A school of fish recently held the aquatic equivalent of a two-party election to choose a leader. There was no electoral college involved, and the outcome wasn't even close.

Instead, the choice of leader was, as is common among fish, a landslide.

Exit polls found that the voters (they were all sticklebacks, by the way, and known to cheat to win) went for the big fish in the pond, preferring the fat and healthy candidate over the thin and quite possibly ill alternative. In fact over the course of several elections, they voted in this same manner.

Further analysis revealed that the larger the electorate the more likely the sticklebacks would choose the healthier leader.

Scientists have explained all this — really — in the Nov. 13 issue of the journal Current Biology.

Fishy voters

In the study, Ashley Ward of Sydney University presented groups of three-spined sticklebacks (Gasterosteus aculeatus) with two fish replicas, with each pair differing in size, fatness, shade or spottiness, all of which reflect something about the health of the fish. For instance, a plump belly can indicate success in snagging food, while spots may signal a parasitic infection.

When shown the fish replicas, the other sticklebacks in the tank would approach and follow one of the two replicas, which were moved around by remote control. Following a certain fish would be their version of casting a ballot.

The researchers conducted the experiment with one, two, four or eight sticklebacks (voters), giving each group the choice between two replica fish. Results showed that as the number of fishy voters increased, the fish made more accurate decisions, better discriminating subtle differences in the replicas' appearances.

When just one fish chose its leader, the fish would make the right choice, picking the healthiest leader about 55 percent of the time. That number went up to 80 percent with the eight-fish electorate, for which all or all but one would follow the same leader.

"Some fish spot the best choice early on, although others may make a mistake and go the wrong way," said researcher David Sumpter of Uppsala University in Sweden. "The remaining fish assess how many have gone in particular directions. If the number going in one direction outweighs those going the other way, then the undecided fish follow in the direction of the majority."

Consensus rules

It makes sense to follow a fish going off toward one of two refuge areas in the tank, Sumpter said, and it makes even more sense to follow a group of fish following the leader.

"If a fish goes somewhere, it's probably going there for a reason, so by following it you have a chance to get in on whatever it knows about," he said.

And when fish copy others, the probability they'll make the correct decision is greater than if each fish makes an individual decision, Sumpter said.

"The chance of the majority being wrong decreases very fast with the number of individuals in the group," he said.

Human jury systems are also based on this consensus approach. Past research has shown the probability that a majority of independent-minded individuals is correct in a decision between "guilty" and "not guilty" increases with group size.

Of course, the follow-the-crowd mentality can lead a fish, or human, astray.

"Now and again these larger groups [of fish] will actually make the wrong decision but they'll all make it together," Sumpter told LiveScience. "When they make a mistake they all make the mistake together, and they all go off following the less attractive fish."