## Raven gangs predicted by maths

Posted on Friday 5 June 2009 at 5:43 pm by Jacob Aron
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Gangs of teenagers roaming the land are generally bad news, but not when it comes to ravens. Juvenile ravens hunting in packs have gotten some scientists very excited, as this behaviour was predicted by a mathematical model before ever being seen in the wild.

Dr Sasha Dall lectures in mathematical ecology at the University of Exeter, and in 2002 set out to solve an evolutionary puzzle: why do young ravens share their food? Natural selection tells us organisms should only help themselves and their relatives. It seems that no one told the ravens.

Typically, juvenile ravens spend their winters drifting in and out of communal roosts. They scavenge for food, usually sheep carcasses, by themselves. Having found a tasty meal they return to the roost and recruit other ravens for a feast the next day. These shared dwellings can house up to 100 individuals, but they don’t stick around. Each bird will move on every few days to another roost and probably won’t encounter their former roommates again.

“From an evolutionary perspective, this is a bit weird,” says Dall. The ravens are unrelated so will not pass on their genes by helping out others. They also don’t encounter the same individuals often enough to build up a sense of co-operation. Using a technique called game theory, in which many different strategies are played out, Dall built a model to explain this unusual behaviour.

The favoured hypothesis amongst ecologists was roosts act as a kind of “information centre” to the advantage of all the juveniles. Individual birds are unlikely to find a carcass by themselves, but if every bird shares information about food locations then they all benefit.

Dall’s model showed that this strategy emerged naturally when ravens try to maximise their access to food. “In the long run, they find more carcasses than they otherwise would,” he says. Bringing a few friends along also allows young birds to chase off any adults who might lay claim to carcasses in their territory.

Problem solved then – except the model didn’t provide just one answer. “I did manage to predict this typical behaviour, but my model came up with another evolutionarily stable strategy,” explains Dall. According to the model, gangs of juvenile ravens should also fly around looking for food together, and never roost in the same place twice. But no-one had ever seen this kind of behaviour.

Perhaps this would have been dismissed as purely mathematical curiosity, if weren’t for Jonathan Wright, professor of biology at the Norwegian University of Science and Technology. Wright was studying a large raven roost in North Wales when he noticed that the juvenile birds were organising themselves into hunting packs, just as Dall predicted.

“I was surprised to discover that this behaviour had been observed somewhere,” says Dall. The variables used in the model, such as the size of the ravens’ search area, matched the real world exactly. The two scientists wrote up their findings in a joint paper, published earlier this year in the journal PLoS One.

So what will Dall turn his mathematical predictions to next? “The evolution of animal personality differences,” he says. Dall plans to investigate why animals of the same species behave differently within social groups. Perhaps game theory has the answer.

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