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GPS sheep prove selfish herd theory

Herd mentality Instead of scattering randomly when faced with danger, sheep selfishly head to the safety of their flock's centre, a new study has confirmed.

The research, which was conducted on a South Australian farm by scientists from the United Kingdom, proves a theory put forward 40 years ago by evolutionary biologist Bill Hamilton.

He suggested that groups of animals as diverse as insects, fish and cattle all react to danger by moving towards the middle of their swarm, school or herd, known as the selfish herd theory. Individuals in a herd benefit from being able to control where they are relative to their group-mates and any potential predator. It also reduces the chances of being the one the predator goes for when it attacks.

Proving this theory has been challenging, because predator attacks are unpredictable and until now it has been difficult to track individual animals in real time.

Dr Andrew King from The Royal Veterinary College (RVC) and colleagues overcame this obstacle by fitting a flock 46 sheep and a Kelpie working dog with mini backpacks loaded with GPS receivers and data loggers.

Previous research by the same scientists had shown that fitting a GPS device on a harness does not affect sheep's ability to move.

The scientists then instructed the dog to herd the sheep towards an open gate. They repeated the exercise twice more, then used the data they collected to calculate exactly where the centre of the flock was, how far the Kelpie was from the middle of the flock, as well as how far each sheep was from the middle of the cluster on a second-by-second basis.

Clear and striking pattern

They found that when the dog got within 70 metres of a sheep, the sheep would try to get to the centre of the flock until eventually they were in a tight cluster.

"The experiments show that the sheep seem to be weighing up the positions of multiple neighbours in order to plot the movements towards the centre of the flock," says King.

"A precise calculation of the flock centroid [centre] may be unlikely, but sheep may be able to approximate where that target location ought to be."

King says the researchers were surprised by how well the study worked. "Normally, you need many more trials than we had to test a theory with any rigour, but the patterns we uncovered were so clear, and striking."

All swarming or flocking animals are likely to show a similar response, he adds.

"However, there are different strategies that animals might adopt. For instance, instead of clumping together, you could spread apart, which might confuse a predator, and make it harder for it to target one individual," says King.

"This might be a better rule in an environment with lots of cover to dive into."

Insights into neurological disorders

King says they opted to conduct their research in Australia because it has "lovely flat, dry open fields ... perfect for doing GPS work."

"We are also working with scientists [based in South Australia] who study sheep that have neurological disorders with the view to understanding human neurological diseases, like Huntington's disease"

He says the sheep used in this study don't have Huntington's disease, but their results will be used to establish a baseline of 'normal' sheep social behaviour.

"Once we have this 'normal' behaviour, [study co-author] Professor Jennifer Morton's team can pinpoint the onset and progression of abnormal social behaviours that may be associated with locomotor or cognitive deficits in their sheep models (the ones with diseases)."

The study appears in the latest edition of the journal Current Biology .