Fungus farming by ants has evolved independently in populations separated by the Atlantic Ocean – the latest example was observed in cocktail ants in Cameroon, which cultivate fungus to build their nests

In warm regions of the Americas, leafcutter ants farm the fungus that they eat, gathering bits of greenery to feed it. An entire ocean away, one species of ant in Africa uses a very similar fungus cultivation technique. The findings suggest that this complex ant-fungus relationship has evolved independently at least twice.

The nearly 50 species of leafcutter ant tirelessly carve and shuttle pieces of leaves back to their expansive subterranean nests, where they feed the bounty to a fungal culture that, in turn, nourishes the ant colony. Until now, the mutualistic partnership – tens of millions of years old – was only known in leafcutter ants from the Americas.

But in a Cameroonian forest, Alain Dejean at Paul Sabatier University in France and his colleagues observed it in tree-living cocktail ants (Crematogaster clariventris). Cocktail ants make cardboard-like “carton nests” out of chewed-up plant material, and researchers already knew that fungi growing in the nests lent structural support to the walls. Other researchers had also seen other Crematogaster species carrying leaf pieces around, says Dejean.

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From a platform suspended in the forest canopy, Dejean and his team observed leaf-cutting in C. clariventris ants and wondered whether the behaviour was linked to the nest fungi. So the researchers cut into the carton nests with a machete and watched over the next few days as the ants repaired the damage, cooperating to make a composite material of cut vegetation and sticky pulp. Using microscopes and genetic analysis, the researchers confirmed that this ant-made cement and the tough walls of the nests contained threads of the same sooty mould fungus, suggesting the ants are growing a fungus in the nest walls using harvested leaves and flowers.

“I don’t think anyone expected to see fungus farming by ants to have evolved independently in another region,” says Corrie Moreau at Cornell University in New York. “This is so shocking and exciting!”

The fungi used by the two distant leafcutter species are not closely related. The African ants also use the fungus in a totally different way: instead of eating it, they use it to fortify their nests against torrential tropical rains. Even after the fungus dies, the dead fungal threads left behind in the walls strengthen the nest’s construction.

“If you have ever seen or touched one of these ant nests you know they are nearly impenetrable fortresses,” says Moreau.

Ted Schultz at the National Museum of Natural History in Washington DC considers the report “a remarkable case of convergent evolution”, referring to the process by which organisms that aren’t closely related independently evolve similar traits in response to similar environmental conditions.

Schultz says it is likely the ants are engaging in a form of animal agriculture for structural materials, similar to human cultivation of lumber and cotton.

Dejean says the team also saw other Crematogaster species using the fungus, including those that build a more fragile “brittle carton” type of nest. He says it is likely that Azteca ants from Central and South America also use fungi to make their own brittle carton. “The repeated evolution of complex behaviours [in ants] is probably much more common than we think,” says Moreau. “I wonder where we will see fungus farming pop up next.”

Journal reference: Ecology and EvolutionDOI: 10.1002/ece3.9904