No Ants Left Behind: Why Do Ants Rescue Their Injured Mates?

Since Matabele ants go out in force, raid and return as a column, any ants that lag behind are at high risk.

An injured ant is ambushed by a jumping spider while returning from the hunting ground alone. Credit: Erik Frank

An injured ant is ambushed by a jumping spider while returning from the hunting ground alone. Credit: Erik Frank

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

As soon as the scout returns after identifying the target, 200 to 500 warriors march out in an orderly column. Once they reach the site, the majors rip up the concrete-hard mud fortifications and minors pour into the vulnerable colony, seizing and killing the inmates. Soldiers of the besieged fight back, ripping the legs off the invading minors. Some cling to the raiders’ legs in a desperate attempt to stall the attack. The battle lasts an hour. As the invaders head back to their encampment with their dead booty, many of their ranks lie wounded at the site. They are far from abandoned as their healthy comrades, the majors, hoist them away from the battlefield. The invaders are Matabele ants (Megaponera analis) and their targets, termites. Scientists discovered that this tendency to help their compatriots is not triggered by empathy but by chemical molecules called pheromones.

Researchers from the University of Würzburg, Germany, investigated why ants rescue their mates and how they sense others’ predicament. They observed 52 colonies of Matabele ants conduct 420 raids in the Comoé National Park, Ivory Coast, for 13 months over three years. They marked the injured ones with acrylic colours to follow their movements and fate.

Twice, sometimes four times, a day, Matabele ants march in formation across the forests, woodlands, and savannahs of sub-Saharan Africa, on the lookout for termite nests. Although these two-centimetre-long ants are one of the largest in the world and are named after the fierce Matabele tribe of southern Africa, the insects suffer high casualties during a raid. Termite soldiers use their strong heads and enormous mandibles to devastating effect.

Helping others is not newly discovered ant behaviour. The larvae of antlions dig little circular pit traps in sand. Any ant that runs over the edge of the pit slips down the slope toward the spiny jaws of the predator. Some species of ants launch rescue operations to free their companions from certain death. Sometimes, they risk being devoured themselves in their attempt to help a fellow ant. Even in experimental conditions, when researchers tied ants with a nylon thread, the captives’ mates tried to free them by biting the thread.

Erik Frank, the main author and a doctoral student, says he was planning to study the raiding behaviour of Matabele ants. “That’s when I noticed some of the ants getting carried back after the hunt,” he told The Wire. “This made me curious and after making sure they were injured, we wanted to know more about what exactly was going on.”

Frank and his research team measured the speed of the ants. When the ants lose their legs, they don’t skilfully run on their remaining legs. Instead, they stumble on their phantom limbs and are considerably slower. The slowest, however, are the ones with termites attached to them. These hurt warriors curl up with their legs tucked, waiting for help. Some ants that were seemingly healthy – not handicapped by the loss of legs or hampered by termites – were also aided by others. Perhaps their injuries were not visible to the researchers or they were not hurt at all.

An ant handicapped by a clinging termite is carried off by a foraging African stink ant. Credit: Erik Frank

An ant handicapped by a clinging termite is carried off by a foraging African stink ant. Credit: Erik Frank

When the researchers forced these ants to return alone, without any aid, to their nests, 32 percent of the injured ants died compared to 10 percent mortality of healthy ones. Spiders took most of the wounded, especially targeting the slowest. Exhaustion killed others, while the predatory African stink ant, Paltothyreus tarsatus, targeted a few.

The researchers removed two legs, one from each side, of a few healthy ants and dropped them in front of an ant column returning from a hunt. The legions hauled the maimed back to the nest. But if the injured belonged to another nest, they attacked and dragged the intruders away from their raiding column.

Back in the nest, ants tend their mates, removing clinging termites. With their mobility restored, the insects recover, sometimes as quickly as an hour, and take part in future raiding parties. Although ants that lost their legs are handicapped for life, they get used to running on their remaining legs in 24 hours. The next day they run almost as fast as healthy ants. Being rescued guarantees the survival of injured ants.

“Individuals that require help are under less imminent threat than in other contexts such as collapsed nests and captures by antlion larvae,” Krzysztof Miler told The Wire. “I’m not sure if I would call what the authors describe rescue behaviour, but it is no doubt extremely specialised behaviour which serves colony-level interests.” Miler, a doctoral student at  Jagiellonian University, Kraków, Poland, wasn’t involved in the study.

What drives ants to help one another? Ants typically make a squeaky sound by rubbing a scrapper over tiny ridges in their rear sections called stridulation. Could this be how wounded ants were calling for help?

