Sick, Immobile Young Ants Send “Kill Me” Signal to Colony Workers

Adult ants that have been infected with deadly pathogens often leave the colony to die so as not to infect others. But, “like infected cells in tissue, [young ants] are largely immobile and lack this option,” said Sylvia Cremer, a researcher at the Institute of Science and Technology Austria (ISTA) who studies how social insects such as ants fight diseases collectively as superorganisms, in a statement.

Cremer’s team previously found that invasive garden ant pupae that have been infected with a deadly fungal pathogen produced chemicals that induced worker ants to unpack their cocoons and kill them, preventing the disease from spreading to the rest of the colony.¹ However, the researchers did not know if the chemicals were simply a result of the fungal infection or if the sick pupae specifically released them to call for their own demise.

In a new study, Cremer and her colleagues discovered that the infected pupae only released chemicals when there were workers nearby, suggesting that the sick young ants put these events in motion.² The researchers’ findings, published in Nature Communications, are the first evidence of altruistic disease signaling in a social insects and share similarities with how sick and dying cells send a “find me and eat me” signal to the immune system.³

To identify fungus-infected pupae, worker ants sense chemicals on the surface of pupae known as cuticular hydrocarbons (CHCs). They then “unpack” the infected ants from their cocoons and disinfect them by creating a hole in the pupae and spraying poison into the hole. The multi-step behavior prevents the fungal pathogen from producing spores that could infect the rest of the colony. Cremer’s team previously noted that certain CHCs were significantly more abundant on the infected pupae than on their uninfected counterparts. They found that washing the pupae with pentane removed CHCs and reduced workers’ unpacking behavior.

In the present study, Cremer wanted to know what triggered the release of these specific CHCs. To address this, the team compared immune gene activation and CHC profiles between infected pupae in the presence and absence of workers.

Infected pupae consistently upregulated immune genes upon infection, regardless of whether workers were around. However, changes in the level of specific CHCs only occurred when the pupae were infected and workers were nearby. Furthermore, the team found that when they coated healthy pupae with extracts from infected pupae, workers unpacked the healthy pupae more frequently than healthy pupae covered in “non-signal” extracts. Altogether, these observations indicate that infected pupae likely release specific CHCs so workers could find and kill them, which means that these young ants sacrifice themselves to benefit the colony.

However, evolutionarily, this behavior likely benefits the young ants too, said Erika Dawson, a behavioral ecologist and a former postdoctoral researcher at ISTA who led the study. “By warning the colony of their deadly infection, terminally ill ants help the colony remain healthy and produce daughter colonies, which indirectly pass on the signaler’s genes to the next generation.”