Ants treat certain leg injuries with life-saving amputations

Lab experiments show that some ants will treat the injured legs of comrades, and when it's necessary will even perform medical amputations. Bart Zijlstra, UNIL hide caption

toggle caption Bart Zijlstra, UNIL

When an ant injures its leg, it sometimes will turn to a buddy who will help out by gnawing the leg off, effectively performing a life-saving limb amputation.

That’s according to some new experiments described in the journal Current Biology, which show that ants are the only animal other than humans known to practice amputation as a medical treatment.

“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,” says Erik Frank, who studies “social wound care” among ants at the University of Würzburg in Germany. “I find that truly remarkable.”

Wound care after battle

In the past, his group has studied how termite-hunting ants in the tropics substantially reduce the death rate among injured nestmates by treating their wounds with antibiotic secretions that come from a special gland.

But that gland doesn’t exist in a common species known as Florida carpenter ants, or Camponotus floridanus. This species nests in rotting wood and will fight rivals to defend its home, so Frank wanted to see how these ants would react when confronted with the kind of injuries that come with battle.

His team quickly observed that these ants would cut off wounded legs, much like miniature Civil War-era surgeons. “And this really piqued my interest,” he says.

An injured ant will present its leg to a nestmate, who will lick the wound and then move up the leg to bite at the shoulder joint for many minutes at a time, until the leg is severed. “You can see the other one not moving, not really flinching and accepting it,” says Frank.

Almost all of the injured ants that got an amputation from a buddy survived. Meanwhile, ants with leg injuries that were kept away from their nestmates, so that they didn’t get this treatment, frequently died.

To see how amputations helped, the researchers experimentally infected open leg wounds on ants with pathogens. They found that the kind of amputations that ants did would stop the infection from spreading and becoming lethal.

Frank went even further and surgically amputated the ants’ injured legs himself — essentially mimicking the ants’ surgical approach — to see how the amputee fared. That confirmed, he says, “these amputations were saving the lives of the infected individuals.”

The research has convinced other ant experts who were not involved in these studies, like Daniel Kronauer of The Rockefeller University. “The experiments are very thorough,” says Kronauer. “To me, it sounds plausible.”

“Here you have an ant species that lives in a log in my backyard where I grew up in Florida, and they’ve been doing clinical amputation for millions of years, much longer certainly than humans have ever done it,” says Clint Penick, a social insect researcher with Auburn University who also was not involved in this work.

“It’s really cool to see something like this, and to see really strong research that backs up that this is actually a medical treatment that ants have evolved to do to prevent infection,” says Penick.

Armed with amputation instincts?

Interestingly, ants would only perform an amputation when an injury was high up on an ant’s leg, close to the middle of its body. Wounds lower down on the leg didn’t elicit this treatment, although ants would lick at the injuries.

That observation prompted Frank’s team to try performing amputations on ants with lower leg injuries that had been infected with bacteria. They found that the ants always died.

Frank and his colleagues were puzzling over why amputation solely seemed to work for upper-leg injuries, until they closely studied the leg’s anatomy. They saw that muscles in the upper leg normally help move blood-like liquid through an ant’s body. It’s those muscles that get damaged when an upper-leg injury occurs. This means that bacteria or other pathogens in an upper-leg injury would spread to the rest of the body more slowly than they would from a lower-leg injury.

The fact that ants only perform amputations in certain circumstances is “really cool,” says Kronauer.

“They can tell where on the leg the injury has occurred, right? And depending on where exactly the leg has been injured, it makes sense to amputate or it does not make sense to amputate,” he notes.

Still, he cautions that it’s not like ant medics are evaluating the wound and consciously weighing the pros and cons of treatment options, like a human doctor would.

“I don't think they have some kind of crazy cognitive capacities,” says Kronauer. “They have basically evolved over like thousands and thousands and probably millions of years to be kind of ‘programmed’ to react to different kinds of injuries in a certain way.”

Even so, says Penick, humans tend to think that their medicine is uniquely sophisticated, and yet this common backyard ant essentially does surgery.

And even when an ant had a lower-leg injury that could not be treated with amputation, he notes, its nestmates would still tend to the wound, apparently applying some kind of antimicrobial secretions that often proved to be life-saving.

“My own work shows that a lot of ants produce antimicrobials,” adds Penick, who says amputation “is just another example of things that are in the public health repertoire that ants have to work with.”