When army ants use their own bodies to plug tiny potholes in rough trails, the whole colony benefits, a new study has found.
Without those instant road repairs, a colony’s daily catch of food can drop by as much as 30 percent, say Scott Powell, now at the Federal University of Uberlândia in Brazil, and Nigel Franks of the University of Bristol in England. As the ants race along paths to and from food, filling holes helps prevent traffic backups, Powell explains.
Many species of army ants send out relentless columns of hunters at night or underground, but Powell and Franks focused on Eciton burchellii, which preys aboveground during the day. Colonies of these ants grow 700,000 strong.
Rather than building nests, they spend the night in ant-gripping-ant balls dangling from an anchor point, such as the side of a tree. As dawn breaks, up to 200,000 foragers swarm out. “The pitter-patter of millions of little feet sounds a lot like rain,” says Powell.
The ants’ goal: to prey on other ant species, various spiders, or even something as big as a scorpion. When the leading edge of a column of foragers catches up to a victim, the army ants form a mass, grab hold of the prey, inject enzyme-rich venom to weaken it, and pull it apart.
Raiders carry bits of the victim back to the main colony, where other workers are tending the youngsters. During an entire day’s expedition, the hunters maintain one principal two-way trail, from 3 to 12 ants wide, back to the rest of the colony.
To examine how pothole filling affects the well-being of the colony, Powell inserted a variety of wood strips, drilled with holes, into the ants’ principal trail.
As the first ant reached a hole, it stretched across and rocked, as if measuring the fit. A big ant didn’t bother to plug a small hole but left it for a smaller comrade. If an ant fit, it would hold a characteristic road-repair posture for as long as traffic continued to race over it. Then the ant would pop out of the hole and rush on.
“As trivial as pothole plugging may seem, Powell and Franks have empirically demonstrated how such behavior can contribute to colony fitness, and that makes it important,” comments army ant specialist William Gotwald Jr. of Utica College in New York.
Relatively large foragers lugging food run at an average of 8 centimeters per second, Powell reports. The pothole repairs help prevent the ants from getting stuck on a narrow stretch of trail where they’d have to slow to the pace of smaller raiders. The smaller ants travel on average about 6 cm/s, the researchers report in the June Animal Behaviour.
“It’s a great example of a novel function where a small investment yields big efficiency gains,” says Sam Beshers of the University of Illinois at Urbana–Champaign, who studies the organization of ant colonies and other biological systems.