Ant colonies operate as tightly coordinated “superorganisms” with individual ants working together, much like the cells of a body, to ensure their collective health. Researchers at the Institute of Science and Technology Austria (ISTA) have now discovered that terminally ill ant brood, like infected cells, release an odor signaling their impending death and the risk they pose. This sophisticated early warning system facilitates rapid detection and removal of pathogenic infections. The study was published in Nature Communications.
In many social animals, group members try to conceal their sickness to prevent social exclusion. Ant brood, however, take the opposite approach. When facing an incurable infection, ant pupae actively emit an alarm signal that warns the colony of the contagion risk they are about to pose.
Upon receiving the signal, worker ants respond swiftly by unpacking the terminally ill pupae from their cocoon, creating small openings in their body surface and applying their antimicrobial poison, formic acid, which functions as a self-produced disinfectant. While this treatment immediately kills the pathogens multiplying inside the pupa, it also results in the pupa’s own demise.
“What appears to be self-sacrifice at first glance is, in fact, also beneficial to the signaler: it safeguards its nestmates, with whom it shares many genes. 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,” explains Erika Dawson, first author of the study and former postdoc in the Social Immunity’ research group headed by Sylvia Cremer at ISTA.
Their collaborative study with chemical ecologist Thomas Schmitt from the University of Würzburg in Germany describes this altruistic disease signaling in social insects for the first time. If a fatally ill ant were to conceal its symptoms and die undetected, it could become highly infectious, endangering not only itself but the entire colony. Active signaling of the incurably infected instead allows effective disease detection and pathogen removal by the colony.
Altruistic self-sacrifice
At the colony level, ants function as a “superorganism,” effectively forming a single living entity. While one or more queens are responsible for producing offspring, the non-fertile workers take on all tasks related to colony maintenance and health. This mirrors cell specialization in the human body, where germline cells in the reproductive organs are dedicated to offspring production while somatic cells carry out all other essential functions.
In both organisms and superorganisms, reproductive and non-reproductive components are fully interdependent, with each essential for the survival of the whole. Cooperation is therefore crucial. Much like cells in our body, individual ants collaborate closely, even engaging in altruistic self-sacrifice for the benefit of the colony.
Find-me and eat-me signal
Why would a complex early warning system evolve if sick animals can simply isolate themselves from the colony? “Adult ants that approach death leave the nest to die outside the colony. Similarly, workers that have been exposed to fungal spores practice social distancing,” explains Cremer. “Yet, this is only possible for mobile individuals. Ant brood within the colony, like infected cells in tissue, are largely immobile and lack this option.”
Body cells and ant brood, such as developing pupae, both rely on external assistance to safeguard the colony. Intriguingly, both address this challenge in similar ways: they emit a chemical signal that attracts either the body’s immune cells or the colony’s workers, allowing these helpers to detect and eliminate them as potential sources of infection. Immunologists call this the “find-me and eat-me signal.”
“The signal must be both sensitive and specific,” explains Cremer. “It should help to identify all terminally-sick ant pupae but be precise enough to avoid triggering the unpacking of healthy pupae or those capable of overcoming the infection with their own immune system.” What properties must such a signal have to achieve this level of precision?
Changes in pupal scent profile
Schmitt, whose research focus is on chemical communication in social insects, explains that workers specifically target individual pupae out of the brood pile. “This means the scent cannot simply diffuse through the nest chamber but must be directly associated with the diseased pupa. Accordingly, the signal does not consist of volatile compounds but instead is made up of non-volatile compounds on the pupal body surface.”
In particular, the intensity of two odor components from the ants’ natural scent profile increases when a pupa is terminally ill. To test whether this odor change alone could trigger the workers’ disinfection behavior, the researchers transferred the signal odor to healthy pupae and observed the workers’ reaction.
“We extracted the smell from the signaling pupae and applied it to healthy brood,” Cremer says in describing the experimental approach. The results were conclusive: Transfer of the signal scent alone was sufficient to induce unpacking by the ants, revealing that the altered body odor of fatally-infected pupae serves the same function as the ‘find-me and eat-me’ signal of infected body cells.
Signaling only in uncontrollable cases
According to Dawson, the fascinating aspect is that ants do not signal infection indiscriminately. “Queen pupae, which have stronger immune defenses than worker pupae and can limit the infection on their own, were not observed to emit this warning signal to the colony,” she explains. “Worker brood, on the other hand, were unable to control the infection and signaled to alert the colony.”
By signaling only when an infection becomes uncontrollable, the sick brood enable the colony to respond proactively to real threats. At the same time, this approach ensures that individuals capable of recovery are not sacrificed unnecessarily. “This precise coordination between the individual and colony level is what makes this altruistic disease signaling so effective,” Cremer concludes.
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