The seed-planting ants

Lift a flat sandstone slab in the Adelaide Hills scrub on a warm afternoon and you will usually find the residents of this story at work: a seething, orange-brown colony of meat ants, tending eggs, hustling food back down a shaft, utterly indifferent to the daylight now falling on their nest. Meat ants are the loudest, most photographable member of a much quieter guild, because most of the seed-planting around them is done by ants nobody photographs — smaller, less aggressive foragers working the leaf litter, one seed at a time, for a reward you would need a hand lens to see.

The reward is an elaiosome: a small, oily, protein-rich parcel glued to one end of a seed, built by the plant for no other purpose than to smell irresistible to an ant. A wattle pod splits, a hakea capsule opens, a grass-tree drops its capsules, and an ant finds the fallen seed within hours, seizes it by the fatty tag and drags it, sometimes many times its own body length, back to the nest. There it strips off and eats the elaiosome and abandons the seed itself, now cleaned and buried in the loose, enriched soil of the nest chamber, precisely the bed a germinating seedling wants. Nobody arranged this as a favour to the plant; the ant is only ever collecting its own dinner. But Australia’s flora has taken up the resulting bargain more than any other on Earth — something like 1,500 native plant species, across 87 genera and 24 families, independently carry an elaiosome, concentrated above all in heath and dry sclerophyll shrubland exactly like the Mount Lofty scrub (Berg 1975).

That a hard-seeded wattle, a woody hakea and a fibrous grass-tree all hit on the same trick is the tell that this is convergent evolution rather than shared ancestry: unrelated lineages arriving independently at the same fatty bribe because they face the same three pressures. Adelaide’s ranges run on old, phosphorus-starved soil and a summer drought that punishes any seed lying exposed on bare ground, and fire, not disaster, is a routine visitor to this country (Keeley et al. 2012). A seed hauled underground by an ant escapes the seed-eating birds and rival ants working the surface, and, as a bonus its planter never intended, it is also insulated from the heat of the next fire — the same soil-stored seed bank that Mediterranean-climate floras worldwide have converged on as a way of riding out a burn (Cowling et al. 1996; Auld 2020).

None of it announces itself. There is no flower to photograph, no birdsong to place it by ear. The whole transaction happens under a rock, under leaf litter, underground, and the only sign you are ever likely to catch of it is the meat ants’ gravel mound at your feet — the visible tip of an unpaid gardening service that has been planting this scrub, one greasy seed at a time, since long before there was a track to walk in on.