Fijian farmer ants live in plants in trees

An ant species on the Fijian islands gathers seeds, slips them into cracks in trees, poops on the seedlings to help them grow, then lives in the hollow chambers formed by the plants – and has been doing so for three million years.

Guillaume Chomicki and Susanne Renner from the University of Munich in Germany describe a housing network criss-crossed by ant trails on a support tree, with the queen at its centre, in Nature Communications – a relationship where the plants and ants are entirely dependent on each other to survive.

That ants cultivate plants and other species isn’t new. Leafcutter ants pluck blades of grass and leaves, drag them back to their nest and feed them to fungi, to which they tend in their vast underground colonies.

The Fijian Philidris nagasau ant doesn’t live in the ground; rather, it inhabits treetops. And instead of burrowing extensive tunnels, they live in at least six species of the plant Squamellaria. Squamellaria is epiphytic, which means it grows on another plant or tree for support and does not have access to soil for nutrients.

Plenty of ant species live in Squamellaria chambers, which look a bit like eggplant fruit hanging from branches.

But most colonies tend to live in the one nest – queen and daughters together – while P. nagasau nest can span several trees as long as their branches touch.

To test if Squamellaria plants were specifically dispersed by ants, the biologists recorded the area of P. nagasau-inhabited plants and found they were highly clustered compared to species not inhabited by the ants.

The pair of biologists watched P. nagasau workers wedge seeds into cracks in branches and patrol the planting sites.

To see if they preferentially “planted” certain species, they gave P. nagasau workers a choice – seeds of one of the six species in which they live, or a different but closely related species.

The workers collected the seeds of the habitable species and ignored the non-residential plant seeds.

As the seedlings grew and formed their first cavity, the biologists saw worker ants constantly shuttle in and out of the developing chamber. While they couldn’t see what was going on in the chamber directly, they thought that perhaps the ants were fertilising their crops.

And when the pair measured the nitrogen content of the seedlings – an indicator of, well, ant poop – they found they contained a massive increase compared to seedlings that weren’t visited.

So how long has this behaviour been going on?

Chomicki and Renner traced the evolutionary history of the plants and ants and conclude the farming began around three million years ago – around the time the plant evolved a way to anchor itself to bark, and the ants started their farming ways.