Even in a bustling ant colony, some workers stand around and do nothing. And yet, despite their generally industrious reputation, inactivity may be part of an ant colony’s natural social structure.

Two University of Arizona scientists, Daniel Charbonneau and Anna Dornhaus, recently examined lazy behavior in some North American forest ants of the species Temnothorax rugatulus. What they found was that there seems to be an entire class of ant workers whose job is to not do very much at all.

For separate three days over three weeks, the researchers observed five T. rugatulus colonies and tracked the behavior of 250 worker ants, as Karl Gruber reports for New Scientist. From video footage, they analyzed and categorized the ants’ activities. Close to half of the ants spent their days not moving or performing jobs. Out of the lazy workers, none seemed more or less lazy than the others. These ants tended to be larger and less social. “Some workers effectively specialize in ‘inactivity’,” the researchers write. The consistency of their behavior suggests that being lazy may be a job in itself, not unlike foraging.

But why are these ants such couch potatoes while others do the heavy lifting? That’s still a mystery, Gruber explains. They could be a back up work squad or serve as guards against raider ants looking for slaves. These ants might also serve as refreshment stations, providing sustenance to tired workers. (A similar phenomenon occurs in honeypot ants.)

Whatever the purpose behind their sloth behavior, it seems that lazy ants are another facet of the colony's social caste system.