For this study, the researchers exposed study ants to six different situations, as described here.
Using a novel experimental technique that binds victims experimentally, we observed the behavior of separate, randomly chosen groups of 5 C. cursor nestmates under one of six conditions. In five of these conditions, a test stimulus (the “victim”) was ensnared with nylon thread and held partially beneath the sand. The test stimulus was either (1) an individual from the same colony; (2) an individual from a different colony of C cursor; (3) an ant from a different ant species; (4) a common prey item; or, (5) a motionless (chilled) nestmate. In the final condition, the test stimulus (6) consisted of the empty snare apparatus.Only active nestmates, ants from the same colony, elicited rescue behavior. Others evoked aggression or no reaction. The authors conclude that a struggling nestmate emits a colony-specific pheromone to which rescuing ants respond.
Moreover, C. cursor ants are able to engage in highly precise behavior directed toward the inanimate object that has entrapped their nestmate. Thus, our findings show that rescuers somehow were able to recognize what, exactly, held their relative in place and direct their behavior to that object in particular, demonstrating that rescue behavior is far more exact, sophisticated and complexly organized than previously observed. That is, limb pulling and digging behavior could be released directly by a chemical call for help and thus result from a relatively simple mechanism. However, it's difficult to see how this same simple releasing mechanism could guide rescuers to the precise location of the nylon thread, and enable them to target their bites to the thread itself.It's the last sentence of this paragraph that interests us. The thread is obviously not emitting any chemical signal, and the ants are not 'pre-programmed' to know that danger is connect to the thread, and that they could break it with their pincers. Anyone who has observed ants for any time at all, including Darwin, must surely be impressed by their sophisticated cognitive, decision-making abilities. They work together to move barriers from their trails, they figure out ways around barriers they can't move, they cooperate to bring prey back to the nest, and so on.
Darwin himself said it very well in Descent of Man (1871):
...the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a pin's head. Under this view, the brain of an ant is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of man.
It's easy to dismiss the rescue behaviors described in this PLoS paper as 'only' biochemical. But cognition and sophisticated decision-making are evolutionarily ancient. And, what are they, really, but the interaction of complex surface molecules and their receptors? We tend to privilege human thought capabilities, but in fact our brains are just signals, ligands and surface receptors, too.