Mildly injured ants may overact their disabilities to get rescued
No wounded left behind — not quite. Ants that have evolved battlefield medevac carry only the moderately wounded home to the nest. There, those lucky injured fighters get fast and effective wound care.
Insect colonies seething with workers may seem unlikely to stage elaborate rescues of individual fighters. Yet for Matabele ants (Megaponera analis) in sub-Saharan Africa — with a mere 1,000 to 2,000 nest mates — treating the wounded can be worth it, says behavioral ecologist Erik Frank at the University of Lausanne in Switzerland.
Tales of self-medication pop up across the animal kingdom. For Matabele ants, however, nest cameras plus survival tests show insects treating other adults and improving their chances of survival, he and colleagues report February 14 in Proceedings of the Royal Society B. For treatment boosting others’ survival, Frank says, the closest documented example is humans.
In Ivory Coast, Frank studied Matabele ant colonies that staged three to five termite hunts a day. He and colleagues at the University of Würzburg in Germany published research last year showing that members of a hunting party carry injured comrades home.
Frank took a closer look at rescues after he accidentally drove over a Matabele ant column crossing a road. Survivors “were only interested in picking up the ants that were lightly injured, and leaving behind the heavily injured,” he says.
When Frank later set injured ants in front of columns trooping home from raids, injured ants minus two legs typically got picked up. Only once did an ant with five missing legs get a lift.
Ants that have lost two legs still have value to a colony, especially in a species where only about 13 new adults a day emerge from pupae. Four-legged ants regain almost the same speed that ants have on six legs, he says. In a typical hunting party, about a third of the ants have survived some injury, but most ants have at least four legs left.
How the ants triage a battlefield evacuation is shaped by the injured ants’ behavior, Frank says. Ants with only moderate injuries, such as two lost legs, emit “help me” pheromones. These ants tuck in their remaining legs and generally cooperate with the rescuers. Not so with ants more seriously hurt, who may not even give off pheromones. Rescuers still stop to investigate. But the seriously injured ants often flail around instead of cooperating, and the rescuers give up.
Frank also has seen ants act more severely injured than they truly are. If the returning fighters bypass them, “they will immediately stand up and run as fast as they can behind the others,” he says. “In humans, it’s a very selfish behavior.” For ants, predators lurk, and the colony benefits by finding the injured first.
WOUNDED IN BATTLE Africa’s Matabele ants, which attack termites in violent battles, offer the first documented case of animals other than humans doing successful wound care and even battlefield triage. When a mildly injured hunter gets carried home, other ants increase her chance of survival by repeatedly “licking” the injured leg (first video clip). An ant with very serious injuries (second clip) at the combat site doesn’t cooperate with attempted rescues and isn’t carried home. A mildly injured ant can act more injured than she is (third clip). If ignored, she will stand up and run (fourth clip) after potential rescuers.
For injured raiders that do get home, another ant — usually not the carrier — steps in to treat the wound by repeatedly moving her mouthparts over it. When Frank isolated the ants to prevent this wound licking, about 80 percent of injured ants died. When he allowed ants an hour of treatment before isolating them, only 10 percent of them died.
Based on Frank’s observations, others who study ants are now wondering if they also have seen such rescue tactics. Andy Suarez of the University of Illinois at Urbana-Champaign wants another look at big Dinoponera australis that he’s frequently seen prowling for prey despite missing a limb. And Bert Hölldobler wonders whether weaver ants he has seen retrieving injured nest mates after battle were rescuing them. The usual interpretation has been cannibalism, says Hölldobler, at Arizona State University in Tempe.
Frank, however, used bright acrylic spots to track the fate of rescued Matabele ants. They weren’t for lunch.
E.T. Frank, M. Wehrhahn and K.E. Linsenmair. Wound treatment and selective help in a termite-hunting ant. Proceedings of the Royal Society B. Published February 14, 2018. doi: 10.1098/rspb.2017.245.
E.T. Frank et al. Saving the injured: Rescue behavior in the termite-hunting ant Megaponera analis. Science Advances. Vol. 3, published April 12, 2017, e1602187. doi: 10.1126/sciadv.1602187.
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