Paper wasps never forget faces

Washington, September 23 : Paper wasps can remember their meeting with an individual for at least a week, even after interacting with many others in the meantime, say University of Michigan researchers.

The discovery made by graduate student Michael Sheehan and assistant professor of ecology and evolutionary biology Elizabeth Tibbetts suggests that the social interaction among wasps may be based on memories of past encounters, instead of on rote adherence to simple rules.

Writing about their findings in the journal Current Biology, the researchers agree that honeybees, too, can remember where they have found nectar.

"But those memories are pretty fleeting. There seems to be a limit to the number of things they can juggle in their head at one time," Sheehan said.

While scientists have long believed that all social insects have similarly limited memories, the new study shows that at least one species of paper wasp, Polistes fuscatus, has a strong, long-term memory, and that it bases its behaviour on what it remembers of previous social interactions with other wasps.

During the study, the researchers 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. They were later separated, and each of them was put in a communal cage with 10 other wasps.

The pair met again one week later, and their behaviour was videotaped once again.

Analysing the videotapes, the researchers observed 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.

With a view to determining that any differences in aggression between the first and second encounters actually were based on memory, not just some general mellowing over time, the researchers introduced each wasp to a new stranger on the day before and the day after the encounter with its old familiar social partner.

They observed that the wasps were just as nasty to total strangers as they had been to each when they first met.

"The interesting aspect of this work is not just that the wasps have a good memory, but that it's social memory. It seems that their specific social history with particular individuals is something they're keeping track of and that it matters to their lives," Sheehan said.

The researchers say that these observations challenge assumptions about social cognition.

Scientists have thought the ability to form social memories and use them as the basis for complex relationships was a driving force behind the evolution of large brains.

Sheehan, however, said that if tiny-brained wasps have such ability, perhaps it does not demand as much brainpower as previously thought.

ANI