Researchers from the University of Florida (UF) have found that when acacia ants, which are known for being fierce and territorial, are short on colony members, they recruit individuals that were once their enemies to defend their newly acquired territory. 

Acacia ants (Crematogaster mimosae) that inhabit acacia thorn trees in Africa use their fearsome bite to deter large animals, such as elephants and giraffes, from eating the leaves of their host trees, as well as keeping predators away. Surprisingly, not even the skin of an elephant's trunk is thick enough to stand up to the ants' bite.

Interested in learning more about these unrelenting ants, study researcher Kathleen Rudolph was more concerned about them raining down on her from the trees and biting her.

"They really seem to have a knack for finding your soft tissue," Rudolph said. "It's a nasty business."

However, acacia ants are also aggressive toward each other, fighting to the death over their tree territories. While there are severe consequences for losing one's territory, and subsequent colony death, Rudolph and UF postdoctoral research associate Jay McEntee were curious about the costs that victorious colonies endure when seizing new territory.

For instance, the winning ants are often left with a depleted colony due to heavy fighting. To overcome this challenge and defend their newly acquired territory, victorious colonies invite members of the loosing colonies to help.

The researchers tested how the ants responded to simulated predator attack using lab experiments based at Mpala Research Centre in Kenya. They instigated wars between ant colonies by tying the trees of unrelated colonies together and counting the number of causalities from tarps placed below. Overall, the research team found that victorious colonies are less able to defend their host trees after fights.

Furthermore, DNA analyses of nearly 800 ants revealed that fighting greatly alters the genetic make-up of victorious colonies. This suggests that non-relatives can become part of a new colony, adapting to defend its residents and territory.

However, in some cases, there was not a definitive winner. The team was surprised to find that colonies would occasionally cease fighting and fuse together when fatal fights had resulted in thousands of causalities - even when the queen of each colony was 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 added. "It's pretty remarkable."

Researchers are still unsure of how the ants know to stop fighting. Perhaps it is that fighting changes the odors that ants use to distinguish fellow colony members from potential invaders.

"If so, the updated or blended cues shared by prior foes may help end aggressive responses," Rudolph said. "Physical combat not only yields biological winners and losers, it can alter the identity of its combatants."

Their study was published in the Dec. 17 issue of the journal Behavioral Ecology.