Ant colonies increase their efficiency by letting workers take time off. New research shows that as the hive becomes more numerous, as many as 80% of workers could be doing nothing at a time.

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We need a nice work-rest balance — although exactly what this ratio is varies wildly from person to person. Up to now, we’ve thought that we get the benefit of rest because we’re smart, while simpler beings such as ants slave away and then they die. We’ve got that one wrong, researchers at the Missouri University of Science and Technology say.

Ant colonies, they showed, can only function because a certain percentage of workers rest at any time.

“It has been a long-standing question in the field as to why large colonies of ants use less per-capita energy than small colonies,” says Dr. Chen Hou, assistant professor of biological sciences at Missouri S&T and lead researcher of the paper. “In this work, we found that this is because in large colonies, there are relatively more ‘lazy workers,’ who don’t move around, and therefore don’t consume energy.”

“We found that the portion of inactive members of a group increases in a regular pattern with the group size,” Hou says.

The team put together specialized computer-imaging software to look at an ant colony and track the motion trajectories of each individual. Previously, similar research only followed the ants for a few minutes. But the team’s algorithm allowed them to follow the movement of ants over large periods of time with better accuracy than anyone before them.

This way, they found that most of the colony ‘sleeps’ to conserve energy. On average, around 60% of workers in a 30-ant group were not moving about. This ratio jumped up to 80% for a 300-strong group of ants.

Rest harder, comrade

So what’s with the vacay? Well, they do it for the common good.

The colony becomes more efficient in the long term by keeping some of its workers on stand-by. While an all-hands-on-deck approach would maximize the speed of resource acquisition, it also requires huge energy expenditure (feeding the ants) and increases foraging time (as nearby resources are over-exploited and workers need to walk to more distant sources). The team explains that off-duty ants help conserve food, energy, and other resources — while the colony gains resources at a slower rate, forage time is reduced and energy expenditure is hugely reduced.

“The simultaneous energetic measurements showed that the per capita energy consumption in the 300-ant group is only 50 percent of that in the 30-ant group,” Hou says.

“We found that walking ants consume five times more energy than resting ants,” he added. “This means that energy wise, one walking ant is equivalent to five resting ants. Thus, if a group has 20 percent active members, this group would consume 180 percent more energy than a similar sized group with all inactive members.”

So the ants try to hit a balance between the need for new resources, and the need to conserve those already harvested. The ‘lazy’ ants are still an asset to the colony. Ants rest by rotation, so there’s always a pool of fresh workers to replace the ones on duty. They can also be called upon in an emergency, kind of like a reserve army or repair team.

“We postulate that ant colonies balance these two optimization rules [income and expenditure] by the coordination of the forager’s interaction.”

“It is intuitive that colonies have inactive members […] But it is unclear why the proportion of the inactive members is not a constant — why larger colonies have relatively more ‘lazy’ workers,” Hou concludes.

Observing how ants maximize efficiency by balancing some work with a lot of rest could help make our society more productive and sustainable.

Fingers crossed on that one.

The full paper “Heterogeneous activity causes a nonlinear increase in the group energy use of ant workers isolated from queen and brood,” has been published in the journal Insect Science.