Press release issued 24 August 2012
A video game designed for predatory fish might have unravelled some lingering evolutionary questions about group formation and movement in animals, according to new research that took a unique approach to observing interactions between real and simulated animals.
In a paper published in Science, the researchers report some of the strongest direct evidence that collective motion in animal groups such as schools of fish can evolve as a finely tuned defence against attack from predators. This dynamic has been suggested by other research, but the many variables that can drive group movement have made it difficult to observe a direct link to self-defence.
Professor Iain Couzin of Princeton University and colleagues developed an evolvable simulation of small prey that allowed the researchers to observe how group formation and movement alone protect against predatory attack. Professor Couzin developed and conducted the study at Princeton with two former postdoctoral researchers from his lab, first author Dr Christos Ioannou, now a research fellow at the University of Bristol, and Professor Vishwesha Guttal, now at the Indian Institute of Science.
The researchers projected the simulated prey - which appeared as small reddish dots - onto one side of a tank containing the famously ravenous bluegill sunfish. The prey interacted spontaneously with one another based on encoded behaviour traits, and the researchers documented in the prey the resulting individual behaviours and group formations. In the end, the bluegills were most likely to avoid attacking simulated prey that had formed coordinated and mobile groups.
These results show that group formation itself can dissuade a predator, even if the prey - as in the simulation - are completely unaware of the danger. This suggests that the specific configuration of animal groups is an evolved defence in its own right. The ideal configurations exhibited by the simulated prey mirror those of many animal groups, wherein individuals follow cues from their near-neighbours to coordinate collective movement.
Professor Couzin said: "This sort of hybrid virtual approach has given us a way of tapping into these long-lasting questions that have really evaded standard analysis for decades."
An important aspect of the simulation is that it let the researchers control the behaviour of individual prey. In nature, animals can respond to a predator in different and unpredictable ways: animals might react as the predator approaches or after the initial attack. In addition, group cohesion depends on the number of animals assembled and environmental factors such as terrain.
For this study, the researchers encoded each individual prey with various strengths of three traits: a tendency to be attracted to, swim in the same direction as, or ignore nearby individuals. Thus, individual prey would either swim alone, group together, or follow other prey, or exhibit a combination of traits.
Once the simulation began the individual prey would swim and form groups spontaneously. Each trial consisted of 16 prey and one bluegill predator. Only the first attack by each bluegill was recorded. A new bluegill was put in the tank after the researchers restarted the simulation and the prey began moving and interacting with each other.
Professor Couzin said: "Effectively, the bluegills were playing an immersive video game in which they hunted. By evolving the prey groupings, the game becomes harder and harder for the predators, as when a video game adapts to the strategy employed by the players. In a similar way, our prey 'evolved' to the mode of hunting that the bluegills exhibited, adapting better strategies that allowed them to evade hunting more effectively."
Naturally, the researchers found that simulated prey that formed groups 'survived' better than those that swam alone. But individuals in groups also needed to strike a balance of closeness and coordinated movement to keep the bluegills at bay.
Large groups that did not move much eventually fell victim to attacks in 'high-risk' areas of the projected space in which bluegills preferentially attacked. Yet when groups of prey moved with coordination, they passed through these high-risk areas too quickly for each individual bluegill to make its attack.
Although the project used bluegills, the catalogue of formation possibilities the researchers developed leave room to understand how individual behaviour and movement influences other predators such as tuna, which target large groups.
A video of the simulation with a description of the project by Dr Ioannou is available on YouTube.
'Predatory Fish Select for Coordinated Collective Motion in Virtual Prey' by C. C. Ioannou, V. Guttal, I. D. Couzin in Science
A simulation of 16 prey was projected (A, green arrow) onto a screen opposite the area from which a bluegill was released. Once the simulation began the individual prey would swim and form groups spontaneously based on three encoded traits: a tendency to be attracted to, swim in the same direction as, or ignore nearby individuals. The bluegills preceded each attack with a characteristic 'hovering' behaviour that allowed the researchers to approximate the time the fish took to make a targeting decision before striking (B, inset).
Image by courtesy of Science/AAAS