Videos Show Ants Chopping Off Nestmates' Legs to Save Their Lives

Life as a Florida carpenter ant can be brutal. These half-inch ants are territorial and have violent clashes with ants from rival colonies in the Southeast.

Fighting can leave ants with leg injuries. But as scientists recently discovered, these ants have developed an effective treatment for the wounds: amputation.

In the journal Current Biology on Tuesday, researchers reported that ants bite off injured limbs of their nestmates to prevent infection. Although other ant species are known to heal the wounds of their victims, typically by licking them clean, this is the first time an ant species is known to use amputation to heal a wound.

The ants in the study performed amputations only on some leg injuries, suggesting that they are methodical in their surgical practices. Aside from humans, no other animal is known to perform such amputations. The prevalence of the behavior among Florida carpenter ants raises questions about their intelligence and 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 another ant’s leg,” said Mr. Buffat, who is now a biologist at the University of Lausanne in Switzerland and one of the study’s authors. His adviser in Würzburg didn’t believe him at first.

“But then he showed me a video and I immediately knew we were on the right track,” said the consultant, Erik Frank.

They began monitoring the survival rate of amputees. Unexpectedly, ants with amputated limbs survived 90 percent of the time.

Even more surprising, the amputations appeared to be consensual. “The ant presents its injured leg and sits there quietly while another ant gnaws it off,” Dr. Frank said. “As soon as the leg falls 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 performed the procedure only on nestmates with thigh wounds.

To understand why the ants performed amputations only on those with injured thighs, the researchers performed amputations on ants with injured lower legs. The survival rate of the experimental amputees was only 20 percent.

“When the wound is far from the body, amputations don’t work, but when it’s closer to the body, they work,” Dr. Frank said.

That was counterintuitive, he said. But an explanation emerged after Dr. Frank and his team performed micro-CT scans on 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 an injury to their thigh, the flow of hemolymph is reduced, making it harder for bacteria to travel from the wound into their body. In these 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 enter its body very quickly. As a result, the window of time for a successful amputation is narrow and the chances of a successful one are slim. The ants, on some level, appear to be aware of this, says Dr. Frank.

“It’s kind of crazy to think that simple animals like ants could evolve 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 are beneficial to the entire colony, as they save lives and slow the spread of pathogens, Dr. Kronauer said.

“About 10 to 20 percent of ants that hunt end up injured in their lifetime. If colonies hadn't developed strategies to help these ants recover, they would need to produce 10 to 20 percent more ants to make up for this loss,” Dr. Frank said. “By saving the injuries, they save a tremendous amount of energy at the colony level.”

Dr. Frank, who has spent his career studying how ants heal wounds, says the findings from his new study have changed the way he looks at insects.

“It made me appreciate the value that each individual ant has in a colony and how valuable it is to take care of those wounds instead of just letting them die,” she said.

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