These ants perform life-saving amputations on each other

In recent years, scientists have discovered many ways in which wild animals play doctor.

Lemurs may use chewed up millipedes to protect against intestinal parasites. Both chimpanzees and orangutans have been seen applying poultices to their wounds. And honeybees are known to line their hives with antimicrobial plant compounds, to name just a few examples. 

Now, researchers have described a medical procedure never witnessed before in the animal kingdom—amputations. While humans have been performing amputations for 30,000 years, the surgeons in this case are unassuming worker ants.

Life in a Florida carpenter ant (Camponotus floridanus) colony can be dangerous, particularly if other colonies are located nearby. It’s not uncommon for neighboring ants to wage war upon each other after the sun sets, and battles leave behind many injured foragers.

“If the injury is further up the leg on the femur, they will proceed to amputate the leg,” says Erik Frank, a behavioral ecologist at the University of Würzburg in Germany. “If the wound is further down the leg, to our surprise, we don’t see amputations.”

What’s more, ants that receive either treatment survive at much higher rates than injured ants that are kept away from their sisters.

“So not only are they doing another kind of medical treatment by amputating the legs, but they’re also somehow able to correctly diagnose the wound and adapt the treatment accordingly,” says Frank, who was lead author of the study published today in Current Biology.

No ant left behind 

While some lizards can abandon their tails and insects and arachnids jettison their legs when a predator attacks—a defensive behavior known as autotomy—the amputations performed by Florida carpenter ants are in another category entirely.

In part, this is because the injured ants seem to be willing participants in the procedure.

“I find it striking to what extent they are freely cooperating in this amputation event,” says Frank. “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.”

While scientists can’t yet say definitively if or how much ants and other insects feel pain, we do know that the ants’ medical choices seem to be rather successful. (Related: Watch ants carry their wounded off the battlefield.)

For lower-leg injuries, colonymates groomed the wound obsessively with their mouthparts, perhaps to remove pathogens that could cause lethal infections. And in those lower-leg cases, ants that were groomed by their sisters survived at rates of about 75 percent, versus just 15 percent for tibia-injured ants that were kept apart from their nestmates, the researchers found.

Similarly, ants with femur injuries that were not treated only survived about 40 percent of the time, versus success rates of between 90 and 95 percent after amputation. Unlike some animals, such as salamanders, ant legs do not grow back once lost, so survival is the goal.

These aren’t the first ants observed caring for wounds. Frank and his colleagues previously found that the termite-hunting Matabele ant (Megaponera analis) has a gland that produces antimicrobial compounds and proteins, which the ants slather upon their injuries. However, while that gland appears in most ants, the Florida carpenter ants seem to have lost it throughout the course of their evolution. And Frank says that this may be why the species has evolved a different approach for wounded warrior care. (Read more about ‘paramedic’ Matabele ants.)

“The diversity in ants is at least as great as between mice and an elephant or a lion,” says Frank. “So, the behavioral repertoire and natural history of different ant species is massive.”

‘The benefits of social living’

“This is such an interesting finding,” says Corrie Moreau, an entomologist and National Geographic Explorer specializing in ants at Cornell University. (Read more about Moreau's efforts to make ants the next big thing in biology.)

“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,” says Moreau in an email.

In addition to defense, brood care, and division of labor, the findings hint at yet another benefit to living in a colony. “Who would have thought that having your sister bite off your leg would be another example of the benefits of social living?” says Moreau.

It’s unclear how common the amputation behavior is among ants, but preliminary evidence suggests that other closely related carpenter ant species may cut off each other’s legs to treat wounds, too. Frank’s team also hopes to tackle other lingering questions raised by the evidence, including what other strategies ants may have for treating injuries and combatting infections and what capacity the animals have for feeling pain.

The study has answered one big question for Moreau, at least.

“As someone that has collected ants all over the world, I have always marveled at ants walking around with only five legs,” she says, “but it never occurred to me that their own sisters might have been the ones to bite them off!”