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The rules of social interaction: attraction, repulsion and one close neighbour



9 November 2011

The Rules: Research led by James Herbert-Read, from the School of Biological Sciences, has found three rules that govern social interactions in big groups of animals, allowing them to move in a coordinated fashion in unison.
The Rules: Research led by James Herbert-Read, from the School of Biological Sciences, has found three rules that govern social interactions in big groups of animals, allowing them to move in a coordinated fashion in unison.

There are three rules that govern social interactions: attraction, repulsion and the actions of one close neighbour. Or at least that's the case for the social interactions of animals in large groups, allowing them to move in synchronised formations - some of the most remarkable sights in nature - as found by James Herbert-Read, Dr Tim Schaerf and Associate Professor Ashley Ward, from the School of Biological Sciences at the University of Sydney.

The research, published in the American journal Proceedings of the National Academy of Sciences, investigates the rules underlying how shoals swim in synch, flocks fly in formation and herds hurry in harmony. In other words, how big groups of animals move in a coordinated fashion, seamlessly moving in unison.

"Some of the most incredible sights in nature happen when animals form into groups and move together as if choreographed," said James Herbert-Read, lead author of the paper and a PhD student in the School of Biological Sciences.

"But how exactly do the individuals in these groups do it? How does each individual know when and which way to turn and how do they avoid bumping into each other?"

The team from the University of Sydney, along with colleagues at the University of Uppsala in Sweden, have answered these questions for the first time using shoals of mosquitofish. They found that each fish in a shoal uses very simple rules to respond to its neighbours.

"We set out to determine which rules individual fish use when they're in a group, so we filmed groups of two, four or eight mosquitofish, Gambusia holbrooki, in a square arena for five minutes and studied the movements of individuals in each group," explained James.

"Using semi-automated tracking software we analysed the trajectories of each fish and how they responded to the position and orientation of their neighbours.

"It turns out the amazing synchronised swimming that fish in shoals exhibit is actually caused by each fish using very simple rules to respond to its neighbours," said James.

"These rules include: 'accelerate towards a neighbour that is far away from you' and 'decelerate when a neighbour is right in front of you' - it's a bit like driving a car! We also found that a fish only responds to a single nearest neighbour at any one time."

This is the first demonstration of the rules that individuals in fish shoals use to interact, and finally allows an understanding of how moving groups perform these coordinated feats.

"There are lots of models that have been developed to describe patterns of collective motion in terms of interactions between individuals, but it's been unclear whether the interaction rules implemented in these models are the ones actually being used by the animals," said James.

The team identified some key similarities with assumptions used in many models, particularly that fish respond to the position of their neighbours through short range repulsion and longer range attraction rules.

"However, we also found some inconsistencies between the classical models of fish motion and our results. For example, we didn't find evidence that fish aligned specifically with their neighbour's orientation - a rule adopted by some models. Instead, it looks like the group aligns in some other way, possibly through attraction and repulsion rules and by fish following individuals in front of themselves," explained James.

"Another difference is how important acceleration in response to a neighbour's position is. Many self-propelled particle models assume that the speed of individuals is fixed and interactions happen through changes in direction. But our mosquitofish actively changed their speed in order to avoid or move towards neighbours, so it's important for future models to incorporate changes and differences in speeds, which is likely to affect a number of group level properties."

"What we found most surprising in our study is the extent to which the single nearest neighbour dominates social interactions. It is possible for single neighbour interactions to produce complex schooling patterns," said James.

"We are beginning to appreciate that even humans might rely on relatively simple rules to effectively navigate their environment."

James E. Herbert-Read et al. (2011). Inferring the rules of interaction of shoaling fish. PNAS. doi: 10.1073/pnas.1109355108

Read the journal paper in the Proceedings of the National Academy of Sciences at: www.pnas.org/content/early/2011/11/02/1109355108.abstract


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