Leaf-Cutter Ants First Insects Discovered To Have Biominerial Body Armor

Atta cephalotes soldier, the largest worker caste within the leaf-cutter ant colonies, extending her mandibles over an Acromyrmex echinatior worker. Caitlin M Carlson

In the ant-eat-ant world of Latin American, scientists have found the first known example of biomineral body armor in a species of insect.

The armor, a super-tough material made from magnesium-rich calcite crystals, is found on the exoskeleton of worker ants belonging to the species of leaf-cutter ant, Acromyrmex echinatior, native across parts of Latin America. Not only does the armor help to fend off attacks from the soldiers of another larger ant species, Atta cephalotes, it also appears to protect the leaf-cutters from disease-causing fungi.

The discovery was recently documented in Nature Communications. In the new study, researchers from the University of Wisconsin-Madison describe how worker ants with the biomineralized exoskeletons were more likely to survive in encounters with A. cephalotes soldier ants than those without. It appears that the biomineral body armor develops as the ants mature and eventually covers the whole body, adding an extra level of protection for its exoskeleton. 

“In direct combat with the substantially larger and stronger A. cephalotes soldier workers, ants with biomineralized cuticles lost significantly fewer body parts and had significantly higher survival rates compared to biomineral-free ants,” the study authors write.

A series of experiments also showed that the ants with the biomineralized exoskeletons had a much lower risk of becoming infected with disease-causing fungi, Metarhizium anisopliae, which establish infection by wheedling into the insect exoskeleton.

This type of armor is known to exist in crustaceans, such as the hard exoskeletons of lobsters, but it’s never been seen before in insects. The material of body armor found in the ants is also notably different from their crustacean cousins. 

Top: the ant's biomineral armor showing as a white cuticular coating. Bottom: Scanning electron microscope of the ant cuticle with crystalline coating. Li et al,. 2020, Nature Communication

With the help of cutting-edge imaging techniques, the team learned that the armor is made of a crystallized calcium carbonate that's rich in magnesium. It’s thought this magnesium component is the secret to the body armor’s toughness. However, it’s extremely rare to find crustaceans with calcite-based body armor that’s rich in magnesium. This, the study says, is likely the result of a shift in the Earth’s oceans that occurred around 550 million years ago that saw significantly lower magnesium-to-calcium ratios.

Even beyond their body armor, leaf-cutter ants are fascinating creatures. They are a relatively rare example of a fungus-farming ant, which actively cultivate fungi much like how humans farm crops. However, the fungi grown by the ant farmers are never found in the wild, becoming so modified by the ants that they're unable to survive independently.

It’s thought ants evolved this ability around 20 to 30 million years ago, which is pretty impressive, considering humans only developed agriculture around 10,000-12,000 years ago.

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