Warring Ants Suddenly Stop Killing and Join Forces

Ants can be aggressive toward each other, fighting to the death over their tree territories. While the consequences for losing colonies are stark—loss of territory or colony death—winning, too, apparently comes at a cost.

After a fight, victorious colonies have to defend their newly gained territory with a workforce heavily depleted by fighting. A new study shows victorious colonies might offset this challenge by recruiting members of the losing colonies to help.

Certain acacia ants, Crematogaster mimosae, use their fearsome bite to defend host trees against large animals such as elephants and giraffes that eat the trees’ leaves. Even elephants’ thick skin can’t protect them from the ants, which bite them inside their trunks.

“They really seem to have a knack for finding your soft tissue. It’s a nasty business,” says Kathleen Rudolph of the University of Florida.

To understand the cost to winners, Rudolph and colleagues conducted experiments at Mpala Research Centre in Kenya, instigating ant wars by tying unrelated colonies’ trees together and counting casualties in tarps placed below.

By simulating the browsing of a large mammal, they discovered that victorious colonies are less able to defend their host trees after fights. After analyzing the DNA of nearly 800 ants, they discovered that fighting changes the genetic make-up of victorious colonies.

Long viewed as fortresses of cooperating sisters, where relatives of the queen work for her benefit, the new findings show that non-relatives can become part of the colony—and potentially defend its residents and territory.

Researchers were further surprised to find that, in some cases, fatal fights with thousands of casualties don’t produce a distinct winner. Instead, colonies cease fighting and fuse together, with the queen of each colony still alive.

“Colonies are battling so aggressively that many individuals die, but then they are able to just stop fighting and form a lasting truce,” Rudolph says. “It’s pretty remarkable.”

How they know to stop fighting remains a mystery. One possibility, Rudolph says, is that fighting changes the odors ants use to distinguish nestmates from potential invaders. “If so, the updated or blended cues shared by prior foes may help end aggressive responses.”

Sorting out these processes could contribute to our understanding of an intriguing aspect of physical conflict—that animal combatants become more similar biologically through combat. That can be true for humans, too: A 2013 study showed that the skin bacteria communities of competing roller derby teams converge during bouts, not unlike Rudolph’s findings in ants.

“Physical combat not only yields biological winners and losers,” Rudolph says. “It can alter the identity of its combatants.”

The study is published in the journal Behavioral Ecology.

Source: University of Florida

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