How Do Ants Farm Fungus? Agriculture Invented Fifty Million Years Early

A mature leafcutter colony runs a production line of considerable sophistication:

• •Foragers cut fragments from leaves with serrated mandibles vibrated at high frequency, and carry pieces many times their own weight back along cleared trails that can run hundreds of metres
• •Smaller workers ride on the carried fragments, defending against parasitic flies that attempt to lay eggs on the carrier's exposed head
• •Inside the nest, successively smaller castes clean the leaf, chew it into pulp, add faecal fluid containing enzymes, and place it onto the growing fungus garden
• •The smallest workers tend the fungus itself, weeding out spores of competing species and removing infected material to refuse chambers outside the garden
• •The fungus produces specialised swollen structures called gongylidia, which exist only in cultivation and which the ants harvest and feed to larvae and to the queen
• •A founding queen carries a fragment of the parental fungus in a pouch in her mouth when she leaves, which means the crop is propagated clonally and can be continuous for millions of years

What makes the system remarkable is that it involves at least four organisms in a stable arrangement. The ants and the cultivated fungus are the obvious pair. The third is a specialised parasitic fungus of the genus Escovopsis, which attacks the garden, is found nowhere else, and can destroy a colony's food supply. The fourth is a bacterium of the genus Pseudonocardia, which the ants carry in specialised pits in their cuticle, fed by glandular secretions, and which produces antibiotics active against the parasite. That is an insect maintaining a culture of antibiotic-producing bacteria on its own body to protect a crop from a specific pathogen, and it has been doing so for tens of millions of years without the resistance that has developed against human antibiotics within decades, which is why the system has been studied intensively by people looking for new antimicrobial compounds and for an explanation of why resistance has not overwhelmed it.

Fungus farming arose once in ants, in South America, roughly fifty to sixty million years ago, and the descendants of that origin include more than two hundred and fifty species arranged in identifiable stages. Lower attine ants cultivate fungi that can still survive outside the nest, grow them on insect frass and plant debris rather than fresh leaves, and maintain smaller colonies. Higher attines, including the leafcutters, cultivate a fungus that has become entirely dependent on them and cannot live independently, feed it fresh vegetation, and build enormous colonies. Genomic work has found that the transition to the fully domesticated crop coincided with the ants moving into drier habitats, which would have made it impossible for the fungus to escape and survive on its own, effectively imprisoning it and completing the domestication. The parallel with human agriculture is close and was not designed to be: a crop that cannot survive without cultivation, monoculture with its attendant disease risk, and antibiotic management of that disease.

Agriculture has evolved independently several times in insects. Termites in Africa and Asia cultivate a different fungus in comb structures inside their mounds, an origin entirely separate from the ants and roughly thirty million years old, with the same result of converting indigestible plant material into food. Ambrosia beetles bore into wood and cultivate fungus in the tunnels, carrying spores in specialised pouches, and this has arisen more than ten times in different beetle lineages. Several ant species farm aphids rather than fungi, protecting them from predators and moving them to good feeding sites in exchange for the sugary honeydew they excrete, which is herding rather than crop cultivation. Marine examples exist, including damselfish that maintain and weed algal gardens on reefs and aggressively exclude other grazers. The recurrence suggests that the arrangement, in which one organism secures food by cultivating another and protecting it, is a solution that a wide range of animals arrive at whenever the conditions allow.

Leafcutter ants cultivate a fungus on chewed leaf paste, weed it, fertilise it with enzyme-containing secretions and eat specialised structures the fungus produces only in cultivation, with new queens carrying a fragment of the crop to found colonies. The system includes a specialised parasitic fungus and an antibiotic-producing bacterium the ants maintain on their own bodies to control it. It arose once in South America around fifty million years ago, the crop is now unable to live independently, and comparable agriculture evolved separately in termites and repeatedly in beetles.