Sarah Lucas

How does a shoal of fish decide where it’s going? Collections of animals like shoals of fish or flocks of birds are clearly able to make decisions about where to go when seeking out food or shelter, without anything that we would recognise as a decision-making process.

Fish school. Source:

For some groups of animals, ‘leaders’ correlate to individuals that are recognisable as having more knowledge or experience, for example, the matriarch in a herd of elephants. Within a group such as a fish shoal there may likewise be more informed individuals, but the capacity of other fish to recognise them as such is perhaps doubtful. The cognitive capacity of a fish for individual recognition aside, fish in shoals are also so close together that their ability to detect an informed individual any distance away is likely to be prevented by crowding. So how can ‘leaders’ inform the direction of a shoal if the other fish don’t know who the leaders are?

This problem was explored by Couzin et al.1, who constructed a model of an individual’s behaviour in a group, applicable to (in our example) fish in a shoal. In this model, the fish have a primary priority to avoid collisions and maintain a region of ‘personal space’. However, if nothing is detected in this region, fish become attracted to neighbours within a certain interaction range, orientating themselves travel in the same direction. ‘Informed’ individuals (leaders) are created by modelling certain fish as having a direction that they want to go. Nowhere in this model is there any mechanism for fish to tell whether or not their nearest neighbours are leaders.

When the model was run it was found that, provided a certain number of individuals were informed, the entire group was able to accurately move in the desired direction. It seems that it is entirely possible for groups such as fish shoals to transmit decisions made by leaders simply by all following one another indiscriminately. Interestingly, the larger the group, the smaller the proportion of informed individuals needs to be. In a group of 200, only five individuals need to be informed for maximum accuracy.

But what about when there are two ideas about where to go? Fish have no way of overtly knowing which is the majority view (it’s not as though there is any sort of discussion or vote) so how do they decide? This was investigated by Couzin et al2. using the same model, and it was shown that the group moves in the direction favoured by the majority of ‘leaders’, again simply through the mechanism of following one another, and even if the number of leaders is small compared to the size of the shoal.

They also took the question a step further – what happens if the strength of the ‘desire’ of the minority to go in one direction is stronger than that of the majority? They modelled this ‘desire’ as the strength of an individual’s preference to go in a certain direction, which trades off against its attraction to its neighbours and hence tendency to stay with the group. The answer is that although a strong enough minority preference can lead to movement in the minority direction if all fish are informed, addition of uninformed individuals tends to return control to the numerical majority. More uninformed fish seems to make the process more democratic!

Finally, group splitting in the model very seldom occurred. This is because fish with no strong directional preference have staying with the group as their top priority and so act as ‘social glue’. The ‘social glue’ tendencies of uninformed fish were observed in real life experiments by Ioannou et al.3, in which leader fish, trained to locate food in one corner of a tank, were placed with groups of untrained fish. Though the leader fish almost always reached the food, either all of the followers followed, or none did. Leaders of fish schools have to tread a balancing act: if they are too keen to get where they’re going, they can leave all of their followers behind.


1. Couzin, I.D., Ioannou, C.C., Demirel, G., Gross, T., Torney, C.J., Hartnett, A., Conradt, L., Levin, S.A., Leonard, N.E., 2011. Uninformed Individuals Promote Democratic Consensus in Animal Groups. Science 334, 1578–1580.

2. Couzin, I.D., Krause, J., Franks, N.R., Levin, S.A., 2005. Effective leadership and decision-making in animal groups on the move. Nature 433, 513–516.

3. Ioannou, C.C., Singh, M., Couzin, I.D., 2015. Potential Leaders Trade Off Goal-Oriented and Socially Oriented Behavior in Mobile Animal Groups. Am. Nat. 186, 284–293.

lucas-s-image-300x300“I am a second year PhD student in the Earth Sciences Department researching rapid climate change in the Cretaceous, though my interests range across the whole span of Environmental Research. I try to blog about things I find ‘quite interesting.”