Ants Take Sick Days, Too

A new study has found that when some members of the colony are exposed to pathogens, they spend less time in the nest

Lasius niger queen and worker ants each got their own individual two-dimensional barcode tags. The tags allowed researchers to track their movement in the colony. Nathalie Stroeymeyt

When you come down with a cold or the flu, you might opt to keep your distance from other people in order to spare them from a similarly sniffly fate—and they in turn might steer clear of you. According to a new study, humans are not alone in their efforts to sequester the sick. In the presence of contagious pathogens, humble garden ants may also change their behavior to keep contaminated critters away from other members of the colony.

Ants are social creatures. They live in large groups, communicating and co-operating with one another to make sure that the colony functions as it should. Because they are often in close contact, ants are also vulnerable to contagious diseases. Studies have shown that ants are able to keep illness at bay through a number of hygienic mechanisms, like removing garbage and the bodies of dead colony members from their nests. Scientists suspected that the insects might also tweak their social behavior to decrease the spread of infections, but this hypothesis was, until recently, hard to prove.

“Ant colonies have hundreds of individuals,” explains Nathalie Stroeymeyt, a post-doctoral researcher at the University of Lausanne in Switzerland, who studies collective behavior in ant colonies. “Up to now, there was just not the technical methodology to measure their interactions at the colony level over extended periods of time.”

Fortunately, an automated tracking system developed by Swiss researchers in 2013 let Stroeymeyt and her colleagues get a detailed look at how 22 lab-reared ant colonies behave when disease is percolating in their midst. The team glued tiny 2D barcodes onto the ants’ thoraxes, which gave each insect a unique identifier—“just like a QR code,” Stroeymeyt says. A camera positioned above the ants’ enclosures snapped two pictures every second, and an algorithm detected and recorded the position of each barcode, giving the researchers’ a wealth of data about the ants’ movements.

For four days, the team let the ants scurry about in their enclosure undisturbed. As with colonies in the wild, some of the ants worked outside the nest to forage for food, while others—like the queen and “nurses” that tend to the developing brood—stayed inside the nest. On the fifth day, the researchers exposed some, but not all, of the foragers from 11 colonies to the fungus Metarhizium brunneum, which is frequently found in the soil of garden ants’ habitats and is known to make them sick. Foragers from the other 11 colonies were treated with a benign solution, to serve as a control group.

Crucially, previous studies have shown that the M. brunneum fungus takes at least 24 hours to infect ants, which in turn gave the researchers time to observe the insects before they were actually sick.

“We wanted to focus on [this] period … so that we could distinguish the active reaction of the ants themselves from side effects of sickness or parasite manipulation,” Stroeymeyt explains.

Writing in the journal Science, the researchers reveal that when the foragers were put back in their enclosure, the contaminated ants spent more time outside of the nest, meaning that they had less contact with the colony’s most valuable members: the queen, who lays all of the colony’s eggs, and the indoor workers, who are younger than the foragers and therefore have more hours to contribute to the colony. (Older ants are tasked with risky foraging jobs outside the nest because, as Stroeymeyt bluntly puts it, they “will die anyways.”)

But the crux of the study lies in the discovery that the contaminated ants weren’t the only ones to change their behavior. Foragers that had not been exposed to the fungus also increased the amount of time spent away from the nest. And the nurses inside the nest moved the young further inward and spent more time overlapping with them, which “could be seen as a spatial isolation from the foragers,” Stroeymeyt says.

How did the colony know to spring into disease-preventing action before the fungal spores had even infected certain foragers? The researchers aren’t certain, but the ants’ keen sense of smell could be key. Ants sniff around with their antennae, which are constantly touching and sampling the insects’ surroundings. It is entirely possible, according to Stroeymeyt, that an ant would be able to detect a festering fungus on one of its colony members, just as easily as it would be able to smell a pathogen on its own body.

Why non-contaminated foragers also decreased the amount of time spent in the nest is another interesting question. As the first line of contact with their soon-to-be-ill workmates, they may have somehow known to stay away from important members of the colony. But it is also possible that, having detected pathogens on their fellow foragers, they simply spent more time treating the contaminated workers outside the nest. Ants produce formic acid through a gland on the tip of their gaster, or abdomen; they can kill fungal spores on one another by picking up formic acid in their mouths and licking the bodies of their pathogen-laden buddies.

Although the researchers recorded fewer interactions between foragers and indoor workers, contact did not cease entirely—and this led to yet another interesting revelation. When they used simulations to model how fungal pathogens spread throughout the colony in the face of the ants’ social network changes, the researchers found that the probability of the queen and nurses receiving a potentially fatal load of the fungus went down, but the probability of these important ants receiving a low load went up.

“That's similar to immunization or vaccination in humans,” Stroeymeyt explains. “These low doses don't lead to mortality, but they allow the ant to develop some sort of protection against later exposure with the same pathogen. That [finding] is also something that's quite new.”

Moving forward, Stroeymeyt plans to investigate how pathogens trigger social changes in wild ant colonies, which can number into the hundreds of thousands; she suspects that segregation between indoor and outdoor workers might be even more pronounced in these large groups.

Megan Frederickson, an associate professor of ecology and evolutionary biology at the University of Toronto who was not involved in the new study, calls the researchers’ conclusions “a novel and exciting finding” brought about by “cutting-edge methods.” She adds that similar technology might help scientists study whether ants also change their social networks to transmit beneficial microbes to one another. And Frederickson thinks “the significance [of the study] even goes beyond ants.”

“I wonder,” she muses, “how often other social animals reorganize their networks to limit the spread of disease.”

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