Rosa Rubicondior: Evolution News - Self-Sacrificing Ants Show the Evolved Genetic Basis of Altruism

ISTA | Ants Signal Deadly Infection

Scientists at the Institute of Science and Technology, Austria, have found that terminally ill pupae in an ant colony emit a chemical signal that prompts worker ants to disinfect them with formic acid — a process that also brings about their death. This behaviour helps keep the colony free from infection and represents a clear example of evolved altruism with a genetic basis. Their findings are reported, open access, in Nature Communications.

One of the criticisms often levelled at evolutionary biology is that it cannot explain altruism, since individuals that sacrifice themselves for others seemingly shouldn’t survive to pass on any genes responsible for such behaviour.

This is plainly untrue. Acts of altruism are widespread in nature: male spiders and mantises are consumed by their mates, providing nutrients for developing eggs; the offspring of social spiders consume their mother, then go on to consume one another. These behaviours persist because they enhance the success of the genes involved.

The key lies in what Richard Dawkins termed the selfish gene. Contrary to creationist misrepresentations, this is not a claim that there exists a gene for selfishness. It refers instead to the way genes appear to act in their own interests. Genes promoting altruistic behaviour benefit when that behaviour increases the reproductive success of individuals carrying the same genes — typically close relatives. The sacrifice of one carrier can thereby enhance the spread of the genes responsible for the altruism.

In humans, altruism arises not only from genetic evolution but also from memetic evolution — the inheritance and adaptation of ideas, norms, and cultural expectations. Human altruism rarely requires life-or-death sacrifice; it more often involves smaller acts such as sharing resources, giving up a seat on a bus, or letting another driver go first at a junction. The advantage, at both genetic and memetic levels, is that such behaviours help build societies where cooperation is reciprocated. Altruism is ultimately an investment in a more stable, supportive environment that may benefit the genes and memes of the individuals who contribute to it.

How Worker Relatedness in Lasius neglectus Supports the Evolution of Terminal-State Altruism. Lasius neglectus is a highly eusocial ant with colonies containing many queens (polygyny) and often many nests (polydomy). At first glance, this might seem to weaken the usual kin-selection explanation for altruism, because workers in polygynous colonies are less closely related to one another than in species with a single queen. Yet altruistic disease signalling still evolves and persists in this species. Understanding why requires looking at the genetic structure of the colony.

Workers are still genetically close enough for kin selection to operate

Even in multi-queen colonies, L. neglectus workers are:

Diploid daughters of the queens,
Frequently fathered by a limited number of males within the colony,
Part of a population where movement between colonies is extremely restricted.

This produces clusters of workers with substantial relatedness, even if not as high as in strictly monogynous species.

Kin selection does not require extremely high relatedness — only that individuals share more genes with their nest-mates than with the wider population. In L. neglectus, this condition is met.
Genetic interests are shared at the colony level

Because colonies reproduce as superorganisms (e.g., through budding rather than long-distance dispersal), selection acts strongly on traits that enhance colony fitness, even if individual relatedness varies.

Genes that:

allow sick pupae to signal their condition, and
allow workers to recognise and “disinfect” them

benefit the overall reproductive output of the colony, which in turn spreads the same genes.

The altruistic signal protects many relatives simultaneously

A fatally infected pupa poses a high risk to all surrounding brood, most of whom are her close relatives — even in multi-queen colonies. Removing a diseased individual:

protects the immediate brood cohort (often full or half-sisters),
reduces pathogen load in a nest where workers and queens overwrite generations,
increases the long-term success of the colony.

From the gene’s-eye view, the benefit of protecting dozens or hundreds of relatives far outweighs the cost of losing one infected pupa.

Co-evolution under moderate relatedness

Studies of eusocial species show that:

Extreme relatedness accelerates the evolution of altruism,
but moderate relatedness is enough to maintain it, provided the cost–benefit ratio is strong.

In L. neglectus, disease spreads extremely efficiently in dense nests. This creates strong selection pressure for any mutation that improves hygienic efficiency — including reliable chemical signalling of terminal infection.

Why this system still counts as kin-selected altruism.

The signal is an adaptive immune–chemical response favoured because:

the signalling pupa shares genes with the colony,
preventing an outbreak boosts the survival of those shared genes,
the effect on colony fitness is large,
and worker behaviours have co-evolved to respond to the signal.

So although L. neglectus workers are not clones of one another and may not be as closely related as in single-queen species, they are still genetically aligned enough that altruism of this kind pays off in genetic currency.

How and why ant larvae signal their diseased state is discussed in an Institute of Science and Technology, Austria, news item.