“We did not really think that stridulation played a role,” Frank told The Wire. “Ants don’t have ears in the classical sense of the word. They can only feel the vibrations through the ground. I believe this signal would have been too difficult to localise, unlike a chemical signal where you just have to follow the gradient.”

To rule out stridulation, the researchers painted black acrylic paint on the stridulatory organ of injured ants. These ants were still able to get aid from their colony mates.

“Chemical communication is, in general, the preferred communication mechanism in social insects,” says Frank. “So we were relatively certain it would be produced in a gland.”

Ants have glands all over their bodies. The researchers extracted compounds excreted in the ants’ rear ends by poison and Dufour’s glands as well as the glands in their mandibles, similar to our salivary glands. They smeared these on dead and live ants and watched how the others behaved toward them.

Although the colony mates attempted to rescue the dead covered in mandibular compounds, they dropped them. “A dead ant has all of its legs sticking out in all directions which makes carrying it a lot more difficult,” says Frank. “We believe that is why they dropped them.”

The ants tried to haul away the live ones covered with mandibular gland secretions. The injured ones cooperated by pulling their legs in, making it easier for their mates to carry them. But just as the rescuers dropped the dead, they also dropped many live ones.

“The healthy ant did not want to get carried back because there was nothing wrong with her,” explains Frank. “So she actively tried to free herself from the ant that picked her up in a few cases. Some allowed the ant to carry them back.”

However, the ants reacted vehemently towards the ones covered in poison and Dufour’s gland secretions. They shoved them off the column, perhaps not recognising them as their own.

The researchers identified the chemical compounds in the mandibular glands as the foul-smelling dimethyl disulfide and dimethyl trisulfide using gas chromatography mass-spectrometry.

Karen Hollis of Mount Holyoke College, Massachusetts, US, told The Wire the study is significant because not only does it show that rescuing injured ants reduces mortality but that injured ants actually call for help. She wasn’t involved in the study.

Why do the Matabele ants go to so much effort to help individuals when they have plenty of warriors? The researchers estimated the size of a colony was between 900 and 2,300 and it produces 13 young per day. But up to 15 of its members are injured in raids a day.

“Losing even half of those injured ants would be a significant burden for the colony to compensate. It would basically have to increase the birth rate by that number,” says Frank. Indeed, wounded veterans form 21 percent of a typical raiding column.

Such rescue behaviour of injured ants is unique in social insects, say the authors. Since Matabele ants go out in force, raid and return as a column, any ants that lag behind are at high risk. An injured ant of a foraging species, where a column of ants runs between nest and food source like nectar or carrion, can return slowly to the nest without running any risk of predation since it’s surrounded by its mates. In such species, rescue behaviour may not evolve.

To sustain their hunting strategy, Matabele ants have to minimise their losses and rescuing compatriots gives them an edge over termites. Only 30 percent of a raiding party carry prey back to the nest, so the others can engage in rescue. Since every rescued ant survives, the benefits of helping others outweigh the loss. Indeed, the researchers estimate Matabele ant colonies are 29 percent larger than other colonies of ants that don’t rescue their mates.

“The authors’ model quantifies the fitness benefits with mathematical rigour,” says Hollis. “Their findings are an important addition to the study of rescue behaviour – helping to show us how we might model our own studies – but also to the study of helping behaviour in ants more generally.”

“I find it curious that this behaviour seems to be low cost,” says Miler, “as opposed to rescue observed in other contexts like collapsed nests and captures by antlion larvae.”

However, these ants don’t indiscriminately rescue any ant seeking help. When researchers dropped injured ants when the column set off from the nest to raid a termite colony, the other ants ignored them. Since ants don’t naturally get hurt on their outward journey, the researchers speculate they disregard the injured ants. Besides, even if they were to rescue them, the rescuers would have to scurry against the tide of the column back to the nest, leaving them open to predation. How could ants reason this is an unnatural situation?

“Obviously ants don’t go through that thought process,” explains Frank. “I would argue that because it never occurs naturally, a response by the ants was never necessary to evolve.”

At the end of the raid, when the minors leave the termite gallery, the majors have a last look around. If they ‘smell’ injured ants, they pick them up. If otherwise-healthy ants haven’t joined the column to march back home, the majors haul them too. If there are no ants that need aid, they grab a termite “so they don’t return empty-handed,” says Frank.

He says this study raises other questions. For example, are the ants able to differentiate between light and severely injured ants? Do they tend injured ants inside the nest? Is this behaviour unique to this species or do other species like the Leptogenys species in India with a similar ecology also behave in a similar manner?

The study was published in the journal Science Advances on April 13, 2017.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.

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