Cooperative prey herding by the pelagic dolphin,Stenella longirostris
Authors: Kelly J. Benoit-Bird and Whitlow W. L. Au
Spinner dolphins rest in shallow bays during the day and feed at night. They move offshore and follow their food as it moves towards the surface and towards the coast. Spinner dolphins have a complex pattern of coordination they use while foraging and feeding. This cooperation pays off as each dolphin gets more food by working as a team than they would on their own. Benoit-Bird and Au describe this group coordination in their later article “Phonation behavior of cooperatively foraging spinner dolphins” (2009). This description follows quite well as a narration of the animation included above.
First, 8–14 pairs of dolphins swam in a widely space line in which they search for an existing patch within the prey field. Second, dolphins reduced their interpair spacing, a stage termed “tight line,” and began to undulate their swimming path up and down. During this phase of foraging, the density of prey began to increase as the prey avoided the dolphins and piled up on itself as snow in front of a plow. During the third phase of foraging, dolphins formed a circle around the increased density prey patch which distributed the prey more evenly over the circle. Finally, individual pairs of dolphins moved inside the circle to actively feed on the most dense regions of the patch while other animals continued to maintain the patch by swimming around it. Two pairs of dolphins moved into the circle at the same time, from opposite sides of the circle for about 10 s of feeding before taking their place at the circle’s edge and allowing the pairs behind them into the circle. Each pair of dolphins got an average of 45 s inside the region of most dense prey in each approximately 5 min foraging bout.
Benoit-Bird and Au used a multibeam echosounder to map the prey field and also map the movement of the dolphins three dimensionally. A simple echosounder that most people are familiar with are fish-finders. By sending out sound and waiting for an echo to return we can essentially “see” with it. Echolocation from toothed whales (Odontocetes) is an extremely advanced “echo sounder.” Analyses of these echoes led to the understanding of this complex behavior.
Benoit-Bird, KJ and Au, WWL (2009), ‘Phonation behavior of cooperatively foraging spinner dolphins’, The Journal of the Acoustical Society of America, 125, 539.