Leaf-cutting worker ants may appear helpless against an enemy soldier ant often of its size. But some of the smaller ants have a secret: their whole body is lined with a thin but hard layer of mineral armor.
It is the first time this type of external mineralization of the whole body has been found in an adult insect, researchers reported online on Nov. 24 at Nature Communications.
“I found rock ants,” recalls evolutionary biologist Hongjie Li who told his colleague, evolutionary biologist Cameron Currie, when the first experimental results of hard coating came in. “I can still feel the excitement now,” Li says.
Currie, of the University of Wisconsin-Madison, found that he was discovered, having spent more than 20 years studying leaf-cutting ants. His lab has been examining the interactions between ants and their external microbes, which are thought to play a key role in ants' farming practices (SN: 23/04/20), when the team found a white glow in the Acromyrmex exoskeletons. echinatior forms workers.
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That coating needed to come off so that researchers could examine the structure of the exoskeleton in more detail. Currie then commissioned Li, then a postdoctoral student in Currie's lab now at Ningbo University in China, to remove him. But nothing seemed to work, suggesting that the coating was not a wax or other carbon-based compound. Then, while brushing his teeth, Li had an epiphany: mouthwash. It helps remove all kinds of food waste without damaging the tongue and can dissolve mineral deposits in the teeth, so Li decided to test the liquid.
The mouthwash did the trick and also gave the team the first clue that the coating was mineral in nature. Other chemical, X-ray and microscopic examinations revealed a thin layer of calcite containing high levels of magnesium.
To see how protective the armor is, the researchers tested the hardness of the exoskeleton of the ants peeking out armored and unarmored pieces until a bleeding formed. The team found that despite being only 7 percent of the total thickness of the exoskeleton, the calcite coating at least doubles the hardness of the exoskeleton.
The discovery is astonishing, says Duncan Murdock, a paleobiologist at Oxford University’s Museum of Natural History, though it may not be entirely unexpected, he adds. This is because ants are distant cousins of crabs and other crustaceans, which usually have mineralized exoskeletons.
It’s unclear how the armor is made, though Currie thinks the bacteria are involved. If so, this could be another way in which the microbiome of ants makes a huge difference in insect survival.
The team also tested how well the armor protected the ants in combat with other larger ant species. In organized battles between a trio of workers and a lone welded ant of a different species, Atta cephalopods, this mineral armor tilted the scale in favor of the workers, the researchers say. Almost all the ants that rose to produce no armor were killed by the soldier ant, while the vast majority of the armored ants survived.
From left: Caitlin M. Carlson; H. Li et al / Nature Communications 2020
Robert Schofield, a biophysicist at the University of Oregon at Eugene, is unconvinced by the battle data and wonders if unarmored ants had other differences that could have led to their demise. Still, he is intrigued and plans to investigate similar coatings he has seen in other leaf-cutting ants.
Currie plans to examine other ants as well, hoping to determine the extent of the armor phenomenon. Finding out how to form thin but durable coatings, he says, could one day be useful for people in developing protective coatings for all types of products.