Ants Signal Deadly Infection
Early disease detection in the colony: Ants signal incurable sickness to save others

Ant colonies operate as tightly coordinated “superorganisms” with individual ants working together, much like the cells of a body, to ensure colony health. Researchers at the Institute of Science and Technology Austria (ISTA) have now discovered that terminally ill ant brood, like infected cells in a body, 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.

Unpacking of a fatally-infected pupa from its cocoon. When an ant pupa signals its imminent death caused by an incurable infection, workers unpack it from its cocoon and disinfect it.
© Christopher D. Pull/ISTA.

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.

Dr Erika H. Dawson, first author.
ISTA (Institute of Science and Technology Austria)
Klosterneuburg, Austria.
Now Université Sorbonne Paris Nord
Villetaneuse, France

Their collaborative study with chemical ecologist Thomas Schmitt from the University of Würzburg 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.

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.

Altruism in the superorganism

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. 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.

Sylvia Cremer, senior author.
ISTA (Institute of Science and Technology Austria)
Klosterneuburg, Austria.

Workers and their pupae. Ant pupae represent the developmental stage between the larval and adult stages. Immobile, they are unable to react to infection by leaving the nest. Instead, when infected and nearing death, they emit an odor signal alerting the workers that they will become a highly contagious infection risk that may threaten the health of the entire colony.

© Line V. Ugelvig & Barbara Leyrer/ISTA

Workers and their pupae. Ant pupae represent the developmental stage between the larval and adult stages. Immobile, they are unable to react to infection by leaving the nest. Instead, when infected and nearing death, they emit an odor signal alerting the workers that they will become a highly contagious infection risk that may threaten the health of the entire colony.

© Line V. Ugelvig & Barbara Leyrer/ISTA

Ant colony in the ISTA lab. Worker ants organize the colony brood into separate nest chambers. Larvae, which have hatched from eggs and require frequent care, are grouped and fed often. Pupae, in contrast, do not eat and are protected from desiccation by their cocoons, so they only need occasional inspections by the workers.
© ISTA

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. 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.

Sylvia Cremer

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.

Thomas Schmitt, co-author.
University of Würzburg
Würzburg, Germany.

Signaling only when needed

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. Worker brood, on the other hand, were unable to control the infection and signaled to alert the colony.

Dr Erika H. Dawson.

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.

Sylvia Cremer

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.

Sylvia Cremer

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.

Destructive disinfection. The video shows worker ants inspecting their brood, selecting a pupa, and initiating the destructive disinfection process.
© Christopher D. Pull/ISTA

Publication:
Dawson, E.H., Hoenigsberger, M., Kampleitner, N. et al.
Altruistic disease signalling in ant colonies. Nat Commun 16, 10511 (2025). https://doi.org/10.1038/s41467-025-66175-z

Abstract
Sick individuals often conceal their disease status to group members, thereby preventing social exclusion or aggression. Here we show by behavioural, chemical, immunological and infection load analyses that sick ant pupae instead actively emit a chemical signal that in itself is sufficient to trigger their own destruction by colony members. In our experiments, this altruistic disease-signalling was performed only by worker but not queen pupae. The lack of signalling by queen pupae did not constitute cheating behaviour, but reflected their superior immune capabilities. Worker pupae suffered from extensive pathogen replication whereas queen pupae were able to restrain their infection. Our data suggest the evolution of a finely-tuned signalling system in which it is not the induction of an individual’s immune response, but rather its failure to overcome the infection, that triggers pupal signalling for sacrifice. This demonstrates a balanced interplay between individual and social immunity that efficiently achieves whole-colony health.

Introduction
Sick animals living in groups are often excluded or subjected to aggression by other group members, both to reduce the risk of infection and to gain social or resource-related advantages over the weakened individual^1,2. As a result, these individuals often conceal their infection status by, for example, suppressing sickness behaviours in the presence of conspecifics^3,4. This is not the case when group members are kin, since relatedness diminishes the conflict of interest between sick and healthy individuals. Therefore, even when group members can detect disease in others, for example, using sickness cues such as altered physical appearance or behaviour, avoidance is often only displayed against non-kin, whilst normal social interactions are maintained with diseased kin⁵.

Diseased individuals can therefore benefit, not only by not concealing, but even from actively communicating their health state to their relatives in order to receive additional care. As such, fungal pathogen-exposed termites display a vibratory signal⁶, inducing grooming behaviour by nestmates⁷ and wounded ants produce behavioural and chemical displays attracting their colony members to care for their wounds^8,9, which in both cases reduces the signalling individual’s infection risk and hence improves its survival. Helping a relative to survive infection can also indirectly benefit the caregiver by increasing its own inclusive fitness via shared genes with the recipient¹⁰. Active disease-state signalling is therefore selected for in groups of relatives, when both the signaller and the signal-receiver gain from the care response. But what happens when the related group member responds not with care, but instead with exclusion or aggression toward the infected individual to protect the group? Should the infected individual still signal its sickness to others, even at the risk of being sacrificed for the benefit of the group?

