Carpenter Ants Perform Life-Saving Amputations to Treat Leg Injuries

A new study provides the first evidence of non-human animals performing amputations on others to improve their odds of survival

A video still of an ant biting off the leg of another
In a still from a video, an ant at the bottom of the image bites off the leg of a wounded ant with a purple marker on its back. Danny Buffat

Researchers have observed Florida carpenter ants (Camponotus floridanus) biting off the injured legs of their nestmates. By removing their companions’ legs, the ants significantly improved their chances of survival, researchers reported Tuesday in the journal Current Biology.

These surgeries are the first known example of non-human animals performing amputations to improve a fellow animal’s odds of recovering, the authors say.

“Not only can they do this, but they are even able to diagnose the wounds and, depending on the location, adapt the treatment accordingly to maximize the survival chances of the injured,” Erik Frank, first author of the new study and an ant researcher at the University of Würzburg in Germany, says to NPR’s Nell Greenfieldboyce. “I find that truly remarkable.”

The finding “pushes the envelope of our understanding of behavioral immune systems in social insects,” James Traniello, who studies social behavior in insects at Boston University and did not contribute to the study, tells New Scientist’s Chen Ly.

Some ant species secrete antimicrobial substances that they apply to wounds to fight pathogens. But Florida carpenter ants lack the glands that produce these secretions—even though they are territorial and might sustain injuries while defending their homes from rivals. So, researchers wondered if the species had any other methods for dealing with infected wounds.

They discovered that when a worker ant injured the upper part of its leg, nestmates would perform amputations around three-quarters of the time. These other ants would lick the wound and then spend about six minutes on average biting at the upper part of the leg until it came off.

“I find it striking to what extent they are freely cooperating in this amputation event,” Frank tells National Geographic’s Jason Bittel. “You can see it presenting the injured leg, and the other [ant], for many minutes at a time, is biting it ferociously … and the injured ant does not seem to complain.”

Ants amputate their nestmates' limbs to save them from infection

But if the injury occurred in the lower part of the leg, other ants would generally not amputate it.

The researchers wondered if the amputations could serve to prevent infections. To test this hypothesis, they manually wounded some ants’ legs. They isolated some, ensuring they would receive no care, performed their own amputations on others and returned a third group to the colony. The team also infected some of the ants’ wounds.

Ants with upper-leg injuries that were isolated died about 60 percent of the time, while ants that received surgeries, either from the researchers or from fellow ants, almost always lived, suggesting the medical care was life-saving. But for the ants with lower-leg injuries, it seemingly didn’t matter whether they received care or not—both the isolated ants and the ants that received amputations from researchers died at high rates.

The experiments confirmed that ants did not perform amputations for lower-leg injuries—and the low success rate of the researchers’ amputations suggested these would be ineffective, anyway. But when ants with injured lower legs were around their colony, the other ants would lick the wounds, per National Geographic. Those injured ants had an approximately 75 percent survival rate, compared to about a 15 percent survival rate for isolated ants with lower-leg injuries.

Researchers theorize that amputation might be more effective for upper-leg wounds because of how they may affect blood circulation.

Florida carpenter ants, like other insects, have muscles responsible for blood circulation in their upper legs, so upper leg injuries could reduce circulation. Lower circulation would, in turn, reduce the spread of infection, buying the ants time to perform amputations and save their nestmates. The researchers found that amputations for lower-leg wounds only improved survival chances if they were performed immediately after the injury occurred, supporting the idea that infections can spread quickly after lower-leg injuries.

“This is such an interesting finding,” Corrie Moreau, an entomologist at Cornell University who did not contribute to the findings, tells National Geographic. “Not only did these researchers show that amputation increases survival, but they also showed that ants in isolation cannot bite off their own leg and are more likely to die.”

The ants “don’t just deploy blanket amputation on any injury—they only do so when it would make sense,” Tomer Czaczkes, who studies ant behavior at the University of Regensburg in Germany and was not involved in the study, says to New Scientist.

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