Fish choose leaders by Consensus

16 November 2008


3 spined sticklebackWhile the world headlines are still full of the news that the Americans have elected Barrack Obama as the first African-American president there is also news this week of how members of the fish world elect their leaders. 

When it comes to deciding which leader to back, most of the time fish reach a consensus to go for the most attractive of two possible leaders. But like a stereotypical sheep following the flock, fish will also follow whatever choice most of the rest of their shoal take - whether it's the right choice or not. 

This is a study from a team of researchers led by David Sumpter of Uppsala University in Sweden and Ashley Ward of Sydney University in Australia, which they published this week in the journal Current Biology. 

They conducted experiments in aquarium tanks with little freshwater fish called three-spined sticklebacks. From previous studies the researchers knew that the fish had certain preferences for leaders; they tend to follow fish that are bigger and plumper, with fewer spots that could indicate they have a parasitic infection. In general they back the fitter more successful fish. 

But what the researchers wanted to know was whether a shoal of fish comes to a group decision on which leader to back by consensus, in other words, do they make decisions that reflect the general opinion of the group?  

A similar thing happens when people are asked to sit on a jury in court. It was an 18th century French Philosopher, Condorcet, who showed that as the size of a jury increases so does the chance that the group will correctly decide if the defendant is guilty or not. And it seems a similar thing does indeed happen with fish. 

Sumpter and Ward made replica fish that had different levels of fishy attractiveness: some were bigger and fatter, some small, skinny and spotty - and displayed various pairs of possible leader fish to groups of sticklebacks. And quite simply, the groups checked out the two possible leaders and swam towards the one they chose. 

The researchers saw that as the size of the shoal increased, the fish got better at accurately choosing the better leader, which is the one that is bigger, better fed and free from infections. 

While the consensus of the group was accurate most of the time, occasionally the shoal of fish would slip up and make the wrong decision, swimming towards the less attractive leader. 

But who can blame the sticklebacks, since sometimes us humans make the same mistakes? Even if copying what the majority of other people around us are doing isn't always the right thing to do, on the whole it tends to be a good strategy.


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