When Ailing Ants Choose to Sacrifice: The Role of Chemical Altruism in Colony Survival

In animal societies, disease represents a major collective risk, particularly in dense and organized groups such as ant colonies. For these social insects, where each individual is closely linked to the others, limiting the spread of a pathogen becomes vital at the group level. In the species Lasius neglectus, researchers from the Institute of Science and Technology Austria (ISTA) have demonstrated extreme but functional behavior. Severely infected pupae voluntarily trigger a chemical signal to cause their own elimination by workers.

This mechanism, detailed in Nature Communications, reveals a collective defense strategy where the individual chooses its own death to preserve the colony. The study questions the limits of biological altruism and underlines the sophistication of social responses to diseases, even among insects which have no consciousness but a formidable collective effectiveness.

A collective strategy against the invisible threat

Among ants, extreme promiscuity — several thousand individuals living in the same confined space — creates an environment highly conducive to epidemics. To deal with this threat, some species have developed what biologists call social immunity. It is a set of collective behaviors aimed at limiting the spread of pathogens. In the case of Lasius neglectusan invasive species of garden ants, this system achieves a striking degree of efficiency.

The study led by Erika Dawson and Sylvia Cremer, from the Institute of Science and Technology Austria, shows that sick adults adopt withdrawal behavior. They voluntarily leave the colony when they feel they are seriously infected. This behavior, already known, makes it possible to limit contact with conspecifics and therefore the spread of the pathogen. But the major discovery concerns the pupae, a juvenile stage still unable to move because they are locked in their cocoon.

In this situation, the strategy of voluntary isolation proves impossible. The researchers then observed a remarkable phenomenon. Severely infected pupae produce a specific chemical signal, triggering their elimination by workers. This behavior, called destructive disinfection, eliminates the pathogen at a stage where it is not yet contagious.

This strategy is not guided by a conscious choice, but it results from a complex evolutionary process. It reflects a form of altruism integrated into collective functioning, where the survival of the individual takes precedence over that of the group. This reinforces the idea that, in insect societies, the colony functions as a single biological entity. It can be compared to a super-organism in which each member plays a well-defined role.

The signal of death: controlled chemical production

The heart of the mechanism revealed lies in an active modification of the chemical profile of infected pupae. Contrary to popular belief, this odor is not a simple byproduct of the fungal infection. Researchers have shown that this production of specific cuticular hydrocarbons (CHCs) constitutes a controlled response, triggered only under certain conditions.

Using an innovative carbon-13 labeling method, the team was able to differentiate the compounds produced by the pupae from those potentially transferred by the workers via grooming. This isotopic labeling made it possible to identify two key molecules: tritriacontadiene (C33:2) and tritriacontene (C33:1). These are only overproduced if the pupa is infected and surrounded by workers. In the absence of workers, even when sick, the pupa does not emit the signal. This indicates a contextual control of chemical behavior, dependent on the presence of potential receptors.

The response from the workers was rapid. In the presence of these signals, they remove the pupa from its cocoon, pierce its outer envelope, then inject an acid produced by their venom gland. This poison acts as a disinfectant agent. It ensures the death of the individual before the fungus becomes spore-forming, that is to say contagious.

The authors point out that this behavior is similar to that of immune cells destroying infected cells in a multicellular organism. The production of the signal by the pupa therefore represents an evolutionarily selected act. It maximizes the chances of survival of genes shared with other members of the colony.

Silent but well-armed queen pupae

While severely infected worker pupae send a chemical signal leading to their own elimination, pupae destined to become queens do not. This lack of reporting initially raised a provocative hypothesis. Would future queens seek to “cheat” the system to maximize their chances of reproduction, even if it means putting the colony in danger?

But the data collected by Sylvia Cremer's team provides a much more nuanced explanation. Unlike worker pupae, queen pupae exhibit significantly higher innate immunity. Gene expression analyzes have shown that, even in the absence of workers, they express immunity-related genes at a high level, such as
Defensin 1 Or PGRP-SC2. Their ability to control fungal infection is therefore much greater.

The results show that the infectious load in worker pupae increases continuously to critical levels. In contrast, in pupal queens, the infection peaks and then declines. A sign that their immune system is often successful in eradicating the pathogen. In this context, sending a kill signal becomes useless, even counterproductive, because it would condemn an ​​individual still capable of survival.

It is therefore not a question of selfish behavior, but of an honest signal of physiological state. Queens don't send alerts because they simply don't need them.

A precise and adaptive chemical language

The study reveals that the sacrifice signal in worker pupae relies on a complex chemical code, much more subtle than a simple change in odor. The compounds C33:2 and C33:1, identified as triggers, are not unique molecules. These are mixtures of isomers, molecular variants having the same formula but a different structure.

The chromatographies carried out by the researchers made it possible to identify at least eight different isomers for C33:2 and six for C33:1. Some isomers are increased specifically when reported, such as 13-C33:1 and 11-C33:1, while others are decreased. This variation in proportions creates a finely coded chemical message, which the workers know how to interpret.

To verify the role of this language, the scientists applied extracts from signaling pupae to healthy pupae. Result: workers treated healthy pupae as if they were sick. This proves that the signal alone, regardless of the individual's actual state, is sufficient to trigger elimination.

Using such a specific signal avoids costly errors. An automatic elimination of any infected individual, without nuance, could lead to the loss of recoverable members, such as queens. This system is therefore based on an individual assessment of risk by the organism itself, relayed by a chemical message readable by others.

This mechanism, comparable to an immune self-assessment followed by a reporting decision, illustrates the extreme sophistication of collective defense strategies in social insects. It shows how biology can produce collective behaviors of remarkable precision, without any individual being aware of it.

Source: Dawson, EH, Hoenigsberger, M., Kampleitner, N. et al.“Altruistic disease signaling in ant colonies”. Nat Common 16, 10511 (2025).