A new study provides the first evidence that ants rescue members of their own colony post-battle even when those ants aren't in imminent danger, reports the Guardian. The observation came about when biologist Erik Frank was watching army ants march out to battle termites in highly patterned formation—"like three ants next to each other, in a 2-meter-long column," he tells NPR—and he noticed some of the ants returned carrying other ants. Those ants, it turns out, weren't dead but injured, missing a leg or two or weighed down by a dead termite whose jaws were clamped onto them. "This [is] behavior 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," says Frank. What's more, 95% survived, and many returned to battle the very next day.
So Frank and colleagues at the University of Würzburg in Germany began to study this behavior more closely, and learned that one difference between soldiers on a battlefield and ants is "these ants are not doing it out of the goodness of their heart," he says, but rather in response to the chemical signals sent off by their injured comrades. Reporting in the journal Science Advances, they note that the behavior does not occur on the way to battle, and not when injured ants are from outside colonies. There's another matter of practicality: They calculate that a colony size is 29% larger when ants carry back their injured than when they don't. Outside researchers agree this is a form of natural selection. "This is an army," one says. "They're going off to attack the termites. It's a battle. And the more numerous you are, the more successful you are." (Ants were farmers well before humans.)