“The level of sophistication with which they have evolved to care for their wounded is unmatched in the animal kingdom. Our human medical system would be the closest,” Erik Frank, a behavioral ecologist at the University of Würzburg who led the study, said in an interview Wednesday. “These amputations prevented infections from spreading through the body … in the same way that medieval amputations worked in humans,” he said, adding that the findings are the first recorded example of a nonhuman animal performing an amputation on a 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 injuries and adapt their healing responses accordingly. It contributes to our growing understanding of the sophisticated strategies ants deploy to care for each other when injured, including sorting the injured and treating the infected with microbial substances.
The scientists observed the amputations in the lab, as done by Florida carpenter ants, a reddish, black or brown ant that is typically less than an inch long. Unlike other ants, Florida carpenter ants lack the ability to produce antimicrobial secretions from their glands to fight pathogens in wounds. “We wanted to see how a species that had lost this gland could still care for its wounded,” Frank said.
The scientists began by deliberately injuring about 100 ants in their legs: either the femur (closer to the body) or the tibia (lower on the leg), in order to compare the reactions of other ants in the colony. They found that the ants did indeed perform amputations when their fellow ants suffered femur injuries, but they never performed amputations when an equivalent injury was sustained to the tibia.
In the first case, an auxiliary nestmate would amputate the entire leg of the injured insect in more than three-quarters of cases.
The ant amputation procedure took about 40 minutes and was performed the same way each time: “They start licking the wound with their mouthparts, then move their mouths up the leg until they reach the shoulder. There they start biting ferociously for several minutes,” Frank said. “The injured ant sits there, calm, lets the procedure happen and doesn’t complain until the leg is cut off.”
Among ants with femur injuries, 95 percent of those that underwent amputation survived, while only 45 percent of those that did not, Frank said.
“The ants – in their world, in their context – have found a strategy that is very effective and has a very, very high level of success,” Frank concludes.
Laurent Keller, an evolutionary biologist who also participated in the study, said the amputations were done very efficiently. “That means when they do the amputation, they have to do it very cleanly to prevent bacteria from getting into the wound,” he said.
Unlike the ants injured in the femur, the ants injured in the tibia (lower on the leg) were never amputated by their peers. “In this case, they just clean the wound,” said Keller, who added that the peers instead engaged in a prolonged wound-care session involving lots of licking.
The wound-cleaning method also proved effective. While about 70 to 75 percent of ants that cleaned their wounds survived, only 15 percent of ants with shin injuries survived when they were isolated from their peers and left unattended, Frank said.
A possible explanation proposed by scientists for the decision to proceed with amputation It has to do with how hemolymph, a fluid equivalent to blood, circulates in invertebrates.
The theory has not yet been tested, but scans show that the tibia area of the leg has a greater flow of hemolymph than the femur area, meaning that pathogens that enter through the tibia spread more quickly to the rest of the body. This significantly reduces the window of opportunity for an amputation to prevent the spread of infection. “If the wound is in the tibia, we do not perform an amputation. This is because normally the blood – or hemolymph for insects – circulates quite quickly. So within 40 minutes, the blood is already carrying the bacteria into the ant’s body,” Keller explains.
The painstaking efforts of ants to heal each other’s wounds illustrate how social insects benefit from their altruistic behavior, Keller said. “By helping each other, they are indirectly helping themselves,” he said.
“From an evolutionary perspective, the colony saves a lot of energy by ensuring that its injured workers remain healthy, rather than 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, regaining their normal running speed in just a day – and are often deployed to perform the most dangerous tasks. He added: “Even in ant societies, the individual has value.”