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High population density, as well as frequent and close contacts between individuals, contribute to a rapid spread of diseases. To protect their colonies, ants have developed disease defense mechanisms, including adaptations to their social organization. Ants do not interact randomly with other colony members, but are organized in sub-groups according to their age and the tasks they carry out. While young worker 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.
The researchers used a "barcode" system developed in the Keller group to follow the interactions between ants, especially to observe their behavior when disease spreads. In a first experiment, they placed digital markers on 2'266 garden ants. Infrared cameras took an image of the colonies every half second, and so the researchers could follow and measure the movement and position of each individual, and their social interactions. The researchers showed that the ants' subdivision into groups acts prophylactically and reduces the risk that disease spreads.
% - Worker - Ants - Foragers - Spores
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. "The ants change how they interact and who they interact with," explains Sylvia Cremer, "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." Laurent Keller adds: "This is the first scientific study that shows that an animal society is...
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