Though many ants spend their lives peacefully tending fungus farms and herds of aphids, others have it much rougher. Such is the case with the ant species Megaponera analis, native to many parts of sub-Saharan Africa, whose days are spent endlessly hunting termites to eat. In fact, a typical M. analis will engage in at least two battles per day with nests of angry termites. They sustain so many injuries that these ants have developed something that is extremely rare in the insect world: M. analis has learned how to rescue and rehabilitate ants that suffer extraordinary injuries on the battlefield.

University of Würzburg ecologist Erik Thomas Frank and his colleagues spent over two years observing M. analis in Comoé National Park in the northern Côte d’Ivoire, tracking 52 colonies that conducted 420 termite nest raids. They recorded their findings in a paper for Science Advances, unveiling a society where combat and altruism have evolved side by side. Their work could also shed light on why rescue behavior has evolved among mammals, too.

The raids always begin with a single scout. Individual M. analis roam the area around their nests up to 50 meters away, looking for likely victims. When the scout finds a good target, she returns to the nest to raise an army (all ants on these raids are females). She recruits 200 to 500 nestmates, leading them in what Frank and his team call "a column-like march formation" to the termites' lair. When they arrive, the ants break into groups. Larger ants dig through the protective layer of soil that covers the colony, then the smaller ants plunge inside, slaughter their prey, and drag it back outside. The bigger ants carry the freshly killed termites home for a feast. To feed an entire colony of roughly 1,000 ants, M. analis conducts two to four raids each day.

The battles with the termites are dangerous and deadly. They often bite off the ants' limbs, heads, or thoraxes. Sometimes the termites will latch onto an ant with their powerful jaws, refusing to let go. Though this doesn't kill the ant outright, it prevents her from moving quickly and takes her out of the melee. Mostly it's the smaller ants inside the termite nest who are injured, and the survivors crawl outside once the hunt is over.

That's when the rescue teams sweep through the battlefield. Though ants are blind, they have an exquisite sense of smell and touch. The larger ants will make their way through the dead and dying, using their antennae to smell their fallen comrades. Injured ants release a special blend of chemicals from a gland in their mandibles, which Frank and his colleagues identified as dimethyl disulfide and dimethyl trisulfide. When rescuers scent those chemicals on an ant, they pick her up and carry her home in the marching column—even if she has several heavy termites dangling off her body. Only ants with fatal injuries are left behind.

Once the M. analis are back in their nest, the wounded are treated. Other ants remove the termites from their bodies, and no lasting harm is done. Ants with missing limbs experience difficulties at first but learn to walk and run again within 24 hours. Frank and his team report that roughly nine to 15 ants are rescued per day, and 95 percent of them quickly return to normal raiding activities. Indeed, at least 21 percent of ants on any given raid have significant, long-term injuries.

Frank et al. Sci. Adv. 2017

Frank et al. Sci. Adv. 2017

Frank et al. Sci. Adv. 2017

The sheer number of disabled yet productive ants in a M. analis colony offers an evolutionary explanation for why rescue behaviors make sense. Obviously, these ants are a tremendous benefit to the colony's survival as long as they can make it back to the nest to recover. But why carry them home? The researchers wanted to know, so they selected 20 injured ants at random and forced them to walk home alone. Thirty percent of them died, usually by predation from spiders or other ant species. One particularly tragic ant died of exhaustion after dragging two attached termites most of the way to the nest. Put simply: the injured are vulnerable, and carrying them home protects them from further harm.

Though it sounds like a basic idea, this kind of rescue behavior is exceedingly rare among ants and other animals generally. Even among M. analis, the behavior is only triggered by a complex set of circumstances. Ants rescue each other only when they are returning from battle. When the researchers put an injured ant in the way of a column marching toward a raid, they ignored her and left her behind. Repeating the experiment with a returning march got the injured ant immediately scooped up. So rescue can only happen when the injured ant releases its distress chemicals in the right context.

The researchers note that they found a similar chemical signal in another species of ant, P. tarsatus, but it didn't result in carrying behavior. Instead, P. tarsatus exposed to the chemical began digging maniacally. It turned out that this type of ant is often the victim of a predator called an antlion, which captures ants by creating quicksand-like traps. For this species, the distress signal meant "hurry up and dig me out!" And that's just what the P. tarsatus did.

But why did the termite-raiding ant M. analis develop such a specific, almost human form of rescue? Frank and colleagues believe this kind of behavior could only have evolved in a group that forages collectively in a relatively small area so that injured ants will always be noticed by their sisters. Also, it seems to make sense for animals that hunt prey that fights back, causing substantial but non-lethal injuries. The benefits of rescue are substantial, at least if your goal is a large hunting party. The researchers write:

The fact that 21% of all ants carry some type of long-term injury in the raiding column substantiates the great value of helping injured nestmates, a conservative estimate because nonpermanent injuries are not included in this estimate. The value of rescue behavior is reflected in the sustainable colony size calculated by our model, which predicts a 28.7% larger colony size compared to colonies without this behavior.

In addition, colony size plays a role. A typical M. analis colony sees about 13 births per day, so each individual matters. One might contrast this with Argentine ants or army ants, who live in nests that may contain tens of thousands of individuals.

Ultimately, Frank and his team write that the rescue squads of M. analis are an excellent example of convergent evolution. Though chemical communication triggers rescue among ants, empathy and other factors trigger it in humans. These behaviors may have evolved separately, but they result in the same basic outcome. Both humans and M. analis have many functional, productive members of their communities who are disabled. And the rescues continue because each one benefits the species as a whole.

Science Advances, 2017. DOI: 10.1126/sciadv.1602187

Listing image by Frank et al. Sci. Adv. 2017