Carry me home Erik Frank

It’s an ant-help-ant world. Ants have been seen carrying their wounded comrades back to the nest after raids on termite colonies ­- an unexpected behaviour in social insects that usually appear to treat individuals as expendable.

The ants don’t help their wounded comrades out of the goodness of their hearts, however: the wounded soldiers are needed for future raids and are soon back in action.

Megaponera analis, a species found in sub-Saharan Africa, feeds exclusively on termites, sending armies of 200 to 500 individuals to raid termite nests. First, large ants called majors break open the nest. Smaller ants, called minors, then rush in to kill and pull out termites.


However, termite nest are defended by soldiers with big jaws, and the invaders often sustain serious injuries. Ants that lose limbs are severely handicapped immediately after the injury, but after a few hours they adjust and can run almost as fast as uninjured ants – if they are carried back to safety.

“At first, they kept tripping over, because they thought they [still] had six legs,” says Erik Frank at the University of Würzburg, Germany. “Inside the nest, they were safe to adapt and change their locomotion.”

Lost limbs

Frank and his colleagues observed 54 raids by these ants in Comoé National Park in Ivory Coast, using infrared cameras to see inside their nests. On average, three ants were carried back after each raid, almost all of them minors. Most had a termite clinging on to them, and some had lost a limb.

“It was completely new and unexpected,” says Frank.

The researchers marked the injured ants with acrylic paint to monitor their recovery. Ninety-five per cent of those that were carried back took part in subsequent raids, sometimes less than an hour after the injury.

Rescuing behaviour appears to be stimulated by a pheromone produced in the mandibular gland of injured ants. When the researchers applied mandibular gland extract onto healthy ants, their nestmates carried them home.

Termites aren’t the only threat to the injured ants. When the researchers forced some injured ants to return alone, a third of them died, mostly from predation by spiders. The team estimate that carrying home their wounded allows the colonies to be 29 per cent larger than otherwise.

Valuable individuals

Frank thinks that several traits of this species made this behaviour likely to evolve: they hunt in groups, they have a high injury rate, and they have very small colonies, which means each individual is valuable.

“There are other species that hunt termites in small groups, where this behaviour could evolve, and I would like to look for this,” he says.

Under these circumstances, the behaviour makes a lot of sense, says Sara Helms Cahan at the University of Vermont. “We often think about ants as being kind of like trees with leaves: every individual doesn’t matter very much because there’s so many of them. But if you have a small colony, that’s actually a pretty important investment.”

The idea of ants helping their comrades isn’t so remarkable, she says. “Ants do many things that could be considered helping. But this type of behaviour – where individuals are cared for after they’re injured – is a new element in the arsenal of ant-helping behaviour.”

Journal reference: Science Advances, DOI: 10.1126/sciadv.1602187

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