Ant Amputation Surgery: How Camponotus floridanus Helps Nestmates Stop Infection
Ant societies are highly organized groups with a striking division of labor, and in some cases their social life even includes something resembling medical care. In the ant species Camponotus floridanus, workers respond to injured nestmates in different ways depending on where the leg wound is located. In one study, researchers experimentally injured different parts of the hind leg of C. floridanus workers and observed how their nestmates treated them. When the wound was located on the femur, nestmates often began by licking the wound, then moved up the injured limb toward the body, eventually reaching the trochanter. There, they repeatedly bit the leg until the injured limb was severed. This amputation usually occurred within a few hours after injury. However, when the wound was located farther from the body, on the tibia, nestmates almost never amputated the limb, even when the wound was infected. Instead, they continued to provide wound care by licking and cleaning the injury site. The ants therefore appear capable of distinguishing wound location and choosing different treatment strategies accordingly. At first glance, one might assume that a more distal tibia injury would allow infection to spread more slowly and could therefore be handled simply by licking. In fact, the opposite is true: bacteria spread faster from infected tibia wounds.
The researchers infected the wounds with Pseudomonas aeruginosa. This bacterium is highly lethal to injured ants, and infected individuals kept in isolation without wound care showed high mortality. Yet when ants with infected femur wounds were amputated by their nestmates, their survival increased dramatically. Measurements of bacterial load supported this result. After femur infection, ants that received no care carried a much higher pathogen load in their bodies; by contrast, ants that received nestmate care or underwent experimental amputation had bacterial loads reduced to levels close to those of uninfected individuals. This shows that amputation can effectively prevent, or at least greatly reduce, pathogen spread from the wound into the rest of the body.
The same approach failed when the injury was on the tibia. If ants with infected tibia wounds were experimentally amputated one hour after infection, their survival did not improve, and their internal pathogen load did not decrease. In comparison, infected tibia-injured ants returned to the colony were not amputated, but the prolonged licking and wound care provided by nestmates did reduce part of the bacterial burden. The researchers then tested the importance of amputation timing and found that, for tibia injuries, amputation was protective only when performed immediately after pathogen exposure. Once there was even a short delay, the pathogen had probably already spread too quickly for amputation to be useful.
Why, then, does infection spread faster from the more distal tibia? The difference may lie in the internal anatomy of the ant leg. Using X-ray micro-computed tomography scans to reconstruct the hind leg, the researchers found that the femur contains a large amount of muscle. They suggest that these muscles are involved in hemolymph circulation within the leg. When the femur is injured, this muscle damage may slow hemolymph flow, delaying the movement of pathogens into the main circulatory system. That delay gives nestmates enough time to perform an effective amputation. The tibia contains far less muscle, so a tibia injury disrupts much less of the leg's circulatory machinery. Its hemolymph channel is also relatively large, meaning that pathogens may enter circulation more rapidly, while the femur muscles remain intact and continue to support flow.
Ants of the genus Camponotus lack the metapleural gland, which many ant species use to produce antimicrobial secretions. Other ants may treat infected wounds with antimicrobial compounds, but C. floridanus does not have this option. Instead, these ants remove the limb at the right time when amputation is likely to help, or intensify wound care when amputation is unlikely to work. The study also found that, for femur injuries, nestmates amputated the limb at similar rates whether the wound had been experimentally infected or treated with a sterile solution. This suggests that, under natural conditions, an open leg wound may already carry such a high risk of infection that nestmates do not need to confirm whether pathogens are present. If the wound is in a location where infection risk is high and amputation can still be effective, they proceed with amputation.
At this point, many readers may wonder why amputation does not outperform wound licking in tibia injuries. After all, removing the injured limb should seem like a more decisive way to prevent bacteria from entering the circulatory system. If licking the wound entrance can reduce bacterial load in tibia-injured ants, one might expect amputation to solve the infection even more effectively. Yet the experiment reached the opposite conclusion, and this question will require further research. As for why C. floridanus responds to infected tibia wounds with prolonged licking rather than amputation, one possible explanation is that workers cannot know how long ago the wound became infected. Over evolutionary time, amputation of infected femur wounds may have often led to successful treatment, whereas amputation of infected tibia wounds may have failed more often because pathogens had already spread too rapidly. Repeated across generations, that difference could have shaped the treatment pattern seen today: amputate femur injuries when the intervention can still save the nestmate, but rely on extended wound care when the injury is on the tibia.
Author: Shui-Ye You
Reference:
Frank ET et al. (2024). Wound-dependent leg amputations to combat infections in an ant society. Current Biology.
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