It was a dramatic rescue attempt: A half-buried ant called for help and her sisters came running. First, they tried pulling at her legs, then they attempted to dig her out. Their digging exposed the problem—the ant was ensnared by a nylon thread that had been tied to a piece of filter paper. So, the ants attempted to bite through the thread and pulled at the filter paper.

That was the scene one day in the lab of University of Paris–North biologist Elise Nowbahari, PhD, working in collaboration with Mt. Holyoke psychology professor Karen Hollis, PhD, and others. It's one of very few animal rescue scenes detailed in the scientific literature (PLoS One Vol. 4, No. 8). The only other animals that have made documented rescue attempts are dolphins, which raise ailing kin to the surface to breathe, and a capuchin monkey that saved another animal from being attacked, says Hollis.

How ants, with their tiny brains, managed such a complex rescue remains a mystery.

"Obviously they are not thinking about it," she says. "Some simple mechanism is going on in their brains," she says.

The researchers unearthed one clue as to how the ants organized their rescue attempts by ensnaring different kinds of victims, including a sister ant from the same colony as the search party, an ant from a different colony, a different species of ant and a motionless (chilled, but not dead) sister of the search party.

They found that ants only rescued their active sister, not the chilled one, suggesting that trapped ants must actively call for help, probably by releasing pheromones. The search party ants, however, did not respond to the ant from a different colony (but the same species), which means that each colony must have its own unique distress call, says Hollis. When the search party found an ensnared ant from a different species, they attacked it—biting it, trying to pull off its antennae and spraying formic acid at it.

From an evolutionary standpoint, it makes sense to attack ants from other species that are roaming in your food-foraging area, says Hollis. And, because ant societies depend on vast numbers of forager ants to bring back food, ants that come upon a sister in trouble have good reason to help her out.

"Even with so many ants in a colony, individual ants are surprisingly important," she says.