Doomed ants send a final scent to save their colony

In many social animals, individuals often hide signs of illness to avoid being pushed aside by the group. Ant pupae take a very different route. When facing an infection they cannot survive, they produce a chemical alarm that warns their nestmates of the danger they represent.

Worker Ants Respond to the Warning Signal

As soon as worker ants detect this signal, they move rapidly. They open the infected pupa's cocoon, create small cuts in its outer surface, and apply formic acid, a natural antimicrobial compound produced by the workers themselves. This treatment kills the pathogens growing inside the pupa but also results in the pupa's death.

"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.

The research, conducted with chemical ecologist Thomas Schmitt from the University of Würzburg in Germany, documents this form of altruistic disease signaling in social insects for the first time. Without such a warning system, a dying ant could become highly infectious and threaten the entire colony. By signaling their condition early, the incurably ill make it possible for the colony to remove the threat before it escalates.

Why Ant Colonies Rely on Extreme Cooperation

At the scale of the colony, ants function as a "superorganism," where different members take on roles essential for group survival. Queens focus on reproduction, while workers manage everything from feeding to nest maintenance and health care. This mirrors the division of labor in the human body, where reproductive cells generate offspring and other cells perform daily biological tasks.

Both organisms and superorganisms depend on the close cooperation of reproductive and non-reproductive components. In ants, this cooperation is so extensive that it includes behaviors that benefit the colony even at the cost of the individual, such as sacrificing fatally infected brood.

Why an Early-Warning Signal Is Needed

A natural question is why ants have evolved such a sophisticated alarm system if sick individuals can simply remove themselves. Cremer explains, "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." However, this option is unavailable to brood, which cannot move independently. Like infected cells in the body, they require outside help.

Developing pupae and body cells face similar challenges and use similar strategies. Both release chemical cues that draw either immune cells or worker ants to them, allowing these helpers to remove potential infection sources. Immunologists refer to this cue as the "find-me and eat-me signal."

"The signal must be both sensitive and specific," Cremer adds. "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." This raises the question of how such precision is achieved.

How Infection Changes the Pupal Scent

Schmitt, whose work focuses on chemical communication in social insects, notes that workers can target a single sick pupa among many. "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."

Two components of the ants' natural scent profile become stronger when a pupa is fatally infected. To test whether this odor shift alone triggers the disinfection response, researchers transferred the altered scent to healthy pupae and monitored the workers' behavior.

"We extracted the smell from the signaling pupae and applied it to healthy brood," Cremer says. The results showed that the scent alone was enough to make workers unpack the treated pupae, demonstrating that the altered odor of terminally infected brood functions in the same way as the "find-me and eat-me" signal seen in infected body cells.

Why Only Certain Pupae Send the Warning

According to Dawson, not all brood send this alarm signal. "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 pupae, however, lacked this ability and did signal for help.

By signaling only when an infection cannot be controlled, brood ensure that the colony responds only to genuine threats. This prevents unnecessary loss of individuals that are still capable of recovery. Cremer concludes, "This precise coordination between the individual and colony level is what makes this altruistic disease signaling so effective."

Information on Animal Studies

To better understand fundamental biological processes, such as those in behavioral biology, immunology or genetics, research involving animals is essential. No other methods, including in silico approaches, can fully replace these studies. All animals used in the experiments are collected, reared, and handled according to strict legal regulations.