Turning Point: Analyzing the Physics of Foraging

In the undersea world of eat-or-be-eaten, speed and agility can make all the difference between finding a meal and becoming one. The Steller sea lion’s finely tuned body, while cumbersome on land, lends a competitive edge when hunting fish and eluding larger predators such as killer whales.

But swimming consumes energy, and out-maneuvering predators and prey is especially taxing. Under normal conditions, a hearty meal will offset the energetic cost of foraging. But conditions are far from normal in Western Alaska, where a shift in ocean climate has impacted fish stocks and caused widespread nutritional stress among sea lions. Under these circumstances, the energetic cost of maneuvering may have a significant effect on how starving sea lions interact with their predators and prey.

Maneuverability — and hence the energetic cost of maneuvering — are not well-researched aspects of sea lion locomotion. To address this knowledge gap, a team of researchers from the University of British Columbia set out to describe the turning dynamics – the forces generated during turning – of three captive sea lions at the Vancouver Aquarium. The study, authored by Olivier Cheneval, Robert W. Blake, Andrew W. Trites, and K.H.S. Chan, was recently published in the journal Marine Mammal Science.

The scientists filmed the sea lions performing 180° turns, and analyzed the principle components of the forces generated by body and flipper movements during each turn. The sea lions decelerated during the first half of a turn, reached their slowest speed in the middle of the turn, and then accelerated out of the turn with a powerful stroke of the pectoral flippers.

Looking more closely, the scientists were able to determine how much thrust and centripetal (rotational) force were generated during acceleration. They found that the most thrust (5 m/s2) was generated halfway through the power phase, or the down-stroke of the pectoral flipper in the second half of the turn. Centripetal force peaked at the beginning of the power stroke, when the body arches through the sharpest point of the turn. Maximum turning velocity was 3.5 meters per second, while turning radius was 0.3 body lengths and turning duration was 1.6 seconds

Four simultaneous underwater photographs showing a Steller sea lion entering a turn.
Cameras were placed on the east, west, north and south walls of the Research Pool

Of particular interest was the minimum turning radius, or the tightness of the turn measured in body lengths, compared to that of fish and marine mammals such as cetaceans (e.g., killer whales) and other otariids (e.g., California sea lions).

The authors noted that California and Steller sea lions have similar body plans featuring highly mobile flippers and very flexible bodies, which afford them an impressive array of maneuvering capabilities within the same stereotypic turning pattern. However, despite similar body plans and comparable turning radii, the study found that California sea lions turn more slowly than their Steller cousins.

When compared to small schooling fish, the preferred prey of Steller sea lions, the fish were found to have a significantly lower (i.e., sharper) turning radius and a higher rate of turn than sea lions. Theoretically, this might allow them to consistently outswim a pursuing sea lion. However, as the authors point out, a sea lion’s higher absolute speed and its use of hunting techniques such as concentrating, disturbing, and disorienting the fish could even the field by reducing the fishes’ relative maneuverability.

When compared to cetaceans such as killer whales, sea lions were found to have superior turning capabilities, although both cetaceans and otariids were less maneuverable than fish. Fish have an arguable advantage in lateral turns due to a laterally compressed body and a natural side-to-side flexure of the spine. Nevertheless, the authors conclude, the Steller sea lion’s ability to execute hairpin turns at high speed places them among the most maneuverable of the marine mammals.

Having deconstructed the turning dynamics of Steller sea lion locomotion, the study provides a firm foundation for future research. Future studies will help to assess how much energy is needed to generate the forces required for out-maneuvering predators and prey in a complex ocean environment such as Western Alaska.

15 February 2007

Publication:
Turning maneuvers in Steller sea lions (Eumatopias jubatus). Cheneval, O., R. W. Blake, A. W. Trites and K. H. S. Chan. 2007. Marine Mammal Science 23:94-109. abstract Steller sea lions are highly maneuverable marine mammals (expressed as minimum turning radius). Video recordings of turns (n=195) are analyzed from kinematic measurements for three captive animals. Speed-time plots of 180° turns have a typical ?V-shape?. The sea lions decelerated during the first half of the turn, reached a minimum speed in the middle of the curved trajectory and re-accelerated by adduction of the pectoral flippers. The initial deceleration was greater than that for passive gliding due to pectoral flipper braking and/or change in body contour from a stiff, straight streamlined form. Centripetal force and thrust were determined from the body acceleration. Most thrust was produced during the power phase of the pectoral flipper stroke cycle. Contrary to previous findings on otariids, little or no thrust was generated during initial abduction of the pectoral flippers and during the final drag-based paddling phase of the stroke cycle. Peak thrust force! at the center of gravity occurs halfway through the power phase while the centripetal force is maximal at the beginning of the power stroke. Performance is modulated by changes in the duration and intensity of movements without changing their sequence. Turning radius, maximum velocity, maximum acceleration and turning duration were 0.3 body lengths, 3.5 m/s, 5 m/s2 and 1.6 s respectively. The relative maneuverability based on velocity and length specific minimum turning radius is comparable to other otariids, superior to cetaceans but inferior to many fish.