The research, by graduate student Michael Sheehan and assistant professor of ecology and evolutionary biology Elizabeth Tibbetts, suggests that the wasps' social interactions are based on memories of past encounters rather than on rote adherence to simple rules.

Until now it was assumed that all social insects had very limited memories. Honeybees can remember where they've found nectar, "But those memories are pretty fleeting," Sheehan said. "There seems to be a limit to the number of things they can juggle in their head at one time."

Tibbetts had previously showed that the wasps recognize individuals by variations in their facial markings and that they behave more aggressively toward wasps with unfamiliar faces. In the new research, Sheehan measured aggression between 50 wasp queens in four different encounters over eight days. On the first day, two wasps that never had met were placed in an observation chamber for a day and their initial interactions videotaped. Then the pair was separated, and each wasp was put in a communal cage with 10 other wasps. A week later, the pair met again, and again their behavior was videotaped.

It was clear from teh results that the wasps treated each other better during their second encounter than when they were strangers, suggesting they remembered each other. "Instead of trying to bite each other and really have a rough-and-tumble encounter, they just sort of hung out next to each other when they met the second time," Sheehan said.

Recognition matters to the wasps because Polistes fuscatus females often share nests, so that it is adaptive for them to be able to recognize nest-partners. Most social insects use smell to identify nest-partners; separate research by Wulfila Gronenberg, associate professor of neurobiology and ecology and evolutionary biology at the University of Arizona, and who had previously worked with Tibbetts, has shown that in the paper wasp, the antennal lobe (used for smell recognition) is smaller than in other wasps, while the so-called mushroom body subcompartments, which integrate information from the senses and help control learning and memory, were not any larger than usual.

Sheehan points out that the findings of the Michigan research challenge assumptions about social cognition, which is generally thought to require a large and advanced brain.