Ant colonies prefer homes infected with fungus

If you walked into a new home you were considering buying and found a dead body covered in fungus, you’d probably run for the door and call 911. You certainly wouldn’t move in. But that’s just what colonies of pharaoh ants do, a new study finds.

Luigi Pontieri of the University of Copenhagen in Denmark and colleagues were curious about how pharaoh ants — an invasive species — choose where to move their colonies. The ants thrive in areas where humans live (they’ll set up house in the voids behind baseboards, inside shower rods and between the pages of a folded newspaper). But they have to frequently relocate. The invasive ants are genetically similar, and have networked colony structures, so the ants are thought to be prone to epidemics. So Pontieri and colleagues figured that the insects would prefer nests that are clean and disease-free.

The researchers set up an experiment in the lab using three types of nests. The first was empty. The second contained ants that had been killed by freezing. And the third had ants that had nestmates killed by the fungus Metarhizium brunneum. The ants were then given a choice between two of the types of nests. The results of the experiment appear November 5 in PLOS ONE.

When faced with nest sites with infected or frozen ants, the pharaoh ants surprisingly chose the infected nests in 26 of 31 trials. They had a slight preference for infected sites over empty ones, and no clear preference for empty sites or those with frozen ants. “The overall preference for infected nest sites brings us to suggest … that [fungus-covered] cadavers may exert an attractive rather than a repulsive effect,” the researchers write.

The scientists aren’t quite sure why the ants prefer sites with fungus-infected ants. But they suggest that moving into such nests may help the ants to “vaccinate” the colony against this potential killer. The ants transfer small amounts of the fungus to each other via grooming, and that may be enough to immunize the insects against the pathogen.

The pharaoh ants may be perceiving the frozen ants as a greater danger, the researchers say, since the ants don’t know what killed their nestmates — it could have been something far more dangerous than the fungus. “Since cadavers killed by undetectable pathogens like viral diseases do not readily advertise their cause of death, the choice of infected nests could be interpreted as an active decision of the colony driven by the likely higher threat posed by unknown diseases,” the researchers write. “That is, it is better to live with a known threat that can be dealt with rather than an unknown threat.”