Abstract
Forces due to propulsion should approximate forces due to hydrodynamic drag for animals horizontally swimming at a constant speed with negligible buoyancy forces. Propulsive forces should also correlate with energy expenditures associated with locomotion—an important cost of foraging. As such, biologging tags containing accelerometers are being used to generate proxies for animal energy expenditures despite being unable to distinguish rotational movements from linear movements. However, recent miniaturizations of gyroscopes offer the possibility of resolving this shortcoming and obtaining better estimates of body accelerations of swimming animals. We derived accelerations using gyroscope data for swimming Steller sea lions (Eumetopias jubatus), and determined how well the measured accelerations correlated with actual swimming speeds and with theoretical drag. We also compared dive averaged dynamic body acceleration estimates that incorporate gyroscope data, with the widely used Overall Dynamic Body Acceleration (ODBA) metric, which does not use gyroscope data. Four Steller sea lions equipped with biologging tags were trained to swim alongside a boat cruising at steady speeds in the range of 4 to 10 kph. At each speed, and for each dive, we computed a measure called Gyro-Informed Dynamic Acceleration (GIDA) using a method incorporating gyroscope data with accelerometer data. We derived a new metric—Averaged Propulsive Body Acceleration (APBA), which is the average gain in speed per flipper stroke divided by mean stroke cycle duration. Our results show that the gyro-based measure (APBA) is a better predictor of speed than ODBA. We also found that APBA can estimate average thrust production during a single stroke-glide cycle, and can be used to estimate energy expended during swimming. The gyroscope-derived methods we describe should be generally applicable in swimming animals where propulsive accelerations can be clearly identified in the signal—and they should also prove useful for dead-reckoning and improving estimates of energy expenditures from locomotion.
Highlights
It is a common assumption in foraging theory that animals adopt strategies to maximize the ratio of energy gained over energy expended [1]—of which locomotion can be the major cost [2]
Overall Dynamic Body Acceleration (ODBA) is normally averaged over longer intervals to provide an estimate of activity, but it has been used to provide sub-second estimates of energy expenditures [4]
Our results show that using gyroscope data combined with accelerometer data makes it possible to clearly identify individual flipper strokes for otariidae swimming in a straight line, and to
Summary
It is a common assumption in foraging theory that animals adopt strategies to maximize the ratio of energy gained over energy expended [1]—of which locomotion can be the major cost [2]. One measured proxy for energy expenditures associated with locomotion is acceleration [3,4], which can be collected using miniature data logging tags that incorporate three-axis accelerometers [5,6,7]. Accelerometers measure the vector sum of acceleration due to gravity and animal movement (Fig 1). A widely used method for estimating animal accelerations generates a metric called Overall Dynamic Body Acceleration (ODBA; [3]). When there is little high frequency tag rotation, the smoothed signals will approximate the gravity signal (static acceleration) and the differences will approximate animal body acceleration (dynamic acceleration). A similar method to ODBA uses frequency filtering to separate the high frequency components of the accelerometer signals separately on each axis [8]
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