Ants are the poster children for eusociality. Each ant has a job, and each ant sticks to its job: the foragers forage, the workers work, the nurses nurse, the queen procreates. It has been assumed that this strict segregation of ant society into age-and-task groups could help prevent epidemics, but this assumption had never been tested. Finally, a group of scientists got together and figured out how to do the experiment using 22 colonies of Lasius niger, the common black garden ant.
A division of more than labor
The scientists hailed from various departments in various universities: Ecology and Evolution, Science and Technology, Biorobotics, Intelligent Systems, and Physics of Complex Systems. They labeled each ant in each colony with what looked like a paper QR code taped to its thorax and used video tracking to follow individual ants, monitoring all of the physical contacts among them.
As predicted, the structure of the colonies inhibited the transmission of pathogens. The colonies were sorted into discrete communities that were connected, but the nodes at which they were connected were limited. This organization also protected the most vulnerable members: the queen and young workers.
Not only did the entire network function to limit disease transmission, but individual ants did as well. Those most likely to pick up an infection are the foragers, since the nest is kept scrupulously clean by the workers, which stay inside. Forager ants have fewer connections within the network and are markedly segregated from the queen and the young nurses.
These observations were made by comparing the network of the ant colonies to other types of networks in simulated infections, but then they were confirmed with real infections. The scientists randomly infected 10 percent of a colony’s foragers with the spores of a fungal ant pathogen, Metarhizium brunneum. (Only 11 of the colonies got infected; 10 percent of the foragers from the other 11 were exposed to a sham solution.) After giving the foragers a day to recover from their treatments, the researchers assayed pathogen loads across all of the ants in the colony. This revealed contact networks, as spores are transferred by physical contact between ants.
Limited contact
Untreated foragers ended up with much higher pathogen loads than the queen and the young nurses, sequestered away in their nest. After nine days, more foragers had died than ants of any other caste; all of the queens were still alive.
Pathogen exposure and not the sham treatment changed the ants’ behavior in ways that further protected the colony. Infected foragers spent more time outside, ventured farther from the colony, and didn’t move around much when they did come inside the nest. Nurses moved the brood deeper into the nest. We don’t know how they knew, but these behavioral changes reduced both the probability of contamination and the number of spores transmitted by touch. They also functioned to vaccinate the queen by allowing her to become exposed to only a low level of pathogen contamination.
One of the practical ramifications of this finding—that the strict regimentation of ant society into castes helps protect their young and their queens from epidemics—is that if you get your kid one of those ant colony toys for Christmas, the resultant ant infestation in your house will be long lived and resistant to epidemics.
Science, 2018. DOI: 10.1126/science.aat4793 (About DOIs).
