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TAKING ONE FOR THE TEAMEach night a handful of Forelius pusillus ants step up to the task of closing the colony’s nest door from the outside. It’s likely the behavior benefits the colony, but the door-closers probably die after securing the nest.Tom Wenseleers

A Brazilian ant colony leaves some members out in the cold each night — literally. Tasked with closing the nest door from the outside, these ants complete their final mission and wander off, never to be seen again, researchers report in the November American Naturalist.

Self-sacrifice for the sake of the colony isn’t unusual in social insects — individuals will often take one for the team, improving the chances that close relatives survive. But unlike a guard bee that dies after stinging an intruder, there is no blaze of glory for these ants. They are probably old workers that meet death alone after fulfilling the door-closing duty.

“If you use the workers for this task, it is not that big of a cost to the colony,” says Adam Tofilski of the Agricultural University in Kraków, Poland, who led the new study.

Forelius pusillus ants nest in the sandy roads that crisscross sugarcane fields near São Paulo, Brazil. During the day, the ants are very active, transporting sand excavated from the nest to a pile surrounding the entrance. But at sundown all the ants go inside, save for a handful of workers who spend nearly an hour dragging and carrying sand from the pile back to the entrance to cover it. In the entrance-closing finale, one to eight ants kick sand backward, doggy-style, until the entrance is completely obscured. Then these ants depart. In the morning the entrance is opened from the inside and the door-closers never return to the nest, the researchers report.

The ants are so small and hard to see, the scientists weren’t able to mark those trapped outside, says Tofilski. He speculates they might be old ants whose days are numbered. Many social insects take on different tasks as they age — in their youth, they’ll stay inside and care for young, graduating to riskier tasks when older.

“If you think of a worker as a bit like a battery — at the beginning of life it is fully charged — you don’t want to lose it then,” comments behavioral ecologist Peter Nonacs of the University of California, Los Angeles.

Nonacs points to the idea of the “disposable caste,” put forth by entomologist Sanford Porter in the early 1980s based on his observations of harvester ants. Porter proposed a life progression through different castes, or classes, whereby individuals graduate to riskier jobs as they age. By the time harvester ants made it to forager, a status held by the eldest ants, they had lost about 40 percent of their weight and their jaws were completely worn down.

“These guys aren’t expected to last that long — they are running very close to empty,” says Nonacs. He has studied other species of ants that shut down the colony at night but have figured out how to do so from the inside, letting the wind completely obscure the opening.

The researchers aren’t sure why the nest must be totally covered. The technique may protect the colony from predators, parasites or rain, says Tofilski.

Figuring out how to track the tiny individual ants could answer some interesting questions, says Nonacs. For example, “How many days can you go as a colony-closer before your number is up?”

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