Scientists discover “rock ants” covered in mineralized armor

(Inside Science) – For the first time, rock-like biomineral armor has been found in the insect world, according to a new study.

Animals have incorporated minerals such as calcite into their skeletons for over 550 million years. Although lobsters, shrimps and other crustaceans are known for their biomineral armor, such shells had not previously been seen in their insect cousins.

Evolutionary biologist Hongjie Li and his colleagues made the discovery while studying the interactions between the leaf cutter ant Acromyrmex echinatior and bacteria. Li, who works at Ningbo University in China, wanted to clean a white coating on these ants that other researchers told him was wax. He tried a variety of solvents and found that the mouthwash worked. This suggested to him that the coating was mineral, not wax. After X-ray analysis of the ants, Li told his colleagues that he found “rock ants,” he recalled.

Scientists found that this coating was made of magnesium-rich calcite that developed rapidly as the worker ants matured, covering almost the entire body. By hitting the biomineralized ant exoskeleton with a microscopic probe, it was revealed that it was more than double the exoskeletons of typical ants, beetles and honey bees.

Researchers found that young workers isolated from their nests and other ants do not develop biomineral armor, possibly because they lacked some source of magnesium or calcium. They then conducted experiments in which 30 A. echinatior The worker ants – half with biomineral armor, half without – were compared with 10 substantially larger and stronger soldier ants of a different species, mimicking the territorial ant wars common in the wild.

The biomineral armored workers killed all the enemy soldiers they were fought against, while the biomineral free ants killed only 20% of the soldiers. Additionally, the biomineral armored workers had higher survival rates – 80% versus about 6%. Armored biomineral ants have also lost far fewer body parts than free biomineral ants. Additionally, exposure of the ants to a disease-causing fungus revealed that biomineral armored ants are significantly more resistant to infection than biomineral-free ones.

Leaf cutter ants collect vegetation on which they grow mushrooms for food. Scientists noted that human agricultural settlements were highly susceptible to bandit raiders, leading to the development of specialized warriors, armor, weapons, and fortified cities. They speculate that the ants that grow mushrooms may have developed biomineral armor for similar reasons, at least in part for defense against other ants.

The fact that biomineral armor is only now being discovered in a relatively well-studied ant suggests that such armor may have gone unnoticed in other insects as well, the researchers noted. In fact, many species of ants that grow mushrooms are covered with white grains similar to A. echinatior’s.

“Biominerals are probably much more common in insects than we currently realize,” said Cameron Currie, an evolutionary biologist at the University of Wisconsin-Madison and one of the researchers who led the study.

Scientists detailed their findings online Nov.24 in the journal Nature Communications.