‘Lazy’ Ants May Be Useless By Design, Study Finds

TUCSON, Ariz. — We often think of ants as hard workers, doing their part to seek out and find food to bring back to the colony. But it turns out a large chunk of ant colonies don’t actually work at all, and as a recent study determined, it may be by design.

Biologists at the University of Arizona studying Temnothorax rugulatus ants native to mountainous regions in the southwestern United States found large numbers of them appearing to laze around the colony, not doing much of anything.

Lazy ants study
By marking ants with tiny dots of paint, Daniel Charbonneau and his collaborators were able to keep track of what individual ants do over the course of two or more weeks. (Photo credit Daniel Charbonneau)

“They really just sit there,” explains Daniel Charbonneau, a PhD candidate at the university studying the so-called “lazy ants,” in a statement. “And whenever they’re doing anything other than doing nothing, they do chores around the nest, like a bit of brood care here or grooming another worker there.”

Based on his studies and others, Charbonneau learned that about 40 percent of ants within a colony are inactive, with some variation based on species, season, and colony. Charbonneau says that similar behavior exists in other social insects, such as honey bees.

Building on these findings, Charbonneau and a team of researchers from around the world observed several colonies of ants maintained in labs. When they removed a fifth of the most active worker ants, previously inactive ants had replaced them within a week. The ants appear to keep that 40 percent of the colony in reserve as “lazy ants” to fill the spots of any lost workers.

“This suggests that the colony responds to the loss of highly active workers by replacing them with inactive ones,” says Charbonneau.

To track the active and inactive ants, the researches painted different colored dots on individuals in their colonies.

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The inactive individuals were just one of four main ant demographics, based on video recordings from inside the colonies and the researchers’ ant-tracking system: walkers that patrol the nest; foragers that gather food and build protective walls from pebbles and mud; and nurses responsible for rearing the brood.

“My speculation is this: Since young workers start out as the most vulnerable members of the colony, it makes sense for them to lay low and be inactive,” Charbonneau says. “And because their ovaries are the most active, they produce eggs, and while they’re doing that, they might as well store food. When the colony loses workers, it makes sense to replace them with those ants that are not already busy pursuing other tasks.”

The full study was published September 6, 2017 in the peer-reviewed, open access journal PLOS ONE.

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