Evasive Maneuvers Reveal That Social Insects Constantly Learn From Bad Experiences
Beside a one-lane unpaved road snaking through dense rain forest some 60 miles east of Panama City, William Wcislo is lying belly-down in the dirt for a close-up view of abduction and murder.
Lurking near the entrance to an underground nest of the soil-dwelling Panamanian sweat bee (Lasioglossum umbripenne), a leggy, big-eyed worker ant (Ectatomma ruidum) waits patiently in ambush.
Before long, a metallic-green female sweat bee, carrying a load of fresh pollen, comes in for a landing. She pauses at the nest entrance as a guard bee checks her credentials, using her scent as a chemical password.
Lunging, the ant plunges her mandibles into the bee's backside, curls her body beneath her victim and jabs her tail-stinger into the bee's throat. The sweat bee is then carried off—pollen and all—to the ant colony as a future meal.
Predator-prey strategies
"Ants are among the world's premier predators and the most important predators of other social insects," explains
Wcislo, an evolutionary biologist at the Smithsonian Tropical Research Institute in Panama. "Throughout their long history, ants have evolved diverse predatory strategies." In turn, insects such as the sweat bee must continually develop new strategies to counter the ants' attacks.
During initial observations of interactions between L. umbripenne and E. ruidum, Wcislo—who has studied social insects for some two decades—noticed the bees using evasive tactics he had not seen or read about before.
By studying how sweat bees recognize ants and the stimuli that trigger their evasive behavior, Wcislo says, he is working to unlock the secrets by which "social insects use past experience and the circumstances of their environment to modify their behavior in precise ways." Such studies may someday help explain the role that psychology and the brain play in evolutionary biology among all animals.
Stimuli and response
With French ant expert Bertrand Schatz and support from the Smithsonian's Fellowship Program, Wcislo set up a simple experiment to study the reactions of sweat bees to ants—both dead ants and live ants—that the scientists placed near their nest entrances. The scientists conducted their study over the course of five years, focusing on some 1,500 different L. umbripenne nests.Wcislo and Schatz wanted to know what senses—visual, olfactory or other—bees use to detect ants.
In normal circumstances, a returning bee will approach the nest entrance and immediately enter. When an ant was placed near a nest entrance, Wcislo explains, "a bee was significantly more likely to abort the first approach flight and re-approach the nest on the side opposite the ant's position."
Other evasive strategies the bees use included flying up to the nest in a confusing zigzag pattern and landing a short distance away from the nest and walking to the entrance.
Dead ants that had their "scent" chemically removed by the scientists elicited the same evasive responses from bees as did live and dead ants with their scent intact. This proved the bees "saw" the ants.
Bees did not react strongly to ant-sized squares and rectangles of black paper the scientists placed near their nests. This indicated that the bees saw some aspect of the ant's morphology or profile that they associated with danger. "Just what aspect of the ant's body is recognized by the bees remains unknown," Wcislo says.
How bees share information with each other about predators also is a mystery. "Evasive bee behavior becomes even more difficult to understand when you consider that a bee that has a bad experience with an ant is usually killed and cannot learn from the experience," Wcislo says.
Counterattack
Among ants, ambush has long been a tried-and-true method of attack. During the study, Wcislo and Schatz observed ants using a strategy to counter the bees' evasive zigzag flying.
As a bee zigzagged, the ant rapidly and excitedly pirouetted across the top of the nest entrance, apparently surveying the 360-degree area surrounding the nest in an attempt to see the bee as it landed. This behavior occasionally resulted in ants capturing bees that had landed away from the nest and were walking in.
For the bees, evasive behavior can have an indirect cost to the hive, Wcislo points out. It decreases the pollen gathering efficiency of the entire nest. Walking in, aborted landings and flying in evasive ways burn up time and energy the bees could be using to gather more pollen and nectar.
"Predator recognition has been little studied in insects," Wcislo says, especially with regard to learned behavior.
"Our work is exciting in that it belies the common view that insects are miniature automatons with hard-wired brains and robotic behavior," he adds. "This is erroneous. The behavioral flexibility of sweat bees shows the complexity of this predator-prey relationship and illustrates the importance of information processing by both species involved."
—Hope Cristol
This article originally appeared in Inside Smithsonian Research, Autumn 2004.
