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In the tropical rainforests of Fiji, a genus of unusual plants has developed a remarkably simple but highly effective way to prevent violence between rival ant colonies: architecture.
In a new study published in Science, an international team led by Professor Guillaume Chomicki at Durham University has revealed how some species of the epiphytic plant Squamellaria (part of the coffee family, Rubiaceae) form peaceful and productive partnerships with multiple aggressive ant species simply by physically separating them within the plant itself.
These plants, which grow on the trunks and branches of rainforest trees, build bulbous structures known as domatia to house their ant partners.
In return for this shelter, the ants fertilise the plant with nutrients from their waste. However, many of the ant species that cohabit these plants are naturally hostile toward each other raising a fundamental question in evolutionary biology: how can such symbioses remain stable without descending into conflict?
The answer, the researchers discovered, lies in the internal design of the plant's domatia.
Using high-resolution 3D CT scans, the team showed that generalist Squamellaria species produce domatia divided into multiple chambers, each with its own exterior opening but sealed off from the others inside.
These compartments act like individual flats in a block of rainforest 'ant apartments', preventing physical contact between rival colonies.
Field observations across three islands over ten years showed that a single Squamellaria plant could house up to five different ant species at once.
Nutrient labelling experiments using nitrogen isotopes confirmed that all the ants, despite their unrelatedness, contributed valuable fertiliser to the plant – making them true mutualists rather than passive guests.
To test whether the physical separation was what kept the peace, the researchers carried out behavioural experiments where they surgically removed the walls between compartments.
The result was immediate: formerly stable cohabitants erupted into fierce and fatal battles, with high mortality rates on both sides. In contrast, when compartments were left intact, the ants coexisted and even shared food sources outside the plant without conflict.
Lead author of the study Professor Guillaume Chomicki of Durham University said: "It is incredible how such an odd group of organisms, restricted to a couple of Fijian islands, can provide general insights into the stability of cooperation between species."
Mathematical models developed by the team confirmed that this compartmentalisation helps maintain stable, long-term partnerships. By preventing early-stage conflicts and enabling each chamber to host a mature, nutrient-providing colony, the system maximises the plant's benefits.
Even when only a small fraction of arriving ants are aggressive, the models show that compartmentalised domatia consistently outperform single-chambered ones.
This discovery helps solve a long-standing mystery in ecology – how multiple, unrelated, and sometimes antagonistic partners can form durable mutualistic relationships. The findings suggest that physical separation may be a widespread, underappreciated mechanism in nature for maintaining cooperation and reducing conflict in symbiotic systems.
