Scientists have long observed the relationship between Japanese oakblue butterfly (Narathura japonica) caterpillars and some nearby ants (Pristomyrmex punctatus). The caterpillars secrete a sugary substance for the ants to feed on, and in turn the ants protect the caterpillar as it grows inside oak tree leaves.

But recently, Masaru Hojo from Kobe University in Japan noticed that each caterpillar was always guarded by the same ants, which “never moved away or returned to their nests,” he told New Scientist. Was there something in the sweet secretions that was rendering the ants unable or unwilling to care about anything beyond attending to the caterpillar—even finding food to sustain themselves? 

To test this theory, Hojo and his colleagues conducted lab experiments in which they allowed some ants to interact with the caterpillars and feed on the secretions, and kept others separate as a control. The control ants that didn't drink the sugar droplets remained free to come and go as they pleased. But those that fed on the secretions stayed with the caterpillar, abandoning their nests. What's more, whenever the caterpillar turned its tentacles inside out, the ants suddenly started acting violently. Meanwhile, the ants who had not consumed the secretion remained unaffected by the caterpillar's behavior. Something in the secretions seemed to be drugging the ants, turning them into zombies that were controlled by the caterpillar.

Hojo speculates that this control over the ants is used in the wild as a form of protection. Rather than fight its own battles, the caterpillar responds to predators by inducing the ants to behave aggressively. “There are glandular cells near the tentacles that could be secreting chemical signals,” Hojo said. “It is possible that both visual and chemical signals are stimulating the ant aggression.”

The researchers, who published the results of their experiment in a recent issue of Current Biologyaren't sure what it is about the secretion that gives the caterpillar mind control over the ants, but it appears to be related to dopamine. The doped-up ants had lower levels of dopamine and, when treated with a drug called reserpine—which blocks the transport of dopamine—even the ants that had consumed the secretion stopped responding to the caterpillars' tentacles.

Hojo and other scientists unrelated to the study say these findings call into question many interspecies relationships thought to be mutualistic—but some people aren't so quick to dismiss the existing beliefs.

“The benefit for the caterpillar is obvious, but we do not know whether the benefit for the ants is as minimal as the authors argue,” Martin Heil, of the Center for Research and Advanced Studies of the National Polytechnic Institute in Irapuato, Mexico, told New Scientist. "If the liquid that the caterpillars secrete is sufficiently nutritious, then it might well be that the overall balance for the ants also is positive."

[h/t New Scientist]