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Why herding sheep is dogs' work

How does a single dog get so many sheep to move so efficiently in the same direction?

The answer - revealed by a team of UK and Swedish scientists this week in the Journal of the Royal Society Interface - is that sheepdogs use just two very simple rules.

The researchers fitted highly accurate GPS tracking devices into backpacks that were placed on a trained Australian Kelpie sheepdog and on a flock of 46 female merino sheep in a five-hectare field.

They used the GPS data to build a computer model of what prompted the dog to move, and how it responded.

The dog's first rule is to bind the sheep together by weaving around side-to-side at their backs. Once this has been achieved, it implements the second rule and drives the group forward.

"We had to think about what the dog could see to develop our model. It basically sees white, fluffy things in front of it," says Dr Andrew King of Swansea University.

"If the dog sees gaps between the sheep, or the gaps are getting bigger, the dog needs to bring them together."

Daniel Strombom of Uppsala University explains: "At every step in the model, the dog decides if the herd is cohesive enough or not.

"If not cohesive, it will make it cohesive, but if it's already cohesive, the dog will push the herd towards the target."

Single sheep dogs can successfully herd flocks of 80 or more sheep in their everyday work and in competitive herding trials.

But the model suggests that, in theory, a dog could herd more than 100 by following the two simple rules.

"Other models don't appear to be able to herd really big groups - as soon as the number of individuals gets above 50 you start needing multiple shepherds or sheepdogs," says Strombom.

But the work goes beyond scientific curiosity.

"There are numerous applications for this knowledge, such as crowd control, cleaning up the environment, herding of livestock, keeping animals away from sensitive areas and collective or guiding groups of exploring robots," says King.