Leafcutter ants march in two rows along the rainforest floor, carrying pieces of leaves back to their nest. It’s a striking operation above ground, but the story doesn’t stop there.
Underground, the ants tend small fungal gardens and perform specialized tasks with shrewd efficiency, according to a study published this week in Royal Society Open Science. The research offers an intimate look into the complex society and intricate behaviors of leafcutter ants throughout Central and South America.
Robert Schofield, a University of Oregon physicist with a strong side interest in biology, became intrigued by ants when he first peered at one through a microscope he helped develop in graduate school.
When he isn’t studying gravitational waves, he observes the three ant colonies in his lab, filming the insects with the help of a microscope and camera. For the new study, he and his six-member team recorded the ants tending to their fungus gardens.
They discovered that most of the ants’ leaf cutting activities occur within the nest, where they use their razor-sharp mandibles to chew leaves into tiny pieces. They don’t eat the leaves themselves; instead, they use them to cultivate a fibrous, stringy fungus called Leucoagaricus gongylophorus, for their food.
In this ant-sized farming operation, each insect has a job, which it performs expertly, Schofield said.
First, they must cut the leaves into many tiny pieces, small enough to eventually cram into the holes of a porous fungal comb. The insects -- all females -- work together, with one ant holding a leaf in place and the other chomping away. If all of their cuts to a single leaf were in a straight line, it would be 3 kilometers long. That's about 1 million times longer than the ant's own body, Schofield said.
After the leaves are chewed to shreds, the ants prepare them for planting by licking, scraping and puncturing them before seeding them with bits of fungus that’s already growing in the nest. Other ants then cram them into the fungal comb, like stonemasons building a wall. In one case, the researchers watched as an ant reinserted a loose leaf into the comb, suggesting a measure of quality control.
They also “weed” by removing bacteria and other fungi from their leaves and garden. They even spread chemicals to control the growth of anything that might hinder their precious fungus.
“I think of them more as craftswomen than assembly line automatons,” Schofield said. “They can solve problems.”
Schofield, whose other research is attempting to detect “wiggles” in space-time caused by colliding black holes, approaches the ants through the lens of a physicist. He accounts for the ants’ behaviors in terms of energy gain versus expenditure, like a currency that can be followed around.
And the ants must be very frugal with their energy expenditures, the study revealed.
The energy required to transport and cut the leaves is about the same as the caloric value of the fungus they grow.
“The reason we eat salads when we’re on a diet is because plants don’t have a lot of calories in them,” Schofield said. “The leafcutters are getting something that has a low caloric content and then they have to carry it long distances.”
The leaves have few calories, but the fungus that grows on them has even fewer, containing only half as much energy, Schofield said. So the ants have developed a number of energy-conserving behaviors.
For example, when given the chance, the laboratory ants eschewed whole leaves in favor of leaves that had already been cut into smaller pieces. Not only did this save some energy, it enabled them to minimize their time spent outside the safety of the nest, the authors wrote.
Previously, scientists thought the ants created a leaf pulp for their fungal gardens, but Schofield and his colleagues found that would require too much energy.
Adding to the complexity, the colony’s largest ants build roads and highways to make others’ work easier. Schofield calls them “bulldozers,” and their job is to move rocks and sticks out of the way of ants carrying leaves.
The ants chew through so many leaves in their lifetime that their jaws start to dull; after that, the ants go to work outside because they’re not as valuable to the colony anymore.
“It’s cool for them to go out and risk death in the outside world,” Schofield said.
With their prime leaf-cutting days are behind them, about 2-3 times less efficient than their sisters with pristine jaws, the ants take on leaf carrying duties. The workers live for about a year.
Future studies should look at how the ants make these changes in behavior, Schofield said. When their mandibles wear out, are they pre-programmed to switch tasks? Do they complete a kind of self-evaluation?
“I think it’s actually self-evaluation,” Schofield said. “If an ant accidentally breaks its mandible … it will stop cutting instead of keep trying.”
With such complex conduct on display, the authors argue leafcutter ants could serve as valuable models for research into the genetic basis of behavior. The ants have more sophisticated brains and nervous systems than, the worm species most favored in neurological research.
“They’re not like robots that have very limited algorithms that they engage in when they’re performing a task,” Schofield said. “I think ants are much more complex organisms than we thought.”
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