In 1907, Sir Francis Galton asked 787 villagers to guess the weight of an ox. None of them got the right answer, but when Galton averaged their guesses, he arrived at a near perfect estimate. This is a classic demonstration of the “wisdom of the crowds”, where groups of people pool their abilities to show collective intelligence. Galton’s story has been told and re-told, with endless variations on the theme. If you don’t have an ox handy, you can try it yourself with beans in a jar.

To Iain Couzin from Princeton University, these stories are a little boring. Everyone is trying to solve a problem, and they do it more accurately together than alone. Whoop-de-doo. By contrast, Couzin has found an example of a more exciting type of collective intelligence—where a group solves a problem that none of its members are even aware of. Simply by moving together, the group gains new abilities that its members lack as individuals.

Couzin–one of National Geographic’s Emerging Explorers–has spent his whole career studying animals that move in shoals, flocks and swarms. His early work involved ants and locusts but when he started his own lab at Princeton, he thought he’d upgrade to a smarter group-living species. Unfortunately, he ended up with the golden shiner—a small, bland, minnow-like fish that’s dumb beyond the telling of it.

Consider this: shiners have a natural preference for darkness. Plop a shoal of them into a pool of water, and they’ll head for the shadiest bits. This is something that animals do all the time: They track gradients in their environment. A migrating robin might follow the Earth’s magnetic field, a moth might follow the scent of a flower, or an ant might track the pheromones laid by its nest-mates. But single shiners are laughably bad at this.

Andrew Berdahl and Colin Torney from Couzin’s team discovered their ineptitude by projecting shifting patterns of light over a shallow pool and adding the shiners in increasing numbers. Overhead cameras tracked their movements, and the team calculated how good they were at chasing the shadows.

The solo fish did so badly that they were almost swimming randomly. Only larger shoals were good at avoiding the shifting light. Even then, Berdahl and Torney found that the shiners’ movements were far more influenced by what their neighbours were doing, than by how bright the environment was.

That’s the key. The individual fish aren’t tracking anything. That would involve realising, for example, that it’s getting darker over there compared to over here, and swimming over there. Instead, they obey one very simple rule—swim slower when it’s dark. Each fish just reacts to how bright it is in its current position. How bright or dark is it right here? That’s a scalar measurement. It’s the shoal that converts these local readings into a vector.

To understand how this works, imagine that a shoal of swimming shiners grazes a patch of shadow. The fish that first enter the shade slow down. But the rest of the shoal doesn’t shoot off into the distance. Shiners have a strong instinct to stick within a certain distance of their neighbours. They almost behave like a rigid block, so if one end slows down, the rest of them swivel… right into the shade.

Once inside, they all slow down. They start bunching up together like cars in a traffic jam. And if the shadows move, so they find themselves in light, they start swimming faster again and leave.

Berdahl and Torney’s discovery flies against the “many wrongs” principle, which biologists have invoked to explain the migrations of natural groups since 1964. The idea is that groups can track gradients in their environment because each individual makes an imperfect decision about where to go. When the crowd pools their estimates, they cancel out each other’s errors, and mutually arrive at the best possible vector. It’s just Galton’s ox again, but applied to migrations.

But the shiners are patently not pooling estimates—the individuals are so bad at tracking gradients of light that it’s hard to believe that they’re making estimates at all. But by adhering to the simple instinct that keep them together as a shoal, the shiners can transform acts of individual detection into an act of group navigation.

That’s collective intelligence! The shiners’ ability to stay in shade emerges from neighbourly interactions of dumb units. The fish aren’t pooling decisions that each individual makes on its own—they’re collectively processing information. By moving as one, they can compute as one.

Couzin suspects that this phenomenon is goes well beyond shiners, and might apply across a variety of migrating animals. After all, the rules that shiners obey are so simple that they should be a doddle for natural selection to produce. You don’t even need a brain to pull off the same trick—just the ability to respond to the environment, and to stay as a group. Cells can do that. All sorts of animals can do that.

This may be important for conservation. Couzin’s team showed that the shiners’ ability to follow the light suddenly collapsed when they shoals fell below a certain size. And we have repeatedly slashed the group sizes of many animals by hunting them or destroying their habitats. If those groups become sufficiently fractured, and their numbers fall below a certain threshold, they may lose abilities that they only have en masse.

Reference: Berdahl, Torney, Ioannou, Faria & Couzin. 2013. Emergent Sensing of Complex Environments by Mobile Animal Groups. Science http://dx.doi.org/10.1126/science.1225883