Body density affects stroke patterns in Baikal seals

Abstract

SUMMARY Buoyancy is one of the primary external forces acting on air-breathing divers and it can affect their swimming energetics. Because the body composition of marine mammals (i.e. the relative amounts of lower-density lipid and higher-density lean tissue) varies individually and seasonally,their buoyancy also fluctuates widely, and individuals would be expected to adjust their stroke patterns during dives accordingly. To test this prediction, we attached acceleration data loggers to four free-ranging Baikal seals Phoca sibirica in Lake Baikal and monitored flipper stroking activity as well as swimming speed, depth and inclination of the body axis(pitch). In addition to the logger, one seal (Individual 4) was equipped with a lead weight that was jettisoned after a predetermined time period so that we had a set of observations on the same individual with different body densities. These four data sets revealed the general diving patterns of Baikal seals and also provided direct insights into the influence of buoyancy on these patterns. Seals repeatedly performed dives of a mean duration of 7.0 min(max. 15.4 min), interrupted by a mean surface duration of 1.2 min. Dive depths were 66 m on average, but varied substantially, with a maximum depth of 324 m. The seals showed different stroke patterns among individuals; some seals stroked at lower rates during descent than ascent, while the others had higher stroke rates during descent than ascent. When the lead weight was detached from Individual 4, the seal increased its stroke rate in descent by shifting swimming mode from prolonged glides to more stroke-and-glide swimming, and decreased its stroke rate in ascent by shifting from continuous stroking to stroke-and-glide swimming. We conclude that seals adopt different stroke patterns according to their individual buoyancies. We also demonstrate that the terminal speed reached by Individual 4 during prolonged glide in descent depended on its total buoyancy and pitch, with higher speeds reached in the weighted condition and at steeper pitch. A simple physical model allowed us to estimate the body density of the seal from the speed and pitch(1027-1046 kg m-3, roughly corresponding to 32-41% lipid content,for the weighted condition; 1014-1022 kg m-3, 43-47% lipid content,for the unweighted condition).

The foraging benefits of being fat in a highly migratory marine mammal

“…In this study, we developed a simple on-board data-processing algorithm by using moving averages to obtain dynamic accelerations, instead of high-pass filtering algorithm, to reduce the battery consumption of loggers (see the electronic supplementary material, figure S1, for more detail about the algorithm). The new algorithm gave us dynamic accelerations (electronic supplementary material, figure S1) and number of strokes (electronic supplementary material, figure S2) that closely matched those obtained by the high-pass filtering method [16,17]. The resulting number of strokes (i.e.…”

“…During descent, the proportion of time spent in prolonged gliding remained high when seals were negatively buoyant (range ¼ 0.4-0.9; figure 2). This indicates that seals largely relied on negative buoyancy as the thrust force for gliding, which kept their stroking effort low [15,17]. In addition, descent pitch angles were steeper when seals were more buoyant (electronic supplementary material, figure S5), which might allow seals to descend with less stroking activity because a steeper pitch angle would bring the vector of forward motion closer to that of the force of gravity [16].…”

“…flipper strokes) of seals (e.g. [16,17]). In this study, we developed a simple on-board data-processing algorithm by using moving averages to obtain dynamic accelerations, instead of high-pass filtering algorithm, to reduce the battery consumption of loggers (see the electronic supplementary material, figure S1, for more detail about the algorithm).…”

“…The relationship between buoyancy and swimming strategy may apply across a range of aquatic animals, from fishes to whales (e.g. fish [13,14], diving mammals [15][16][17], birds [18][19][20] and sea turtles [21]), representing transitions from & 2014 The Author(s) Published by the Royal Society. All rights reserved.…”

Hyperbaric computed tomographic measurement of lung compression in seals and dolphins

“…It is important to realize that the depthDiving mammal lung compression affected by drag (Miller et al, 2004;Watanabe et al, 2006) and lift (Fish, 1996). Therefore, direct imaging is required to actually document the process of lung compression.…”