“The level of sophistication with which they have evolved to care for their injured is unrivaled in the animal kingdom. Our human medical system would be the closest match,” said Erik Frank, a behavioral ecologist at the University of Würzburg who led the study, in an interview Wednesday. “These amputations stopped infections from spreading into the body … the same way medieval amputations worked in humans,” he said, adding that the findings mark the first recorded example of a nonhuman animal performing an amputation on a fellow member of its species to save its life.
The study, published Tuesday in the journal Current Biology, suggests that Florida carpenter ants (Camponotus floridanus) are able to differentiate between types of wounds and adapt their healing responses accordingly. It adds to our growing understanding of the sophisticated strategies ants deploy to care for one another when injured, including by triaging the wounded and treating the infected with microbial substances.
The scientists observed the amputations in laboratory conditions as performed by the Florida carpenter ants, a reddish, black, or brown ant which typically measure under 1/2 an inch in length. Unlike some other ants, Florida carpenter ants do not have the ability to produce antimicrobial secretions from their glands to combat pathogens in wounds. “We wanted to see how a species that lost this gland would still care for their injured,” said Frank.
The scientists set out by deliberately injuring around 100 ants on the leg: either the femur (closer to the body) or the tibia (farther down the leg), to compare how fellow ants in their colony responded. They found that the ants effectively performed amputations when their nestmates had sustained femur injuries, but never performed amputations when an equivalent injury was sustained on the tibia.
In the former, a helper nestmate performed an amputation on the injured insect’s entire leg in over three-quarters of cases.
The ant amputation procedure lasted around 40 minutes and followed the same pattern each time: “They start licking the wound with their mouth parts and then they move up the leg with their mouth until they reach the shoulder. There, they will start to bite quite ferociously for many minutes at a time,” said Frank. “The injured ant will sit their calmly, allowing the procedure to occur and not complaining until the leg is cut off.”
Among the ants with a femur injury, 95 percent of those that received an amputation survived, while only 45 percent of those who did not receive an amputation survived, Frank said.
“The ants — in their world, in their context — have found a strategy that is highly efficient and has a very, very high level of success,” concluded Frank.
Laurent Keller, an evolutionary biologist who also worked on the study, said the amputations were performed very effectively. “It means that when they do the amputation they must do it in a very clean way to prevent bacteria from entering the wound,” he said.
In contrast to the treatment received by ants that sustained a femur injury, ants that sustained a tibia injury (further down the leg) were never observed receiving an amputation from fellow nestmates. “In this case, they only clean the wound,” said Keller, who said the nestmates instead provided an extended wound care session involving lots of licking.
The wound cleaning method also proved effective. While around 70-75 percent of those who received wound cleaning from fellow ants survived, only 15 percent of the ants with tibia injuries survived when they were isolated from their fellow ants and left unattended, Frank said.
One possible explanation offered by the scientists for the decision on when to perform an amputation has to do with how hemolymph — a fluid equivalent to blood — flows within invertebrates.
The theory has not been tested yet, but scans show that the tibia area of the leg has greater hemolymph flow than the femur area, meaning that 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 stave off an infection from spreading. “If the wound is at the level of the tibia then they don’t do an amputation. This is because normally the blood — or hemolymph for insects — circulates quite rapidly. So within 40 minutes the blood will already carry the bacteria into the body of the ant,” explained Keller.
The painstaking efforts adopted by ants to care for each others’ wounds illustrates how social insects reap benefits from behaving altruistically, said Keller. “By helping each other, they are indirectly helping themselves,” he said.
“Evolutionarily speaking, the colony saves a massive amount of energy by making sure their injured keep well, rather than just throwing them away 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 in as soon as one day — and are often deployed to perform the most dangerous tasks. He added: “Even in ant societies, the individual holds value.”
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