Friday, July 13, 2012

Elusive Killer Ants Explained; Bop Insects on Heads

Vicious hunters also live in giant treetop colonies, study shows. A trap-jawed ant seizes a butterfly after hitting it on the head.

An D. armigerum ant seizes a butterfly after striking it on the head.

Photograph courtesy Alain Dejean, CNRS/ECOFOG

Rachel Kaufman

for National Geographic News

Published July 3, 2012

High in the trees of French Guiana (map), the trap-jawed ant Daceton armigerum dominates other ants and takes down prey a hundred times its own weight, according to a new study.

Researchers found that the elusive ant hunts by ambushing a larger insect and bopping it on the head, stunning the prey for several seconds. Then nestmates arrive, grab the unfortunate insect in their powerful jaws, and pull backward, spread-eagling the victim.

(See "Pictures: Wasps Turn Ladybugs Into Flailing 'Zombies.'")

If that's not enough, the ants also have stingers that can inject paralyzing venom, as well as viselike mandibles that can tear a victim apart.

Study leader Alain Dejean of the the French National Center for Scientific Research (CNRS) and colleagues spent two decades searching forests for the rare ant.

"Each time we were lucky to find a colony, we stopped everything," Dejean said.

For example, whenever wind or people felled a tree on which D. armigerum lived, the scientists gathered downed branches, he said.

The team would then transport the branches to a small study garden and attach them to trees to observe how the ants interacted with other ant species—in addition to studying the insects in the wild.

(See "Pictures: Fire Ant Swarms Form Living Life Rafts.")

Killer Ants' Uneasy Peace

Unexpectedly, the team observed huge colonies of the ants in the forest canopy—up to 950,000 individuals lived in one colony spread across 200 tall trees.

In these crowded habitats, the team also discovered that the trap-jawed ants can coexist with other territorial species—an unusual behavior for ants.

"In Africa we have a situation we call no-ant land" where territorial-ant colonies have clearly defined borders, Dejean said. Should any "foreign" ant wander into the wrong area, the interloper will be attacked.

But in South America, D. armigerum and other ant species have brokered a tenuous peace—mostly on D. armigerum's terms.

Dejean observed large-scale farming of scale insects in the tree canopies, behavior common in other species of ants but seen only once before in D. armigerum. In the trees, D. armigerum and other ant species provide the scale insects with protection in exchange for the honeydew they produce from plant sap.

(More scale insect news: "Cloned Fathers Mate With Insect Daughters—From Inside.")

In the experiment, a species of carpenter ant had a scale farm in the study garden.

The carpenter ants "were attending the insects day and night. But after D. armigerum was introduced in the garden, the [carpenter ants] were obliged to leave during the day, otherwise there was a fight," Dejean said.

Mostly, though, when other ants came across D. armigerum, they'd back down rather than risk a fight.

The research appeared June 21 in the open-access journal PLoS ONE.

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