Sneaking eavesdroppers gain insight into potential opponents’ combat skills

Paper wasps listen to rivals struggling to quickly assess potential opponents without personal risk. This new finding adds to the growing evidence that even mini-brained insects have an impressive ability to learn, remember, and make social deductions about others.

Many vertebrate animals, including some birds and fish and numerous primates, minimize the costs of conflict by using “social eavesdropping” to learn about potential rivals’ fighting ability before interacting with them personally.

Tracking a network of individually differentiated social relationships is believed to be a cognitive challenge and, until recently, was considered out of reach for humble insects like paper wasps, which have brains a million times smaller than the human brain.

But mounting evidence suggests that insects’ miniature nervous systems don’t limit sophisticated behaviors. The ability for complex insect behavior may be determined more by social environment than by brain size, according to University of Michigan biologist Elizabeth Tibbetts, lead author of an article scheduled for publication June 25 in Magazine. Current biology.

“It is surprising that wasps can observe and remember a complex network of social interactions between individuals without directly interacting with them,” said Tibbetts, a professor in the UM Department of Ecology and Evolutionary Biology. “Complex social relationships are believed to favor the evolution of large brains and greater social intelligence, but the brains of paper wasps are relatively small.”

In the study, Tibbetts and her students collected female paper wasps Polistes fuscatus from sites around Ann Arbor, Michigan, in early spring.

Unlike a honey bee colony, which has a single queen and several equally classified workers, paper wasp colonies contain several breeding females called founders. These females fight their rivals and form complex and linear dominance hierarchies based on the results of those fights. The rank of a wasp in the hierarchy determines its participation in reproduction, work and food.

In the laboratory, the researchers used enamel to mark all the founders with unique color patterns on the chest. Then, two at a time, “fighting” wasps were placed in a small container known as the battlefield, while two “bystander” wasps observed the pair through clear plastic partitions.

All trials were videotaped, and a research assistant assigned scores to each wrestler using an aggression index that awards points for behaviors such as biting, riding, grabbing, and stinging. The dominance range was determined using the number of mounts, a dominance behavior in which the dominant wasp drummed its antennas on the subordinate while the subordinate ducked and lowered its antennas during a fight.

Later, the spectator wasps were paired in the combat arena with a wasp they had observed (experimental test) or a fighter they had never seen before (control test). Tibbetts and his students compared the behaviors in the experimental and control trials to determine the role of social eavesdropping.

They found that bystander wasps were more aggressive when combined with an individual who was the victim of much aggression in a previous fight, as well as individuals who initiated very little aggression in the previous fight.

By comparing experimental and control trials, the researchers were also able to reject non-espionage explanations for the observations, including phenomena called priming and winner / loser effects.

“The results show that P. fuscatus wasps use social eavesdropping,” said Tibbetts. “Spectators watch other people’s fight and use the information about the fight to modulate subsequent behavior.”

In previous studies for more than a decade, Tibbetts and colleagues showed that paper wasps recognize individuals of their species by variations in their facial markings, and behave more aggressively towards wasps with unknown faces.

They also showed that paper wasps have surprisingly long memories and base their behavior on what they remember from past social interactions.

But previous work focused on how wasps use individual recognition during direct interactions, and did not assess, as this new study did, whether wasps find out about other individuals only through observation.

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