These sick baby ants sacrifice themselves to protect their colony

Some baby ants don’t ask for help when they contract deadly infections — they ask to be killed.

Terminally ill worker ant pupae actively emit a “find me and destroy me” chemical signal, prompting other workers to eliminate them, behavioral ecologist Sylvia Cremer and colleagues report December 2 in Nature Communications. This final altruistic act may be an attempt to save the colony from contamination and ensure its survival.

“Just as cells in a body coordinate to maintain the health of the whole organism, individual ants work collectively to protect the colony,” says Cremer, of the Institute of Science and Technology Austria in Klosterneuburg.

In earlier work, Cremer and her team showed that workers of the ant species Lasius neglectus quickly sniff out fungal disease spreading among their brood of baby ants. After detecting cuticular hydrocarbons — chemical alarm signals the pupae emit via their hard exteriors, or cuticles — worker ants engage in “destructive disinfection,” prying open cocoons, puncturing cuticles and spraying formic acid inside. This antimicrobial spray kills both the fungal spores and the sick pupae.

In the new study, the researchers discovered that the pupae’s immune genes activate after contracting a fungal infection, implying an immune response. But infected pupae emit the hydrocarbons only in the presence of a mature worker, not when they are alone.

“We were fascinated to discover that pupae can sense their surroundings and adjust their chemical signals based on whether workers are present,” says coauthor Erika Dawson, a former biologist in Cremer’s lab who is now a grant writer at Sorbonne University in Paris.

The team also found that a higher infection load did not trigger more chemical release. Together, the results indicate that the cues aren’t passive — “the ants are proactively signaling their destruction,” says Mark Bulmer, a molecular ecologist at Towson University in Maryland who was not involved with the study.

Infected queen pupae, on the other hand, do not emit this self-sacrificial signal, because their advanced immune systems curb the infection before it gets out of hand, the researchers found. Because of their ability to reproduce, queens are more valuable than workers, which could potentially explain their better immune system.

This altruistic signaling shows how ant colonies function as a single living entity, or superorganism. Ants in a colony are more like cells in our body, says Erik Frank, an animal ecologist at the University of Würzburg in Germany. Immune cells sometimes self-destruct to prevent the spread of infection, thereby protecting other cells.

“Humans are very much interested in the individual fitness; because we can all reproduce, we want to maximize our own genes, while social insects, all of them [except the queen] are sterile, and they can only really benefit by helping the queen reproduce as much as possible,” Frank says. “It makes sense for individual workers to sacrifice themselves for the inclusive fitness of the colony rather than their own selfish individual fitness.”

While adult ants exposed to pathogens can practice social distancing or leave the nest to die when terminally ill, pupae confined to their cocoons must rely on the distress signal. Workers, the researchers note, are able to sniff out and destroy only the infected pupae, and do not kill the brood indiscriminately. Cremer wants to check whether pupae would sacrifice themselves so readily if their infection levels were much lower and they could recover. “Our expectation is that the ants should not take that risk,” she says. Overlooking even a single pupa that could potentially shoot out fungal spores “can have devastating effects for the colony,” she adds