mean of 2 seconds from a 60 second swim with a comparison of the overall mean ODBA estimated using different thresholds.