Ant colonies function as highly coordinated “superorganisms,” with each ant playing a role akin to cells in a body, ensuring the collective health of the colony. Researchers at the Institute of Science and Technology Austria (ISTA) have unveiled a fascinating discovery: terminally ill ant brood emit a unique odor signaling their impending death and the threat they pose. This sophisticated early warning system enables the rapid detection and removal of pathogenic infections. The study was recently published in Nature Communications.
In contrast to many social animals that conceal illness to avoid social exclusion, ant brood adopt a different strategy. When faced with an incurable infection, ant pupae actively emit an alarm signal to alert the colony of the contagion risk. Upon receiving this signal, worker ants spring into action by unpacking the terminally ill pupae from their cocoons, creating small openings in their bodies, and applying formic acid, an antimicrobial poison that acts as a self-produced disinfectant. While this treatment effectively kills the pathogens within the pupa, it also results in the pupa’s death.
Altruistic Self-Sacrifice for the Greater Good
What may appear as self-sacrifice is actually beneficial to the signaler. By warning the colony of their deadly infection, terminally ill ants help maintain the colony’s health, ensuring the production of daughter colonies and indirectly passing on the signaler’s genes to future generations. Erika Dawson, the study’s first author and former postdoc in the Social Immunity research group led by Sylvia Cremer at ISTA, explains, “By warning the colony, terminally ill ants help the colony remain healthy and produce daughter colonies, which indirectly pass on the signaler’s genes to the next generation.”
This collaborative study, conducted with chemical ecologist Thomas Schmitt from the University of Würzburg in Germany, marks the first description of altruistic disease signaling in social insects. If a fatally ill ant were to hide its symptoms and die undetected, it could become highly infectious, endangering the entire colony. Instead, active signaling allows for effective disease detection and pathogen removal.
The “Find-Me and Eat-Me” Signal
At the colony level, ants operate as a “superorganism,” effectively forming a single living entity. While queens are tasked with reproduction, non-fertile workers manage all aspects of colony maintenance and health. This mirrors the specialization seen in human cells, where reproductive and somatic cells are interdependent, each essential for the organism’s survival. Cooperation is crucial, with individual ants engaging in altruistic self-sacrifice for the colony’s benefit.
Why would such a complex early warning system evolve if sick animals could simply isolate themselves? Sylvia Cremer explains, “Adult ants that approach death leave the nest to die outside the colony. Similarly, workers exposed to fungal spores practice social distancing. Yet, this is only possible for mobile individuals. Ant brood, like infected cells in tissue, are largely immobile and lack this option.”
Both body cells and ant brood rely on external assistance to protect the colony. They emit a chemical signal that attracts either the body’s immune cells or the colony’s workers, enabling detection and elimination as potential infection sources. Immunologists refer to this as the “find-me and eat-me signal.”
Changes in Pupal Scent Profile
Thomas Schmitt, focusing on chemical communication in social insects, notes that workers specifically target individual pupae within 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 consists of non-volatile compounds on the pupal body surface.”
Particularly, the intensity of two odor components from the ants’ natural scent profile increases when a pupa is terminally ill. To test if this odor change alone could trigger the workers’ disinfection behavior, 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 describes. The results were conclusive: 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, 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,” she explains. “Worker brood, unable to control the infection, 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, ensuring 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.
Implications for Future Research
The discovery of such sophisticated communication in ants opens new avenues for understanding social immunity and disease management in superorganisms. It also raises intriguing questions about the evolution of altruistic behaviors and their genetic underpinnings. As researchers continue to explore these dynamics, the insights gained could have broader implications for studying disease management in other social species.
Meanwhile, the study underscores the importance of animal research in uncovering fundamental biological processes. Despite ethical considerations, such studies remain indispensable for advancing knowledge in behavioral biology, immunology, and genetics.
