Crowded shopping malls, fully booked airplanes, daycare; these places are hot-beds for spreading illness. The frequent and close-knit nature of human interactions only helps disease to hop from person to person. How we behave when we are sick (staying home from work) or how we behave towards others who are sick (running away) affects the spread of disease by changing how often and with whom we interact. Interestingly, such changes in individual behaviour can also change the social structure of entire communities to reduce disease transmission, according to a new study by Nathalie Stroeymeyt from the University of Lausanne, Switzerland and colleagues.
Before you ask, no, Stroeymeyt and her co-workers did not go around experimentally infecting human societies to reach their conclusion. Instead, the researchers used the highly social garden ant as their species of choice. Humans are very similar to group-living animals like ants or bees when it comes to social organization. We come in frequent contact to communicate with each other and we should reduce this contact when sick. Also, like humans, ants live together in structured societies where everyone has a job. Foragers leave the colony to find food; nurses stay in the colony to care for the brood; and the queen – who produces all the offspring – is the most important ant in the colony. The queen cannot get sick.
Stroeymeyt's team assessed how ant colonies reacted to a spreading disease using a nifty research tool called social network analysis. They attached very small ID codes to each ant, returned them to the colony and followed them for 24 hours. Then, they used software to visualize and analyse the network of ant interactions. The team found that healthy, uninfected ant colonies are structured to prevent disease transmission in the first place. For instance, forager ants, who are more likely to pick up pathogens while roaming outside, had less contact with ants inside the colony. The colonies were also very ‘modular’, meaning the ants tended to congregate together and mainly interact with other ants in job-specific cliques.
Next, the researchers infected some ants to see how the colony responded to a disease outbreak. They dusted 10% of the foragers from each colony with fungal spores to make the ants sick. With the colony on the cusp of an epidemic, the structure shifted to reduce pathogen exposure: the ant cliques became even more tightly aggregated and the groups moved further apart. The ants never lost sight of their goal to save the queen and protect the brood. Infected foragers distanced themselves from the others and spent more time outside the colony. Nurses also moved the brood deeper into the nest, reducing contact with the infected foragers. This new anti-infection social organization reduced spore transfer to the nurses and the queen. And, when the researchers tracked the ants’ survival for 9 days after the infection, they found that more foragers than nurses perished, but the queen never died.
We tend not to think that ants have many similarities with humans, but Stroeymeyt's findings give us insight into the spread of disease in complex social groups. We can all identify with how the ants changed their behaviour in a time of sickness. The researchers show that such changes are reflected in the social network of an entire community, which reduced the spread of disease and protected valuable members of the population. Long live the queen!