Sheep and the Centripetal Force of the Herd. (July 24, 2012)

ScienceNow's Sarah C. P. Williams reports on a study, unfortunately not done in Australia but rather by Andrew King and colleagues at the Royal Veterinary College of the University of London who attached GPS backpacks to 46 sheep and to a trained Australian Kelpie in order to address the question of what are sheep thinking when as part of the flock they are menaced by predators.

Theodore Stankowich of the University of Massachusetts, Amherst, who was not involved in the study work told Ms Williams: "It's really difficult to measure 2D spatial information on large animals in the wild [this study has]  taken advantage of a unique opportunity to work with the sheep to answer these types of questions in a controlled environment."

Upon releasing the Kelpie the positions of the sheep and the dog were recorded at 1-second intervals. King and his colleagues found that the model that best predicted the movement of the flock was the motion of the centre of the flock.

As Ms Williams reports: "Rather than run in a line away from the dog, scatter in all directions, or follow their nearest neighbors, the sheep all hurried toward the flock's center. The sheep began to converge when the dog was 70 meters away. Even as the flock as a whole moved, each sheep continuously competed to be as near the middle as possible."

The researchers summarise there findings:

Flocking is a striking example of collective behaviour that is found in insect swarms, fish schools and mammal herds. A major factor in the evolution of flocking behaviour is thought to be predation, whereby larger and/or more cohesive groups are better at detecting predators (as, for example, in the ‘many eyes theory’), and diluting the effects of predators (as in the ‘selfish-herd theory’) than are individuals in smaller and/or dispersed groups. The former theory assumes that information (passively or actively transferred) can be disseminated more effectively in larger/cohesive groups, while the latter assumes that there are spatial benefits to individuals in a large group, since individuals can alter their spatial position relative to their group-mates and any potential predator, thus reducing their predation risk. We used global positioning system (GPS) data to characterise the response of a group of ‘prey’ animals (a flock of sheep) to an approaching ‘predator’ (a herding dog). Analyses of relative sheep movement trajectories showed that sheep exhibit a strong attraction towards the centre of the flock under threat, a pattern that we could re-create using a simple model. These results support the long-standing assertion that individuals can respond to potential danger by moving towards the centre of a fleeing group.

Dr King says: "The fact that they're running toward the center reduces the chances of their being on the edge and being picked off by a predator."  In short it's classified as selfish behaviour.

At present the work is being cautiously accepted. Professor Stankowich considers the work to be "an advance on previous efforts. I'd like to see the same technique applied to other large mammals." And Dr King notes that trained sheepdogs may not be the best mimic of a dangerous predator, "It could be a downgraded level of threat, because it's something the sheep have experienced before."

Click here to access the paper as well as the supplementary data which contain videos of the movement of the GPS packs worn by the sheep (red dots) and the Kelpie (black dot).