It is assumed that social life can lead to the rapid spread of infectious diseases and outbreaks. In ants, disease outbreaks are rare and the expression of collective behaviors is invoked to explain the absence of epidemics in natural populations.
If a pathogen enters in the colony of ants, ants change their behavior to avoid the outbreak of disease. In this way, they protect the queen, brood and young workers from becoming ill.
For the defense of their colony, ants have created disease protection mechanisms, including adjustments to their social association. Ants don’t communicate randomly with other state individuals, however, are composed in sub-bunches as according to their age and the tasks they complete.
On the other hand, young ants so-called “nurses”, look after the valuable brood at the center of the colony, older worker ants become foragers that collect food outside the nest. These forager ants are more exposed to pathogens.
To observe the ant behavior when outbreaks, scientists at the IST Austria used a barcode system and followed the interactions between ants.
In a first experiment, they placed digital markers on 2’266 garden ants. Infrared cameras took an image of the colonies every half second.
Doing this, they were able to observe and measure the movement and position of each individual, and their social interactions. They found that the ants subdivision into groups acts preventive and thus reduces the risk that disease spreads.
10% of the worker ants (all foragers) were then exposed to fungal spores which spread easily through contact. Comparing the ant colonies before and after pathogen exposure showed that the ants quickly detect the presence of the fungal spores and change their behavior to strengthen already existing defenses.
Sylvia Cremer at the Institute of Science and Technology Austria (IST Austria) said, “The ants change how they interact and who they interact with. The cliques among ants become even stronger, and contact between cliques is reduced. Foragers interact more with foragers, and nurses more with nurses. This is a response by the whole colony – also animals who are not themselves treated with spores change their behavior.”
Scientists then used a highly sensitive qPCR method to quantify exactly how many spores an individual ant carried on its body. qPCR monitors how a targeted DNA molecule is amplified during the so-called polymerase chain reaction. This allows researchers to draw a conclusion about how much of the DNA, and by inference how much of the fungal spores, were present in the beginning.
Laurent Keller at the University of Lausanne said, “This is the first scientific study that shows that an animal society is able to actively change its organization to reduce the spread of disease.”
Because the ants changed how they interact, spores transfer patterns also changed. Only a few individuals received a high pathogen dose, which could cause disease. In addition, more ants received a low dose, which Cremer and her group previously showed to not cause disease but instead be protective in the face of future infections – similar to variolation in humans.
Keller said, “The analyses also showed that the colony protects especially valuable animals. The queen, the only individual that reproduces, and the nurses, young worker ants that can still contribute many hours of work to the colony, received less of the pathogen.”
Scientists also conducted a survival experiment, to see how pathogen load 24 hours after exposure correlates with death from the disease. There was a high correlation.
Scientists also calculated a predictive spore load for each individual ant, based on its interaction with other ants in the first 24 hours after pathogen exposure. Ants with a high predicted spore load were more likely to die nine days after exposure than ants with a low predicted spore load.
Cremer said, “How ants collectively deal with problems, such as the risk of an epidemic, could give insights into general principles of disease dynamics. Social interactions are the routes on which diseases travel and define how epidemics may spread. Basic research on ants can help us to deeper understand epidemiological processes, which can be relevant also in other social groups.”
The results of the study are published in the journal Science.