Call them nature’s own luxury high-rise condo rentals.
Squamellaria plants, from the same family as coffee and quinine, are fat aerial tubers perched high in tropical trees. In Fiji, one of the traditional names translates as “testicle of the tree;” they can grow to about the size of a basketball and sprout some leafy shoots. But that’s not the quirkiest thing about the plants: Some also invite different species of aggressive, easy-to-war ants to nest in the same plant’s multi-chamber innards. This setup is perfect for humans studying cooperation and mutualism, relationships in which both partners benefit in some way.
As many as five genetically different colonies of ants have turned up living side by side in the same treetop tuber, says Guillaume Chomicki, an evolutionary biologist at Durham University in England. This peaceful coexistence among such different partners is puzzling. Evolutionary theorists predict that one ant-plant combo should eventually take over.
Instead, thousands of potentially violent ants with genetic differences live just a wall apart in plant-based multiunit housing.
The colonies’ twisty suites of connected inner chambers squiggle through the plant’s innards but don’t open into each other, the researchers’ dissections and CT scans show. Each has a private entrance, but no inner doors allow neighbors to mingle, the team reports in the July 10 Science.
But “the minute you break these compartment walls, you have a deadly conflict,” Chomicki says. Within half an hour, every colony in the plant “is dead.” Yet with private entrances plus the lack of indoor connections between suites, the architecture of the plant itself acts as the peacekeeper.
Western scientists since at least the 1870s have studied the more famous mutualism of ants tending fungi as food. Both fungi and ants get shelter out of this arrangement. The mutualisms of the various Squamellaria plants look different: plant shelter for the partner ants; ant-provided nutrition for their mutual plant.
Chomicki sees no sign the ants even bite off snacks from the plant, which he suspects is doping itself with some nasty anti-ant ingredient such as calcium carbonate. That could also explain why ants don’t nibble through walls themselves in sectarian attacks.
What the plant gets out of this, the researchers found, are nutrients seeping into its tissues from droppings as well as the underfoot layer of discarded plant bits, meal scraps and other detritus that litter the tunnels. Chomicki has fed the ants with nitrogen and phosphorus tagged for tracing and found both nutrients capable of moving from the duff of underfoot household debris into plant walls. So the arrangement benefits both sides.
In some more recently evolved Squamellaria-ant partnerships, a tuber hosts just one species of ant, with colonies that can sprawl over many plants. With no conflict, there’s no need for walls or separate entrances, just one shared cavity. And the ants provide an extra benefit, the new paper reports. They have the additional farmer skill of planting seeds, tucking them in appropriate cracks in tree bark. Chomicki has even seen these ants standing guard over the tender early sprouts.
So is the mutualism the ants farming plants, or plants farming ants? Entomologist Ted R. Schultz of the National Museum of Natural History in Washington, D.C., knows a lot about the topic. He and his colleagues spent years putting together the deep history of how fungi and ants turned to farming and survived together in the near-apocalypse of the end of the Age of Dinosaurs.
In nature’s extravagant variety in two-way relationships, he sees Squamellaria and their ants as leaning more toward “the plants manipulating the ants, suggesting that the ants are being domesticated by the plants.” Yet most of the genetic changes, creating those discreet compartments, take place in the plant. And this suggests that also to some degree “the plants are being domesticated by the ants.” Nature doesn’t care about humans’ tidy categories.
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