“The level of sophistication with which they have evolved to care for their injured is unmatched in the animal kingdom. Our human medical system would be the closest match,” Erik Frank, a behavioral ecologist at the University of Würzburg who led the study, said in an interview Wednesday. “These amputations stopped the spread of infections in the body … in the same way medieval amputations worked in humans,” he said, adding that the findings mark the first recorded example of a non-human animal performing an amputation on another member of its species to save life.
The study, published Tuesday in the journal Current Biology, suggests that Florida carpenter ants (Camponotus floridanus) are able to distinguish between types of wounds and adjust their healing responses accordingly. This adds to our growing understanding of the sophisticated strategies ants use to care for each other when injured, including tripling the injured and treating the infected with microbial substances.
The scientists observed amputations under laboratory conditions as performed by Florida carpenter ants, a reddish, black, or brown ant that is typically under 1/2 inch in length. Unlike some other ants, Florida carpenter ants do not have the ability to produce antimicrobial secretions from their glands to fight pathogens in wounds. “We wanted to see how a species that lost this gland would still care for their injured,” Frank said.
The scientists set out by deliberately wounding about 100 ants on their legs: either the femur (closest to the body) or the tibia (farthest from the leg), to compare how their ants reacted in their colony. They found that ants effectively performed amputations when nestmates had sustained femur injuries, but never performed amputations when an equivalent injury had been sustained to the tibia.
In the first, a helper nestmate performed an amputation of the injured insect’s entire leg in over three-quarters of the cases.
The ants’ amputation procedure took about 40 minutes and followed the same pattern each time: “They start licking the wound with their mouthparts and then move up the leg with their mouths until they reach the shoulder. There, they will start biting very ferociously for many minutes at a time,” Frank said. “The injured ant will sit quietly, allowing the procedure to take place and not complaining until the leg is cut off.”
Among the ants with a femur injury, 95 percent of those who received an amputation survived, while only 45 percent of those who did not receive an amputation survived, Frank said.
“Ants – in their world, in their context – have found a strategy that is very efficient and has a very, very high success rate,” Frank concluded.
Laurent Keller, an evolutionary biologist who also worked on the study, said the amputations were performed very effectively. “This means that when they do the amputation they have to do it in a very clean way to prevent bacteria from getting into the wound,” he said.
Unlike the treatment received by ants that sustained a femur injury, ants that sustained a tibia (further leg) injury were never observed receiving an amputation by nestmates. “In this case, they just clean the wound,” said Keller, who said the nestmates instead offered an extended wound care session that included lots of licking.
The wound cleaning method also proved effective. While about 70-75 percent of those who received wound cleaning from other ants survived, only 15 percent of ants with tibial injuries survived when they were isolated from their ants and left unattended, Frank said.
A possible explanation offered by scientists for the decision when to perform an amputation concerns how hemolymph—a fluid equivalent to blood—flows within invertebrates.
The theory has yet to be tested, but scans show that the tibial area of the leg has greater hemolymph flow than the femoral area, meaning pathogens that enter through the tibia will spread more quickly to the rest of the body. This, in turn, significantly shortens the window of opportunity for an amputation to prevent the spread of an infection. “If the wound is at the level of the tibia, then they do not perform an amputation. This is because normally the blood – or hemolymph for insects – circulates quite quickly. So within 40 minutes the blood will already be carrying the bacteria into the ant’s body,” explained Keller.
The painstaking efforts adopted by ants to tend to each other’s wounds illustrates how social insects reap benefits from altruistic behavior, Keller said. “By helping each other, they are indirectly helping themselves,” he said.
“Evolutionarily speaking, the colony saves a huge amount of energy by making sure their injured ones stay well, rather than just discarding them and replacing them with a new worker,” he said. Previous studies show that ants that have lost one or even two legs can still be productive members of their colony, returning to their normal running speed within a day – and often settling for the most dangerous tasks. He added: “Even in ant societies, the individual has value.”
