Published online 3 July 2007 | Nature | doi:10.1038/news070702-4

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Rats do unto others as they have been done to.

A helping paw: rats are more generous when another rat has helped them out. Punchstock

Rats that benefit from the charity of others are more likely to help strangers get a free meal, researchers have found.

This phenomenon, known as 'generalized reciprocity', has only ever been seen before in humans. A good example, says Michael Taborsky of the University of Bern, Switzerland, is what happens when someone finds money in a phone box. In controlled experiments such people have been shown to be much more likely to help out a stranger in need following their good luck.

In humans, such benevolence can be explained by cultural factors as well as by underlying biology, says Taborsky. But if similar behaviour can be found in other animals, he reasons, an evolutionary explanation would be far more likely.

To test for this behaviour in animals, Taborsky trained rats to pull a lever that produced food for its partner, but not for itself. Rats who had received a free meal in this way were found to be 20% more likely to help out an unknown partner than rats who had received no such charity1.

Taborsky believes this behaviour isn't confined to just rats and humans. "I'm convinced generalized reciprocity will be very widespread and found in many different animal species, as our study suggests that an underlying evolutionary mechanism is responsible."

"Generalized reciprocity is certainly underappreciated in animals," notes Laurent Keller, an evolutionary biologist at the University of Lausanne, Switzerland. But with so little data in this area, he adds, "it is very difficult to make predictions about how prevalent it's likely to be."

Survival of the friendliest

Explaining why animals cooperate has long been a tricky area for evolutionary biologists. According to darwinian evolution, organisms are adapted to maximize their individual chances of survival, so how is it possible to account for acts of charity?

Among family members, it makes sense for one animal to help another in order to help their shared genes to get on in the world.

But when it comes to helping non-family members, things are more complicated. It might make sense for one animal to help another in exchange for receiving help themselves later on. But this is open to abuse by cheats who are happy to take, but not to give in return.

Highly intelligent animals — including humans — can judge whether or not to give help based on the individual track record of a specific potential partner. But this kind of 'direct' reciprocation between two individuals only happens under restricted conditions, says Taborsky. "Animals have to meet frequently and have to remember what other individuals have done and how they acted in the past. That means animals have to possess high cognitive abilities. And for these reasons it hasn't been demonstrated very often."

Another strategy, called 'generalized reciprocity', is for an animal to assume that its most recent interaction with any other individual is representative of how the whole community usually behaves. In that case, an animal only has to remember its last experience. "It's a simpler mechanism and therefore more likely to be evolutionarily important," says Taborsky.

Not-so-dirty rats

Taborsky thinks he has seen both types of reciprocity in his rats. In addition to being 20% more likely to help out an unknown partner if they had received a bit of charity, his study shows that rats were more than 50% more likely to help a specific rat who had helped them in the past1.

Taborsky thinks that it is likely that the two forms of reciprocity work together. If an animal can remember how generous another individual is it will use this information to decide whether it's wise to cooperate. But if not, then it can base its decision on more general recent experience — how helpful other animals have been towards it in the recent past.

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Other researchers point out that because rats typically live in family groups, it's hard to discount the idea that they're just trying to help their relatives out.

"What we need to understand now is the physiological and neurological mechanisms responsible," says Taborsky. He suspects that hormonal changes in response to charity might be behind the tendency to cooperate. "We are going to look at hormonal regulation, and we're also looking at other animals — cichlid fish at the moment."

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