Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Swarm Savvy” by Susan Milius (@2009 by Society for Science & the Public).

Only a few millimeters long, rock ants (Temnotho-rax albipennis) prove difficult to track in the wild but excellent for the tabletop world of the laboratory.

When something terrible happens to a rock ant home, such as a researcher lifting off the roof, the majority of ants cluster in the ruins. A quarter to a third of the colony scurries out looking for new possibilities.

“I think of the ants as a sort of search engine,” ant biologist Nigel Franks says. In one set of tests, he and his students disrupted a nest and watched to see what the ants would make of a series of new possibilities that improved with distance. The best nest was almost three meters distant, nine times as far from the original home as a nearby but less appealing choice. “It was just such fun doing this experiment because the ants won,” Franks says.

In spite of the epic distances, the ants typically found and agreed to move into the best nest. “They’re fantastic at it.” Franks says.

Franks and Elva Robinson, both of the University of Bristol, monitored rock ants by fitting them with radio-frequency identification tags. The data suggest that each scout follows a simpler rule than previously thought, Robinson, Franks and their colleagues reported online in Proceedings of the Royal Society B.

Instead of making direct comparisons between sites, a scout follows a threshold rule. If she finds a poor site, she keeps searching. When she finds a site that exceeds her “good enough” threshold, she returns to the original nest.

Next, previous work shows, the scout recruits a new scout to join her on a trek to the good site. She dashes around tapping her antennae on other ants and releasing a pheromone from her sting gland, explains Stephen Pratt of Arizona State University in Tempe. Usually she finds a volunteer within a minute or so, and the two set off tandem running.

Scout A, who knows the way, runs back toward the nest while her follower, B, jogs closely enough to tap antennae against the leader. Should A sprint a little too fast and dash beyond antennae range, she slows until her partner catches up. Periodically the two ants stop, and the newbie looks around as if learning landmarks. It’s a slow way to get to the site, and Franks argues that it qualifies as animal teaching.

When the ants do reach the possible site, the recruit explores it and, depending on her assessment, returns to recruit yet another scout.

As with bees, it’s the quorum of scouts at the sites that matters. When enough of them gather at a particular place to encounter each other at a sufficiently high rate, they’ve got a decision.

Once scouts reach that decision, their behavior changes. Each scout dashes back to the nest, but instead of coaxing a nest mate for a tour, she just grabs somebody. She uses a mouthpart hook, an over the shoulder throw, and off she goes with the passive nest mate curled on her back in an ant version of the fetal position. Carrying takes about a third as long as leading would, and scouts can haul the rest of the colony to a new home within hours. The ants shift from the independent info gathering of scouts to group implementation of the quorum’s decision.

Rock ants’ willingness to thrive in the lab allows experiments on finer points of collective decision making, Pratt says. For example, forcing a crisis among the ants demonstrates that they will, in a pinch, trade accuracy for speed. When researchers destroy an old nest so that ants are completely exposed, the ants scope and relocate within hours. Other experiments that just offer the ants a better nest but don’t ruin their current one can result in days of deliberation. Speed has its costs, and ants in a hurry now and then make mistakes, such as splitting the colony between two nests. Slower moves prove more accurate.

The quorum system could be widespread in group behavior in nature, Pratt says. Overall it’s a beautiful tool, allowing for carefully balanced independence plus some shortcut speed. Yet the system “has a dark side,” he acknowledges. Once individuals have made their independent assessments and then a quorum has reached agreement, fellows copy the quorum behavior. The chances are low that the whole quorum will reach the same wrong decision. But flukes can happen. In most uses of a quorum, “it’s going to make a decision more accurate,” he says, “but it also slightly increases the incidence of these rare events when you get it really spectacularly wrong.”