The Ants Go Marching … to the Rescue

The remarkable rescue behavior of ants has proved to be a rich source of research for psychology and education professor Karen Hollis and her students.

In a series of studies, Hollis, her collaborator Elise Nowbahari of the University of Paris, and their students have investigated the rescue behavior of several species of sand-dwelling ants. In the wild, these ants coexist with antlions, larval insects that ambush prey from funnel-shaped pits dug into sand.

“We’re interested in the predator-prey relationships between ants and antlions,” says Hollis. “We want to learn more about how predators find better ways to catch prey, and how the prey gets better at evading predators.”

It’s been known for some time that ants, after detecting pheromones, a chemical call for help, will try to rescue trapped nestmates by digging sand and pulling on the victim’s limbs. But in a 2009 lab experiment, Hollis and Nowbahari observed a new behavior: After discovering a trapped nestmate—tethered in place by a tiny snare—the rescuer ants would remove debris from around the ant and bite the snare to try to free the victim.

This behavior piqued the team’s interest. In attempting to rescue a nestmate, an ant is potentially risking its own life, which is an extreme form of altruism, explains Hollis.

“This behavior raised many questions,” she says. “Do all ants exhibit this behavior? Do ants learn from their encounters with predators, and if so, do they apply this learning to future rescue attempts?”

Last year, Hollis and her collaborators answered some of these questions by comparing rescue behavior across five species of ants that live in different types of sand in the wild. They found, as described in a feature published by a British journal, Animal Behaviour, that the two species inhabiting loose sand exhibited the full range of rescue behavior toward nestmates. The three species that lived in habitats with more compact sand, away from antlion pits, rarely tried to rescue nestmates.

To further test this behavior, Hollis asked her student research group to conduct new experiments in a basement lab in the Reese Psychology and Education Building. The students ordered hundreds of predators from an antlion “farmer,” placed each antlion in its own habitat—a container filled with sand—and collected colonies of ants from around the campus.

In the first student-led experiments, Katie Taylor ’12 and Allison Visvader ’12 studied the level of danger ants are willing to experience to rescue nestmates.

Taylor and Visvader found that rescuers directed their rescue behavior to address a specific threat. In trials using snares to hold down victims, the rescuers moved sand from around the victim and bit the snare.

But the ants’ rescue behavior changed in trials with actual predators. In these tests, the students dropped a victim into an antlion pit and then placed a second ant in the pit. In most cases, the second ant attempted to rescue its nestmate by stinging or trying to dismember the antlion by pulling on its mandibles. In one memorable trial, a rescuer ant managed to wrestle the victim away from the antlion, which promptly grabbed the rescuer instead. The freed ant, instead of fleeing, then tried to rescue its nestmate.

Taylor and Visvader documented the results of their research—including the first instance of rescue behavior observed in North American ants—in their paper Extreme Altruism: Precision Rescue Behavior in North American Ants, soon to be published in the Journal of Evolutionary Biology.

This semester, students in Hollis’s lab investigated whether ants learn from their experiences with predators. In particular, the students wanted to understand how trapped ants’ pheromones trigger rescue behavior in nestmates.

Kelsey McNew ’13 placed an ant in an antlion pit. After much scrambling and wriggling, the ant escaped the predator. While it recovered from the ordeal, McNew dangled another ant by a tether in a different antlion pit. This ant released its distress pheromones into the trap. Then, the first ant was put into this pit. Confronted with a chemical call for help from a nestmate, but having just survived a traumatic encounter with an antlion, how would the ant behave?

The results have yet to be collated, but McNew says the students have observed a range of behavior: from some rescuers fleeing to others diving into the pit looking for a nestmate.

Hollis’s research is generating plenty of buzz in science and mainstream media, in part because some scientists are offering rescue behavior as evidence that animals feel empathy toward their kin.

Hollis disagrees. Far from showing empathy for trapped nestmates, the rescuer ants are acting based on genetic programming for the good of the colony, she says.

“Taken together, our results indicate that rescue behavior is an effective antipredator strategy that is triggered and directed by biological mechanisms,” she explains.

“Ants’ ecological success stems from their social behavior, such as division of labor for particular tasks. It makes sense that, under particular environmental conditions—such as living in a sandy habitat with voracious predators—ants would have evolved to take risks to rescue their nestmates and help them get back to work.”