To Protect Colony, Sick Baby Ants Sacrifice Themselves

Researchers have observed sick adult ants leaving their colony to avoid sickening others. Now, they've discovered that terminally ill young ants or pupae, unable to move in their silken cocoons, have a similar strategy: They ask to be killed for the good of the group. According to a study published in Nature Communications, ant pupae infected with a lethal pathogen release what NPR calls a "kill me" scent that prompts worker ants to open their cocoon and coat them in a self-made disinfectant (formic acid) that kills the pathogen and ultimately the developing ant, before the illness becomes contagious. The behavior, called "destructive disinfection," had been documented before, but it wasn't clear whether it was an automatic colony response or triggered by the sick young ants themselves.

To test that, researchers led by Sylvia Cremer of the Institute of Science and Technology Austria and University of Würzburg chemical ecologist Thomas Schmitt focused on Lasius neglectus, an invasive garden ant. They found that infected pupae alone did not emit the signal; they produced it only in the presence of workers able to carry out the disinfection. Producing the scent appears costly for the pupae, so it makes sense, the team says, that they call for destruction only if someone is around to do the job. The signal also came only from worker-destined pupae, not future queens, who can fight infection on their own.

In a final test, the team established cause and effect by transferring the chemical scent from a sick pupa to a healthy one. Worker ants then began to dismantle the healthy pupa's cocoon as if it were infected, confirming the smell is the trigger. A release notes this is the first time this form of altruistic disease signaling has been observed in social insects. But the study authors note the behavior isn't entirely altruistic as there's a genetic payoff for the terminally ill ants: because most workers are sterile, their best chance to pass on shared genes is by protecting the colony, even if it means self-sacrifice. (Ants have also been found to amputate limbs to curb infections.)