The evolution of such ‘altruistic signalling’ of infectious disease should be promoted in social groups by two factors: (i) high relatedness, leading to a large indirect fitness gain to the sacrificed individual, if signalling promotes group health, and (ii) low direct fitness loss to the sacrificed signaller, i.e., when its expected future reproductive value is small. Both are fulfilled in the non-reproductive workers of social insect colonies. Workers are typically highly related with one another and produce no offspring of their own, but instead rear the queen’s brood and maintain the colony¹¹. Therefore, the fitness of workers depends on the fate of the colony as a whole. Here, we experimentally test whether altruistic signalling of one’s own sickness indeed evolved in social insect colonies.

Social insects, particularly termites, ants and social bees, show sophisticated collective disease defences when colony members come into contact with pathogens. These social immunity measures range from sanitary care of individuals that can still be rescued, to elimination of fatally-infected colony members^12,13. The latter can occur via ‘hygienic cannibalism’ as observed in termites^14,15 and in ant queens during their nutrient-deprived colony founding phase¹⁶. In bees¹⁷ and ants¹⁸, it often involves the removal of typically immobile larvae or pupae from the nest, while the mobile adults often leave the colony when approaching death^19,20,21. Here we study the invasive garden ant, Lasius neglectus, in which adult workers care for the brood, yet switch to destructive disinfection of worker pupae that suffer from deadly infections of the fungal pathogen Metarhizium brunneum²². Since pupae of this ant species are enclosed in a cocoon, the first step in this multicomponent behaviour is that workers prematurely unpack them from their cocoon, followed by biting and disinfection. This process prevents pathogen replication in the host and ultimately limits the spread throughout the colony. Notably, this destructive disinfection is performed by the workers during the non-infectious incubation period of the fungal pathogen, therefore not inducing any disease risk to the workers²². The expression of this behaviour depends on detecting disease-related changes in the pupal surface chemistry. In particular, unpacked pupae showed increased abundance of four cuticular hydrocarbon (CHC) peaks (tritriacontadiene, C33:2; tritriacontene, C33:1; pentatriacontadiene, either alone as C35:2, or co-eluting with pentatriacontene, C35:2 + C35:1), and experimental CHC removal prevented unpacking behaviour²². Yet, it remains unknown whether these chemical changes are simply passive cues resulting from infection or an induced immune response, or whether they represent active signals produced by infected pupae to trigger their own destruction for the colony’s benefit – thereby increasing their inclusive fitness. In this study, we manipulate the context in which pupae could signal their disease status, enabling us to disentangle these two scenarios.

Dawson, E.H., Hoenigsberger, M., Kampleitner, N. et al.
Altruistic disease signalling in ant colonies. Nat Commun 16, 10511 (2025). https://doi.org/10.1038/s41467-025-66175-z

Copyright: © 2025 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

Far from being a problem for evolutionary biology, altruism is exactly what we should expect in species where individuals share a large proportion of their genes with those around them. Whether it is a worker ant tending a sick pupa, a bee sacrificing herself to defend the hive, or a meerkat giving alarm calls that attract predators, these behaviours persist because they increase the survival of the genes responsible for them. The individual may pay a cost, but the genes benefit when they are carried into the next generation by relatives who have been protected or assisted.

What creationists often overlook is that natural selection operates at the level of genes and lineages, not at the level of isolated individuals. Any behaviour that improves the overall reproductive success of those carrying the same genes will be favoured, even if it demands a personal sacrifice. In tightly knit colonies such as ants, where relatedness is high and life is lived in close quarters, the selective pressure for effective hygienic measures is enormous. A mutation that allows a terminally infected pupa to signal its condition — and so prevent a catastrophic outbreak — directly enhances the long-term success of that gene within the colony.

Altruism, then, is not an anomaly that undermines Darwinian evolution but one of its most elegant outcomes. It arises naturally wherever cooperation among relatives enhances collective survival. Far from contradicting evolutionary theory, examples such as this new work on Lasius neglectus highlight how subtle and powerful selection can be in shaping behaviours that, at first glance, appear selfless. In reality, they serve the deeper logic of heredity: genes that contribute to the success of their carriers’ kin will flourish, and the societies built around such strategies can become some of the most sophisticated and resilient in nature.

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