Assessment and treatment
The researchers found that in the case of all the femur injuries, nest mates would first lick the wounded area with their mouthparts, presumably to clean or disinfect the wound, and then chew off the leg to amputate it. However, if the injury was to the tibia, the nest mates only licked the wound clean and didn't subsequently amputate. Both classes of injured ants showed remarkably high survival rates upon receiving either treatment: 90–95 percent for amputee ants (compared to less than 40 percent for untreated injuries) and about 75 percent with just the wound cleaning (compared to 15 percent for untreated injuries). Surviving amputee ants were able to resume their full range of duties despite losing one of their six legs.
The fact that the ants selectively treated two different kinds of wounds suggests that they can assess the nature of those injuries and tailor their treatment methods accordingly, per the authors. Frank et al. wondered why injuries to infected lower legs (tibias) weren't amputated and conducted additional experiments in which they amputated the infected tibias themselves. They were surprised to find that when they did so, only about 20 percent of the ants survived.
The CT scans helped explain why. It turns out that ants don't have a heart to centrally pump blood throughout their bodies; instead, they have several heart pumps and muscles distributed throughout their bodies to ensure circulation of the hemolymph (ant blood). The thigh area has many such muscles, so if that area is injured, the muscles are impaired, hindering circulation. This, in turn, lowers the risk of infection since bacteria cannot spread from the wound into the body as fast. So it's worth the effort to amputate the leg, a labor-intensive process that can take 40 minutes or more.
That's not the case for injuries to the lower leg (tibia), which doesn't have circulatory muscles. So bacteria can spread through the body very quickly; there simply isn't enough time to perform an effective amputation.
"Because they are unable to cut the leg sufficiently quickly to prevent the spread of harmful bacterial, ants try to limit the probability of lethal infection by spending more time cleaning the tibia wound," said co-author Laurent Keller, an evolutionary biologist at the University of Lausanne—a treatment that has a much higher survival rate than if the ants attempted to amputate legs with wounded tibias. "Our study proves for the first time that animals also use prophylactic amputations in the course of wound treatment. And it shows that the ants orient the treatment to the type of injury."
DOI: Current Biology, 2024. 10.1016/j.cub.2024.06.021 (About DOIs).
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