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The life of a Florida carpenter ant can be brutal. These half-inch ants are territorial and have violent bouts with ants from rival colonies in the Southeast.
Combat can leave the ants with leg injuries. But as scientists recently discovered, these ants have evolved an effective wound treatment: amputation.
In the journal Current Biology, on Tuesday, researchers report that the ants bite off the injured limbs of their nest mates to prevent infection. Although other ant species are known to tend to the wounds of their injured, typically by licking them clean, this is the first time that an ant species has been known to use amputation to treat an injury.
The ants in the study performed amputations on only certain leg injuries, suggesting that they are methodical in their surgical practices. Aside from humans, no other animal is known to conduct such amputations. The prevalence of the behavior among Florida carpenter ants raises questions about their intelligence and their ability to feel pain.
In early 2020, Dany Buffat, a graduate student at the University of Würzburg in Germany, was observing a colony of Florida carpenter ants in his lab when he noticed something strange. “One ant was biting off another ant’s leg,” said Mr. Buffat, who is now a biologist at the University of Lausanne in Switzerland and is an author of the study. His adviser at Würzburg didn’t believe him at first.
“But then he showed me a video and I knew immediately that we were onto something,” the adviser, Erik Frank, said.
They began tracking the amputees’ survival rate. Unexpectedly, the ants with amputated limbs survived 90 percent of the time.
Even more surprising, the amputations appeared consensual. “The ant presents its injured leg and calmly sits there while another ant gnaws it off,” Dr. Frank said. “As soon as the leg drops off, the ant presents the newly amputated wound and the other ant finishes the job by cleaning it.”
After observing dozens of amputations, the researchers noticed that the ants would perform the procedure only on nest mates with thigh injuries.
To understand why the ants performed amputations only on those with injured thighs, the researchers performed amputations on ants with wounded lower legs. The survival rate of the experimental amputees was only 20 percent.
“When the wound is further away from the body the amputations don’t work, but when it’s closer to the body, they do work,” Dr. Frank said.
That was counterintuitive, he said. But an explanation emerged after Dr. Frank and his team performed micro-CT scans on the amputees.
Ants have several muscles throughout their bodies that keep hemolymph, their version of blood, flowing. Florida carpenter ants have many such muscles in the thighs. When they sustain a thigh injury, the flow of hemolymph is reduced, making it more difficult for bacteria to move from the wound into the body. In such cases, if the entire leg is amputated quickly, the chance of infection is very low.
But when a Florida carpenter ant injures its lower legs, bacteria can penetrate its body very quickly. As a result, the time window for a successful amputation is narrow and the chance of it being successful is slim. The ants, on some level, seem to be aware of this, Dr. Frank says.
“It’s pretty crazy to think that animals as simple as ants could have evolved such a complex behavior,” said Daniel Kronauer, an associate professor at Rockefeller University in New York who studies ants and other highly social organisms but was not involved in the research. “But I wouldn’t be surprised if other ant species had similar behavior.”
Such amputations benefit the entire colony by saving lives and curbing the spread of pathogens, Dr. Kronauer said.
“Roughly 10 to 20 percent of ants that go out hunting eventually get injured in their lifetime. If the colonies had not developed strategies to help these ants recover, they would need to produce 10 to 20 percent more ants to compensate for this loss,” Dr. Frank said. “By rescuing the injured, they save an enormous amount of energy on the colony level.”
Dr. Frank, who has spent his career studying how ants treat wounds, says his new study’s findings have changed how he looks at the insects.
“It made me appreciate the value an individual ant has in a colony and how beneficial it is to care for the injured instead of just leaving them for dead,” he said.
