IISc study sheds light on fish schools, bird flocks

When studying animal group behaviour such as a school of fish or a flock of birds, the appearance of “noise” — statistical deviations from expected patterns — is often a source of annoyance for scientists. A new study shows that noise may have value after all.

While studying the cichlid species of fish, Indian Institute of Science (IISc) scientists discovered that “noise” can actually help make sense of how complex group-level behaviours — such as swimming together in a synchronised manner to avoid predators and forage efficiently — emerge from simple individual behaviours.

“It is very counter-intuitive,” said Vishwesha Guttal, Associate Professor at the Centre for Ecological Sciences, IISc, and senior author of the paper which was published in Nature Physics. “We all usually think noise reduces order.”

Understanding how such emergent behaviour arises in nature can be useful for studying other processes, such as robotic swarms or how information propagates in crowds, explained PhD student Jitesh Jhawar, the first author of the paper.

“People are actually using these kinds of models to understand the dynamics there.”

The findings were made after Jhawar and others closely tracked the cichlid fish Etroplus suratensis — a popular estuarine fish known locally as karimeen — in large water tanks. Both the direction in which the fish moved and the degree to which they were aligned towards each other were studied.

Crucially, the researchers also tracked how these behaviours fluctuated over time.

“What we find is that when the fish are moving in a misaligned state, the fluctuations are high,” said Danny Raj M, co-author of the paper and an INSPIRE Faculty Fellow at the Department of Chemical Engineering.

And when fluctuations were high, it had surprising effects on the behaviour of the group: they became more synchronised in their swimming. This was because each member of the group was copying the direction of one of its neighbours, chosen randomly. This contrasts with classical models which suggested that each fish copied what the overall group was doing on average.