While humans have been performing amputations for 30,000 years, the surgeons in this case are ants.
In his 11 years studying ant behaviour, biologist Erik Frank had never seen anything like it. He and his colleagues at the University of Würzburg brought Florida carpenter ants (Camponotus floridanus) – a centimetre-long insect native to the southern United States – back to their lab in Germany to learn how they respond to injury. In July 2024 they published the results of their fascinating study in Current Biology.
Most ant species treat the injured or severed limb of a comrade by coating it with an antimicrobial goo. But, for whatever reason, the reddish-brown carpenter ant lost that gland over evolutionary time. So, they took a different tack: They bit the remainder of the limb off, effectively amputating it. But not in every case!
The treatment depends on where on the leg the injury is located … high up on the femur, or lower down on the tibia.
For lower-leg injuries, colonymates groomed the wound obsessively with their mouthparts, perhaps to remove pathogens that could cause lethal infections. And in those lower-leg cases, ants that were groomed by their sisters survived at rates of about 75%, versus just 15% for tibia-injured ants that were kept apart from their nestmates, the researchers found.
Similarly, ants with femur injuries were quickly approached by one or two comrades, who would then proceed to gnaw the leg above the femur, amputating it entirely … with a procedure that sometimes took as long as 40 minutes to complete. Ninety percent of the ants that got this “surgery” survived. In contrast, only 40% of those left alone lived.
An analysis of DNA in the ants’ bodies revealed that amputations following femur wounds halted bacterial infections in their tracks, whereas amputations following tibia wounds did not. Tibia amputations didn’t improve the insects’ survival, and this is what the ants just “seem to know,” Frank says.
“Not only did these researchers show that amputation increases survival, but they also showed that ants in isolation cannot bite off their own leg and are more likely to die,” says says Corrie Moreau, an entomologist and National Geographic Explorer specialising in ants at Cornell University.
In addition to defence, brood care and division of labor, the findings hint at yet another benefit to living in a colony. “Who would have thought that having your sister bite off your leg would be another example of the benefits of social living?” adds Moreau.
The study might also answer one big question for Moreau. “As someone that has collected ants all over the world, I have always marvelled at ants walking around with only five legs,” she says, “but it never occurred to me that their own sisters might have been the ones to bite them off!”
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References
cell.com/current-biology/fulltext/S0960-9822(24)00805-4
nationalgeographic.com/animals/article/carpenter-ant-amputation-wound-treatment-first
science.org/content/article/ants-may-be-only-animal-performs-surgical-amputations
Images
1. Ant surgery. Photo credit: Bart Zijlstra
2. Florida carpenter ant amputating the leg of an injured comrade (marked with yellow)
3. Video: Amputation behaviour. Injured individual ant (marked in yellow) has its leg removed at the trochanter by a nestmate.
4. Illustrations show amputation behaviour. Varying results for femur v. tibia injuries.
5. Percentage of time receiving wound care for femur v. tibia injuries
6. Anatomy of an ant's leg. The red dashed line shows where the ants amputate.
