Scientists have discovered another clue to the evolutionary success of leaf-cutter ants: they have biomineral armour, the first time this has been discovered in insects.
The calcite biomineral has been extremely important in animal evolution, according to Cameron Currie from the University of Wisconsin-Madison, US, senior author of a study published in the journal Nature Communications.
Commonly found in crustaceans such as lobsters, molluscs and other marine animals, calcium carbonate skeletons evolved more than 550 million years ago when the oceans’ ratios of magnesium to calcium dropped dramatically.
The calcite found in leaf cutter ants (Acromyrmex echinatior) is rich in magnesium, which Currie says is rare in the biosphere.
“This higher magnesium content in our ants’ biomineral is very important for the function, as it increases the hardness of the armour,” he says. “So, our ants have really unique and strong armour.”
Until now, magnesium enrichment of calcium carbonate structures was only known from sea urchin teeth, the authors note. “The ability of fungus-growing ants to facilitate the formation of magnesium-rich biominerals on their epicuticles is thus surprising.”
Given the ants’ ubiquity, the find suggests high-magnesium calcite biominerals might be more prevalent in insects than previously thought, they add, paving the way for new research avenues.
Currie and colleagues had been investigating the intriguing way that leaf-cutter ants work with bacteria to produce antibiotics that help ward off diseases. When searching for what the ant might produce for the bacteria, they found crystals on the ant’s surface.
Lead author Hongjie Li became fascinated with the crystals and discovered they were a biomineral. This led to a series of experiments with leaf-cutter ants, originally collected in Costa Rica and Panama and kept in the lab.
First, the researchers used cutting-edge spectro-microscopy to identify the mineral. They confirmed that the ants form the magnesium calcite themselves, as it could be seen coming from the inside of the cuticle and deposited on the exoskeleton, explains co-author Pupa Gilbert.
They found the biomineral develops as the ants mature, increasing the hardness of their exoskeleton and covering virtually their whole body, and showed that it acts as armour in staged fights with Atta cephalotes soldier ants.
“Having found that the biomineral really increased the hardness, we conducted ‘ant wars’ with and without the biomineral to show that the armour helped the ants in their battles with other ants,” explains Currie.
Finally, they found that worker ants with biomineralized exoskeletons were less prone to infection by pathogenic fungi than those without, showing a further protective role for the armour.
Leaf-cutter ants evolved into complex societies around 20 million years ago from the ancient fungus farming Attini tribe. Dominating the new world tropics, their success in domesticating crops parallels the key role of agriculture in human dominance over the past 10,000 years.
Like us, they’ve also evolved ways to control crop infection, using bacteria to fight pathogens. This study adds a further intriguing resemblance to our own fledgling species’ farming history.
“Our finding of biomineral armour in a leaf-cutting ant provides another exciting new parallel to humans,” says Currie. “The evolution of protective armour for engaging in wars with other agriculturalists.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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