Decision-making in ant disease management
Triage in the Animal Kingdom
Ants are a perfect model for studying cooperation in the animal kingdom. They allow us to draw conclusions about how a group prevents the spread of disease. Similar to a hospital, individual members of an ant colony care for their sick nestmates. But while a hospital has fixed rules for triage, the individual decisions about who cares for whom and when in an ant colony have remained unclear until now.
To decipher the care decisions of an individual ant, experimental biologist Sylvia Cremer and her research team at ISTA collaborated with her colleague, theoretical physicist Gašper Tkačik, and mathematician Katarína Boďová from Comenius University in Bratislava. In their multidisciplinary study, published in the journal Nature Communications, the scientists examined garden ants and fungal spores and discovered what information the ants consider when making their individual care decisions.
The behavior of the ants and the spore load—the amount of fungal pores—of individual colony members were analyzed over a specific period. This revealed that ants preferentially selected the most infectious colony members for care. However, an ant will stop caring for others after it has just been cared for by its fellow colonists. Thus, the ants not only assess the risk of infection in others but also respond to the social feedback they receive from the colony regarding their own risk of infecting others. This unique combination of simple rules results in the most infectious colony members being cared for by the least infectious, ultimately leading to highly efficient disease control at the colony level.
Success Together
Social ants are masters of cooperative disease defense. This enables them to achieve colony-level protection known as "social immunity." This refers to the collective measures taken to reduce the risk of disease and its transmission within the colony. Previous studies have shown how colony members care for one another. Among other things, they nibble infectious spores off infected nestmates and disinfect them with chemicals. But how do they know whom to disinfect?
Ants Care for the Most Infectious Members of the Colony
To answer these questions, the scientists studied the behavior of healthy ants in relation to two nestmates, both carrying varying amounts of infectious fungal spores. In the experiment, the caring ants could decide how to divide their care between the two colony members. After careful behavioral observations, Barbara Casillas, a former PhD student in Cremer's group, discovered a fascinating phenomenon in the ants' caregiving behavior.
The ants preferentially target individuals with the highest spore count—that is, the ant that currently poses the greatest risk to the group. "As a rule, ants select the one with the highest current spore load, even though the spore load is constantly changing due to the germination process itself," explains Cremer. "This allows the ants to dynamically respond to changes in the disease threat."
Understanding Ant Behavior Through Mathematics
However, experimental approaches have their limitations. While researchers were able to observe how the ants behaved, they couldn't determine the reasons behind their actions. The individual decision-making process that determines group behavior remained a "black box." Mathematician Katarína Boďová, assistant professor at Comenius University, and Gašper Tkačik, theoretical physicist at ISTA, took on this challenge together.
Together, the team deciphered the information the ants use to decide when and on whom to begin their grooming behavior. Boďová explains: "The ants follow a simple rule of thumb: If they encounter an ant with a high spore load, they are more likely to disinfect that ant." The advantage is that the ants don't need to remember the spore load of all colony members; instead, they can rely entirely on the information they gain from contact with the ants in their environment.
The team also deciphered what information the ants use to decide when and on whom to begin grooming. The system is not perfect, however. The ants sometimes tend to the less infectious individual. But the many small tendencies toward tending to more heavily infected individuals in each decision made by individual ants add up to a clear decision and efficient elimination of pathogens at the colony level. The ants can react to minimal differences in spore load, but they make more precise decisions when the discrepancy is greater.
"We don't yet know how the ants perceive the difference in spore count. Perhaps the ants with the higher load have a stronger fungal odor," Cremers hypothesizes. The group's most recent work suggests that ergosterol—an essential membrane component of all fungi—could be a possible recognition marker for the ants.
Infectious ants do not participate in nursing care
Mathematical modeling revealed another factor relevant to an ant's nursing activity: its sensitivity to social signals from its nestmates. Cremer describes this as a social feedback loop that prevents highly infectious individuals from caring for others, thereby reducing the risk of transmission during nursing.
More than just an interesting observation of ant behavior, this publication aims to understand individual decision-making within a colony. Cremer summarizes: "Collaboration with our colleagues in theoretical science has provided us with new insights into the individual decision-making processes of ants that underlie their social immunity and cooperative disease defense."
Publication:
B. Casillas-Pérez, K. Boďová, A. V. Grasse, G. Tkačik & S. Cremer. 2023. Dynamic pathogen detection and social feedback shape collective hygiene in ants. Nature Communications. DOI: https://doi.org/10.1038/s41467-023-38947-y
Project Funding:
The ISTA part of the project was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant No. 771402; EPIDEMICSonCHIP to Sylvia Cremer) and the Human Frontier Science Program (Grant No. RGP0065/2012 to Gašper Tkačik).
Information on Animal Experiments:
This study used a non-protected ant species. The collection of the ants from the wild, their rearing in the laboratory, and all experimental work comply with European and Austrian law, as well as institutional ethical guidelines.