Leafcutter ants are able to grow fungi, and recently researchers have discovered a new type of biological mineral on the exoskeleton of this insect that acts like armor.
Photography: EDUARD FLORIN NIGA / EDDIMAGE PHOTOGRAPHY
Written by: DOUGLAS MAIN
Leafcutter ants get their name for their arduous feat: they chop leaves and then carry these bulky broken leaves (blades that look like green flags many times larger than them) on their backs to their territory. Once back in the nest, they continue to cut and process leaves to feed the underground fungal farms. Along the way, these insects have to bravely fight against various predators and often fight other ants. New facts have proved that leafcutter ants are more tenacious than we previously thought. In a new study, researchers found a natural carapace on the exoskeleton of a Mesoamerican leafcutter ant. This armor-like coating is made up of calcite with a high magnesium content, which has previously been found only in another biological structure: the teeth of the sea urchin, which can grind limestone. Many animals have calcium-containing minerals in their bones and teeth, and the shells of crustaceans such as crabs and lobsters and other body parts also have mineralized components. But before this discovery, no calcite had been found in any of the adult insects.
Leafcutter ants have armor that helps them survive battles with other ants.
Photo credit: EUGENIA OKONSKI, SMITHSONIAN INSTITUTION
This coating of leafcutter ants is made up of thousands of tiny flake crystals capable of hardening exoskeletons. A paper published Nov. 24 in the journal Nature Communications said the "armor" helps prevent ants from losing limbs in battle with other ants and fights off fungal infections.
We are all too familiar with leafcutter ants, so this finding is particularly surprising. "There are thousands of papers on leaf-cutting ants," said Cameron Currie, an evolutionary biologist at the University of Wisconsin-Madison and co-author of the study.
"We're really excited to find this substance in the most thoroughly studied insects in nature," he said.
Although the paper looked at only one species, leafcutter ants, Currie and his colleagues speculated that other related ants may also have this biological mineral.
Ants in rock armor
Long before humans or their immediate ancestors evolved, about 60 million years ago, leafcutter ants invented their own way of agricultural production. These underground fungal farms are the product of a symbiotic relationship that both feeds ant larvae and protects fungi, each with its own unique fungus.
Some of the nearly 50 species of leafcutter ants, including the study subjects, also possess a symbiotic fungus that protects their fungal farms from infection by other harmful fungi. The microbe attaches to young worker ants, and as they snake through fungal farms, the bacteria secrete chemicals that kill the invading fungus.
Hongjie Li, a former postdoctoral researcher in Currie's lab (now at Ningbo University in China), began studying the bacteria and soon became interested in the strange tiny crystals that covered the ant exoskeletons. He persuaded geologists to help him study these mineral-like materials, using several imaging techniques, including electron microscopy, to analyze their composition.
One morning in the fall of 2018, when Li Hongjie got the results, he was ecstatic. Because the results showed that the ant was covered with a biological mineral that had not been found in any insect before.
"The ants have stones on them," Mr. Li said. "I found rock ants!"
Li Hongjie said that the carapace on the leaf-cutting ant is very similar to the composition of the mineral dolomite, but the hardness is slightly higher.
Like all insects, the exoskeleton of an ant is made of chitin, which is tough and elastic. To see if this extra biological layer also acts as a protective armor, Li hongjie and his colleagues first raised two kinds of ants in the laboratory, one with a biological mineral layer and the other without. (If ants are separated from the colony as they pupate and are reared under certain conditions, they do not form a mineral layer.) Then they did a couple of tests.
Ant Wars
One of the experiments was to pit the ants against a slightly larger but closely related type of ant in "Ant Wars." In an hour of fighting, those ants without mineral layers lost 3 times more body parts than "rock ants".
Next, the researchers exposed the insect to a pathogenic fungus that can infect ants and is associated with the species of fungus that causes "zombie-like" behavior. After 6 days, all the ants without a mineral layer died. But the mortality rate of ants with biolayer armor is only 50%.
Another experiment showed that the hardness of these ant exoskeletons was more than tripled when they contained biological minerals than when they were not.
The mineral layer also increases as the ant ages. Young ants tend to take care of fungal gardens and are not at high risk of being attacked by other ants or predators, so they don't need it. When they begin to forage in the world beyond the fungal garden, the mineral layer will be thicker than when they were younger, Li hongjie said.
Andrew Suarez, an entomologist at the University of Illinois, said finding this type of mineral in insect exoskeletons is very exciting because similar minerals have only been found in more isolated special structures such as teeth before.
"It's like your body is covered with enamel (made of tiny crystals)," said Suarez, who was not involved in the study.
"I love this paper because it found something new, the biomineralized bones of insects," says Andrew Knoll, a biologist at Harvard University.
"A lot of arthropod exoskeletons are made of calcium carbonate, such as crabs, lobsters, and extinct trilobites, but it's really interesting that this structure extends into fully land insects."
Undiscovered armor
The scientists also say that such biocrystals may be used in manufacturing in the future, such as coatings or the manufacture of nanocrystals that can enhance the strength of various materials or prevent corrosion.
The focus now is to understand the role these minerals play in ants and see if there are other armor and biological minerals on them that have not yet been discovered.
Currie thought it was possible. "If you don't understand the biological makeup of this species, what does that mean for 99.9 percent of insects that have little or no study?"
(Translator: Strange Flowers Blossom)