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Issue 477, 18 February 2002

Crowds, gangs and best behaviour scientists find that our actions could be a little fishy

How do you behave in a crowd? Do you stand meekly behind the next person, shuffling forward when they do, or push past other people to get through faster? You might think that you act in an individual way, and there's no pattern to your movements. But researchers at Leeds have found that our behaviour bears striking resemblances to the behaviour of animals, following certain rules that can be mapped, explained and even predicted.

Dr Iain Couzin (pictured left) explains: "Ants follow chemical trails to food sources along which they follow set paths to avoid congestion. Similarly human beings in large crowds, moving in both directions along a walkway, for example, will spontaneously form 'lanes' travelling in one direction or another. By following local rules in this case the easiest route through a crush of people we create larger scale patterns which on the ground, with our restricted field of vision, we are unaware of."

The key to understanding crowd behaviour is computer simulation. While it's sometimes possible to see wider patterns among a large group, it's harder to track one individual, or quantify their relationship with their companions.

However, a computer can do all of these things if provided with the right information. Biologist Dr Jens Krause and his research team focus their work on fish, but maintain that the findings can be applied to other animals, and even humans.

Dr Krause (pictured right) : "Fish are useful for our work because they are easy to study. We can maintain laboratory populations, and make observations of their behaviour. We then feed this into complex computer simulations, to try and determine patterns, and the final stage is to test this on wild populations in the field."

The scientists film fish in the laboratory and then digitise the film, feeding this into a computer. They're just beginning a project with a research centre in Kenya, where herds of zebra have been filmed from the air. Just as with the films of fish, this information will be put into the computer model to analyse the animal's behaviour.

Dr Couzin handles the computer modelling: "When you're dealing with shoals, it is impossible to track each fish with the naked eye. Instead I have programmed a PC to automatically track the fish and determine their patterns of behaviour. From this information we can develop computer simulations of fish schools (pictured below). Then we can change the parameters in the model distance between fish, degree of alignment or attraction to see how that affects behaviour."

All behaviour is basically ruled by balancing out costs and benefits: how great a risk or advantage does taking an action involve, in terms of safety from or exposure to predators and losing or gaining access to mates or food?

Herds of animals, flocks of birds or shoals of fish stay together for safety, but the researchers have found that far from being a stable group, fish move between groups, and change their position within the group to satisfy their needs, generally in terms of food. When hungry, fish will move to a smaller group; when fed, they will move back to the larger. And the fish are able to tell how big a group is, by the distance between the members: smaller groups are more spaced out; bigger groups, closer.

Their findings have wider implications, as Dr Krause explains: "The commercial fishing industry needs to know how fish shoal. In the open ocean, with fish like tuna, for example, it's impossible to study them in this much detail. But understanding how the group functions is very important, for estimating population levels, setting fishing levels, and for conservation.

"Understanding group interaction and how much movement goes on between groups is also useful for predicting the spread of disease, particularly in fish farms."

Just as with humans, there is a debate as to whether animal behaviour is determined more by nature or nurture.

Using zebra fish, Dom Wright (pictured left) has been trying to test whether genetics or environment has the most influence on behaviour. Using two populations, one free population collected from Nepal which has a high tendancy to come together in shoals and a laboratory bred population which doesn't, he has been selectively mating and testing the offspring's shoaling behaviour. His results seem to show that patterns of behaviour are 50% determined by genetics, and 50% by other factors. The genome of zebra fish has nearly been sequenced, so Dom Wright is now trying to link the genes with the inherited behaviour.

Dom Wright said: "When you cross the lines, you can work out the genetic architecture of the trait. It's hard to find an exact gene for 'shoaling behaviour' but should be possible to work out roughly how many are involved, and their general location."

Other aspects also come into play, such as gender differences in behaviour, and even metabolic rate. Fish with a faster metabolic rate require food more often, and so are willing to take more risks in order to get it.

But many other aspects of behaviour clearly relate to environment, such as level of predators, attacks by parasites or even temperature which again affects the amount of food required.

While human beings might not face the necessity of finding food, or fear predators, the reason why we form groups, and how we behave in large crowds could be analysed using the same methods. It might even explain how we choose our friends.

Dr Krause: "We've noted a species in Canada which seems to form 'gangs'. In experiments in the field in Canada, we marked fish, followed their movements, and saw that they came back to form the same groups. We believe they must find it safer to stay with fish they know because they know how they will react."

Even among fish it's worth knowing who your mates are.

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