When house hunting, many animals look for certain creature comforts, whether it’s safety from predators, access to food, or a comfy temperature range. The pharaoh ant, however, shops around for a morbid must-have: dead bodies visibly ravaged by disease .
The study, recentlypublished in PLoS ONE , marks the first time that scientists have seen whole colonies of ants actively choosing to live among pathogens that could infect and kill them. When given a two-way choice, upstart colonies of Monomorium pharaonis , the pharaoh ant, prefer candidate nest sites containing ants killed by an infectious fungus over nest sites containing harmless ant corpses, possibly helping to build the colony’s immunity to disease, say the study’s authors.
“We were quite sure we’d find the opposite results of what we did,” said Luigi Pontieri, a researcher at the University of Copenhagen and the study’s lead author. “Many people in the field couldn’t believe that there’d be an attraction.”
Pontieri and his colleagues began studying the pharaoh ant because it was ideal to test how social groups deal with disease. The yellow-brown ants, each just a few millimeters long, are used to living in humans’ shadows : They thrive in dirty environments ranging from “the wall of [a] house” to “between the pages of a book,” says Tim Linksvayer, a University of Pennsylvania biology professor and the study’s senior author.
As a preliminary study, the team needed to evaluate pharaoh ant colonies’ baseline reaction to Metarhizium brunneum , a fungus that the researchers knew could infect the ants. In separate trials, the researchers placed pharaoh ant colonies the same distance away from two of three possible nest sites: a completely empty space, a space filled with harmless pharaoh ants corpses, and a space filled with pharaoh ants already killed by M. brunneum— their bodies were coated with the fungus’ telltale white fuzz. Upon detecting the fungus, the team hypothesized, the ant colonies would run the other way, setting up shop wherever the fungus wasn’t. Otherwise, says Linksvayer, it’d be like choosing “a house [filled] with dead humans…that you could see were killed by some disease.”
To the researchers’ surprise, however, the ant colonies moved towardthe fungus: When choosing between areas filled with harmless or infectious corpses, over 80% of colonies preferred to live among the ants killed by M. brunneum . The pharaoh ants weren’t using the fungus as a food source and probably weren’t lured into a trap laid by M. brunneum , either, since colonies of other ant species the fungus infects don’t show any attraction to it. Instead, the researchers suspect that the ants are exposing themselves to low levels of M. brunneum , in an effort to prime their immune systems to prevent full-blown infections later on in life—an ant vaccination, of sorts.
If so, the study could complicate some pest management strategies: Many use M. brunneum to repel or kill invasive ant species like the pharaoh ant. If the team’s hypothesis is right, Pontieri says, human efforts to eradicate pharaoh ants with pathogenic fungi like M. brunneum “may get [the ants] immunized instead.”
The researchers don’t yet have evidence, though, that this “immune priming” is why the pharaoh ants are attracted to the fungus. Linksvayer says they would have to show that pharaoh ants living among the infectious corpses have heightened immunity to M. brunneum , which they haven’t yet demonstrated. But the team’s results fall in line with other research demonstrating social immunity , the group-wide building of disease resistance. And even if the pharaoh ants’ attraction to M. brunneum remains mysterious, says Timothée Brütsch, a Université de Lausanne researcher who studies individual ant queens’ attraction to fungi , “it is positive that more and more studies are mentioning this surprising behavior,” especially because it’s so counterintuitive to human intuition.
Linksvayer agrees: Unlike humans, “ants have been living in these very large societies for millions of years,” meaning that “humans could potentially learn things from…how they organize their societies,” even if ants’ practices sometimes seem strange from our point of view. “If you [could] go into a room with a bunch of corpses and somehow gain immunity by hanging out with them, that might make sense,” he says. Otherwise, he adds, the ants’ morbid vaccination method might not be one worth adopting.