Ants with terminal illnesses send out a distress call to their colony mates, urging them to poison them with acid to protect the entire colony. This discovery challenges the notion of an ant colony as a mere collection of individuals, instead presenting it as a unified 'superorganism'.
In a recent study published in the journal Nature Communications, researchers uncovered that the pupae of the Lasius neglectus ant species actively release a chemical signal, prompting other colony members to destroy them. This behavior is a form of self-sacrifice, as sick individuals often conceal their condition to avoid social exclusion or aggression. However, pupae, being immobile and encased in cocoons, resort to a more drastic strategy: emitting a chemical signal that essentially calls for their own demise.
The study's co-author, Sylvia Cremer, explains that adult ants approaching death leave the nest to die outside the colony, while workers exposed to fungal spores practice social distancing. Yet, this is only feasible for mobile individuals. Ant brood within the colony, akin to infected cells in tissue, lacks this option and must rely on other means.
Upon receiving the signal, worker ants remove the pupae from their cocoons, puncture holes, and inject them with formic acid, an antimicrobial poison. This process not only kills the pathogens but also the pupae. While previous research had shown that worker ants can recognize sick pupae and kill them to disinfect the nest, the new study reveals that the sick pupae actively trigger this behavior.
During the experiment, sick worker pupae emitted a modified body smell, warning adult ants to destroy them. This signal was only produced by sick ants near adult worker ants, indicating it's not just an immune response. When the smell was applied to healthy pupae, they were also destroyed, confirming the chemical's role in triggering the response.
Thomas Schmitt, another co-author, explains that the scent cannot simply diffuse through the nest chamber but must be directly associated with the diseased pupa. The signal is composed of non-volatile compounds on the pupal body surface.
The behavior of terminally ill pupae mirrors the human body's cellular response. Our cells release chemical cues, known as 'find-me and eat-me signals', for our immune cells to identify and destroy them, preventing infection. Similarly, terminally ill ants safeguard their nestmates by warning the colony of their infection, ensuring the colony's health and the production of daughter colonies, which indirectly passes on the signaler's genes.
Interestingly, queen pupae did not emit the chemical signal. They possess stronger immune defenses and can restrict the infection independently. Worker pupae, however, couldn't control the infection and warned the colony. This precise coordination between individual and colony levels makes the altruistic disease signaling highly effective.
