The life of a Florida carpenter ant can be brutal. These inch-long ants are territorial and engage in violent battles with ants from rival colonies in the southeastern United States.
Combat can leave ants with wounds on their legs. But as scientists have recently discovered, these ants have evolved an effective treatment for wounds: amputation.
In the magazine Current Biology On Tuesday, researchers reported that ants are biting off the injured limbs of nestmates to prevent infection. While other ant species are known to heal the wounds of fellow ants, usually by licking them, this is the first time an ant species has been known to use amputation to treat a wound.
The ants in the study only amputated certain leg wounds, suggesting they are methodical in their surgical practices. Other than humans, no other animal is known to perform such amputations. The prevalence of this behavior among Florida carpenter ants raises questions regarding their intelligence and ability to feel pain.
In early 2020, Dany Buffat, a graduate student at the University of Wurzburg in Germany, was observing a colony of Florida carpenter ants in his lab when he noticed something strange. “One ant was biting another ant’s leg,” said Buffat, who is now a biologist at the University of Lausanne in Switzerland and a co-author of the paper. His advisor in Wurzburg didn’t believe him at first.
“But then he showed me a video and I knew right away we had something on our hands,” Erik Frank said.
They began tracking the survival rate of the amputees. Unexpectedly, the ants with amputated limbs survived 90 percent of the time.
Even more surprising, the amputations seemed consensual. “The ant presents its injured leg and sits quietly while another ant bites it off,” Frank explained. “As soon as the leg falls off, the ant presents the newly amputated wound and the other ant finishes the job by cleaning it up.”
After observing dozens of amputations, the researchers observed that the ants performed the procedure only on nestmates with thigh wounds.
To understand why the ants only amputated those with wounds on their thighs, the researchers performed amputations on ants with wounds on the underside of their legs. The survival rate of the amputees in experiments was only 20 percent.
“When the wound is further away from the body, amputations don’t work, but when it’s closer to the body, they do work,” Frank explained.
This was counterintuitive. But an explanation emerged following 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 of these muscles in their thighs. When they suffer a wound to the thigh, the flow of hemolymph is reduced, making it difficult for bacteria to pass from the wound into the body. In these cases, if the entire leg is quickly amputated, the chance of infection is very low.
But when a Florida carpenter ant injures the underside of its legs, bacteria can penetrate its body very quickly. As a result, the window for successful amputation is tight and the chances of success are slim. The ants, in some ways, seem to be aware of this, Frank said.
“It’s crazy to think that the evolution of animals as simple as ants might have produced such 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 slowing the spread of pathogens, Kronauer said.
“Approximately 10 to 20 percent of ants that go out to hunt will be injured during their lifetime. If colonies had not developed strategies to help these ants recover, they would have to produce 10 to 20 percent more ants to compensate for this loss,” Frank explained. “By rescuing the injured, they save a huge amount of energy at the colony level.”
Frank, who has dedicated his career to studying how ants treat wounds, said the findings of his new study have changed the way he views insects.
“It has made me appreciate the value of an individual ant in a colony and how beneficial it is to tend to wounds rather than leaving them for dead,” she concluded.
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