It sounds like a heroic tale from a classic wartime movie: the rescue of an injured warrior on the battlefield so that they can fight another day. In fact, it is behaviour seen in ants.
Researchers have found that a termite-eating species of ant, called Megaponera analis, carries wounded compatriots back from the field.
“This [is] behaviour you don’t expect to see in ants; you always imagine an individual ant as having no value for the colony and that they sacrifice themselves for the good of the colony,” said Erik Frank of the University of Würzburg and co-author of the study published in the journal Science Advances. But the new research shows that “the good of the individual is for the good of the colony in this case,” he added.
While in humans “rescuing” is ascribed to empathy, the team say the study shows there are other mechanisms in nature for such behaviour, revealing that the injured ants signal their plight through secretions of pheromones from their mandibular glands.
“These ants do not have empathy, they use chemical communication,” said Frank.
The study, the team adds, is the first evidence of ants rescuing nest-mates even though they are not in imminent danger of suffocation or being eaten.
Frank and colleagues made their discoveries through a series of observations in the Comoé national park on the Ivory Coast, involving 52 different colonies of ants which made 420 raids on termites. Larger ants break open the soil covering the termite site while smaller ants charge in, kill the termites and haul them out.
The team found that as well as carrying dead termites back to their nest, the ants also scooped up their peers who had lost legs or antennae, were rendered immobile by termites clinging to their bodies or lagging behind for another reason. More than 96% of those rescued were smaller ants.
To check whether the rescue behaviour was focused on injured nest-mates, the team cut off two legs each from a total of 40 ants over a number of raids and watched how others responded. They found that if the ant was from a different colony, it was either attacked or dragged away from the ant column, but if it was from the same colony it was carried back to the nest – although this only occurred at the hunting ground or on the return march to the nest.
In a further experiment, when the team prevented 60 ants from being carried home, they found that 32% died on their return journey, primarily by being snapped up by spiders.
By contrast, if ants were rescued and allowed to recover or have clinging termites removed, they lived to scrap again. “What we found was that nearly all of these ants, around 95%, participated again in future fights,” said Frank. About a quarter of all ants in a hunting party showed some type of long-term injury, the authors note, pointing out that those who lost legs soon recovered their speed of movement.
Using a model, the team found that the rescuing behaviour reduces the cost of foraging and is linked to a colony size about 29% larger than without it. “By saving these injured ants which participate again in future raids, they don’t have to replace them by producing new workers,” said Frank.
Francis Ratnieks of the laboratory of apiculture and social insects at the University of Sussex, who was not involved in the study, welcomed the research.
“As the rescued ants can be useful again to the colony, and the rescue is not expensive and is practical, it would be favoured by natural selection,” he said.
“These ants are specialist termite predators, and termites have specialised defenders with large heads and mandibles,” he added. “So it is an interesting evolutionary arms race in which by rescuing ants that have been injured, and especially those that have been immobilised by a termite defender who is clinging on to an ant, the ants have reduced the cost of their predation method.”
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