When you get a flu or cold, you can choose to keep away from other people in order to spare them from a similarly sniffly fate – and they, in turn, can stay away from you. According to a new study, humans are not alone in their efforts to kidnap patients. In the presence of contagious pathogens, humble ants can also change their behavior to keep contaminated creatures away from other members of the colony.
Ants are social creatures. They live in large groups, communicating and cooperating with one another to ensure 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 the disease under control through various hygienic mechanisms such as removing litter and bodies from members of the dead colony from their nests. Scientists suspected that insects could also adjust their social behavior to decrease the spread of infections, but this hypothesis was, until recently, difficult to prove.
"Ant colonies have hundreds of individuals," explains Nathalie Stroeymeyt, a postdoctoral researcher at the University of Lausanne, Switzerland, who studies collective behavior in ant colonies. "Until now, there was not just the technical methodology to measure their interactions at colony level for long periods of time."
Fortunately, an automated tracking system developed by Swiss researchers in 2013 has allowed Stroeymeyt and his colleagues to get a detailed look at how 22 colonies of lab-bred ants behave when the disease is infiltrating their environment. The team pasted tiny 2D bar codes into the ants' thorax, which gave each insect a unique identifier – "like a QR code," says Stroeymeyt. A camera positioned above the ants 'cabinets took two photos per second, and an algorithm detected and recorded the position of each bar code, giving researchers a wealth of data on ants' movements.
For four days, the team left the ants running in their enclosure undisturbed. As with the colonies in the wild, some ants worked out of the nest to look for food, while others – like the queen and the "nurses" who care for the developing chicks – were in 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 often found in the soil of ant habitats and is known to make them sick. The forages of the other 11 colonies were treated with a benign solution to serve as a control group.
Crucially, previous studies have shown that M. brunneum The fungus takes at least 24 hours to infect the 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 the side effects of the disease or manipulation of the parasite, "Stroeymeyt explains.
Writing in the newspaper Science, the researchers report that when the foragers were put back into their enclosure, the contaminated ants spent more time out of the nest, meaning that they had less contact with the colony's most valuable members: the queen, who lays all the eggs colony and the internal workers, who are younger than the fodder and therefore have more hours to contribute to the colony. (Older ants are in charge of risky foraging works out of the nest because, as Stroeymeyt puts it directly, they will "die anyway.")
But the crux of the study lies in the discovery that contaminated ants were not the only ones to change their behavior. Forages that were not exposed to the fungus also increased the amount of time spent away from the nest. And the nurses inside the nest moved the young people inside and spent more time overlapping them, which "could be seen as a spatial isolation of foragers," says Stroeymeyt.
How did the colony know that it would become a preventive action before fungal spores had infected certain forages? Researchers are not sure, but ants' sense of smell can be critical. Ants smell with their antennae, which are constantly touching and sampling the insects' surroundings. It is quite possible, according to Stroeymeyt, that an ant be able to detect a purulent fungus in one of its members of the colony, as easily as it would be able to smell a pathogen in its own body.
Why uncontaminated forages also decreased the amount of time spent in the nest is another interesting question. As the first line of contact with your co-workers who are about to get sick, they may have somehow managed to stay away from important members of the colony. But it is also possible that, having detected pathogens in their foraging colleagues, they simply spend more time treating the contaminated workers out of the nest. Ants produce formic acid through a gland at the tip of the gaster, or abdomen; they can kill fungal spores in each other, taking formic acid in their mouths and licking the bodies of their pathogen-laden fellows.
Although the researchers reported fewer interactions between foragers and internal workers, the contact did not stop altogether – and this led to another interesting revelation. When they used simulations to model how pathogenic fungi spread through the colony in the face of changes in the social network of ants, the researchers found that the probability of the queen and nurses receiving a potentially fatal fungus burden decreased, but the likelihood of these important ants getting a low load went up.
"This is similar to immunization or vaccination in humans," explains Stroeymeyt. "These low doses do not lead to mortality, but they allow the ant to develop some protection against further exposure with the same pathogen. what [finding] it's also something quite new. "
Moving forward, Stroeymeyt plans to investigate how pathogens trigger social changes in colonies of wild ants, which can reach hundreds of thousands; she suspects that segregation between internal and external workers may 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' findings "a new and exciting discovery" brought on by "cutting-edge methods." She adds that a similar technology can help scientists study whether ants also change their social networks to transmit beneficial microbes to each other. And Frederickson thinks "the meaning [of the study] even goes beyond the ants. "
"I wonder," she reflects, "how often do other social animals reorganize their networks to limit the spread of disease."
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