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?”
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?”
The paper version ends: "These guys aren't expected to last that long — they are running very close to empty," says Peter Nonacs of the University of California, Los Angeles.
Well, let me say, Nonacs, you've got it all wrong. We Closers have our own lore. We are a special caste, and you don't have the right to talk about us. In the days before we Close, we are fed specially by just-born worker ants who fellate our antennae and groom our pincer hairs whenever we motion. Then the morning comes, and everyone makes way for us, a small movement of the abdomen, a bowing of the antennae briefly by a young upstart, and we rest. It is said that once twenty or twenty five generations ago the Queen herself came out and opened an eye on the Closers. And we rest some more, or perhaps pretend to help load for a while, and then the night comes and we rest by the Entrance and wait, and bid goodbye, and we are off. Out — at night. Ah yes, we put the last boulders in. Big fucking deal. Then we are gone.
And we scout at night. We look for the next week's harvest. We leap over precipices and mountains, nothing stops us. If we Return at all it will be a miracle, we will bring news to the others, the Queen may intimate something to her hordes of frightening harpies, and a Direction may be supplied. Such things are known to have happened. Meanwhile, it is true, the survival rate is not high. The things of the night are not kind to Trappers. They scheme and they slide. They come as ghosts with poison and sprays and fangs. We stay perfectly still under a leaf. If it rains we look at God and we say, "If I Return I will be famous." If the mammoth horror of the pupae comes, "Let me provoke indigestion." I myself have seen the night sky and lived to tell of it. It is true that lanterns hang in a cavern.
The Birds need no introduction. They take everyone, the brave, the strong. There is sleep for us now. But if —
in a miracle —
a Closer survives the rising of the sun and my friends I am not lying I have SEEN THE SUN RISE FROM THE SOIL with my own eyes —
and if, in a double miracle, after such an ancient departure, HE IS RECOGNIZED BY THE YOUNG GUARDS and not killed immediately —
the fanfare, the triumphant blaze of a return! To bring news of a burr of subsoil fungus the size of a barn, or a dead opossum wedged improbably in a rotting tree crevice!
Feast with us and then and only then shall you speak of the Closers.
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