Ants, at a glance, have an understandable reputation for industry. But it turns out they're not all obsessed with work: Some ants are lazier than others, a new study finds, and that's actually a good thing for the colony.

Researchers from the Missouri University of Science and Technology studied ants in motion and at rest. At issue was a question of energy expenditure in a colony. Scientists knew that larger ant colonies expended less energy than smaller ones, per capita, but they did not know why. Until now.

"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," explained the study's lead researcher, Chen Hou, in a statement.

As ant group sizes grew, so, in a regular pattern, did the number of inactive members, the scientists found. In groups of 30 ants, 60 percent of ants were not moving around, while in 300-ant groups 80 percent of the critters were doing their best to do nothing.

What's more, the 300-ant group used only half the energy, per capita, of that expended by the 30-ant group.

By not consuming energy, the researchers said, the layabout ants saved resources for the colony overall, which made it a more productive foraging unit.

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Efficient, coordinated foraging activity seems to be how the ants can be more productive gathering resources while expending less energy to do so.

"Maximizing resource acquisition would require most individuals to be highly active, but would also result in high energy expenditure and long average foraging time," Hou explained. "In contrast, minimizing time and energy expenditure would require most individuals to be inactive, but would also result in low resource acquisition. Thus, we postulate that ant colonies balance these two optimization rules by the coordination of the forager's interaction," said Hou.

The researchers used an automated vision analysis computer program and algorithm to track ant motion over long periods of time – on the order of two hours at a stretch. Previous studies, they said, only analyzed ant motion in one-minute intervals and thus may have missed out on true depictions of ant behavior and energy use.

When the scientists studied the ant motion videos, they found some interesting nuggets about the insects. For one thing, they move at highly variable speeds – from 0.2 centimeters per second to 1.4 centimeters per second. For another, they got a clearer picture of individual ants' energy expenditure when working vs. resting.

"We found that walking ants consume five times more energy than resting ants," said Hou. "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."

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Yet to be answered, Hou said, is the question of proportion: why bigger ant groups have more shiftless ants.

"It is intuitive that colonies have inactive members, because these members may serve a backup role or buffer, which would be activated when colonies are under stress, such as an urgent need for nest maintenance or defense," Hou said. "But it is unclear why the proportion of the inactive members is not a constant, why larger colonies have relatively more 'lazy' workers. Thus, our study calls for future research on ant interaction networking and behaviors."

The findings have been published in the journal Insect Science.

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