A Fijian ant that actively cultivates plants and then inhabits them for protection is described in a paper published online this week in Nature Plants. The study involves an evolutionary analysis of this behaviour that suggests the ants have been farming the plants for millions of years, long before human agriculture.

Several species of animals, such as leafcutter ants or beetles cultivating fungi, have developed mutually beneficial relationships in which they grow and nurture - or farm - other organisms.

Guillaume Chomicki and Susanne Renner now show that on the Fijian islands, the ant Philidris nagasau actively farms at least six species of the plant Squamellaria (epiphytic plants that grow above ground on another plant or tree for support and do not have access to soil for nutrients). They find that the insects gather seeds from the plants’ fruits and insert them into cracks of the host tree. Seedlings then form hollow chambers that are constantly visited by the ants, which defecate inside to fertilize the young plant and help it grow without access to the rich tropical soil below. The chambers grow and offer nesting space and protection for transient workers and permanent ant colonies. Unlike previously observed ant-plant mutualisms, the authors note that this interaction is not facultative but obligate: the ants and plants are interdependent and one cannot survive without the other. The authors show that the created monoculture of plants forms a housing network criss-crossed by ant trails on the support tree, with the queen at its centre.

Finally, the authors reconstruct the evolutionary history of both the ants and plants to suggest that this farming behaviour arose around three million years ago by co-evolution: the plants developed a specific adaptation for bark anchoring, and the ants started their planting behaviour.