Among the most surprising things about the Melastomataceae is that a number of species live in partnership with ants. The partnership is built into the plant's anatomy: swollen pouches on the leaves that serve as ready-made nests.
Quick facts. Plants: Tococa, Maieta and several related genera in the Neotropics. Key structure: leaf pouches called domatia. Partner: ants, sometimes specific ant species. Habitat: humid lowland and foothill forest, especially the Amazon basin.
What is a domatium?
A domatium (plural domatia) is a small chamber on a plant that shelters animals, usually ants or mites. Plants with ant domatia are called myrmecophytes, meaning ant-plants. Several melastome lineages have independently evolved them, which suggests the arrangement has clear advantages.
In Tococa and related plants, the domatia are inflated, hollow structures at the base of the leaf blade where it meets the stalk. They look like two small bladders at the base of each leaf. In Maieta, the leaves in each opposite pair are strongly unequal in size, and the larger leaf bears the pouch at its base.
The ant side of the partnership
Ants move into the chambers, rear their young inside and patrol the plant. In return for housing, the ants behave as a bodyguard. They attack or drive off leaf-chewing insects and may trim or kill climbing vines and seedlings that would shade the host. Some ants also bring in food scraps and produce waste inside the domatia, which the plant may be able to absorb as nutrients, though the extent of this benefit varies and has been debated.
A well-studied example involves the ant Pheidole minutula, which is associated with Maieta guianensis and with some Tococa species in the Amazon. Colonies live in the leaf pouches and forage over the plant surface.
How strict is the relationship?
Not every ant-plant relationship is a tight, exclusive partnership. Some melastomes host a particular ant species almost exclusively, while others house a mix of opportunistic species. Researchers distinguish between true mutualism, where both sides clearly benefit, and looser associations where one partner might gain more than the other.
Why does it evolve?
Tropical forests have intense insect pressure. A plant with a resident ant guard can invest less in chemical defences, which are costly to produce. Meanwhile ants gain a protected nesting place in an environment where suitable cavities may be limited. Such mutual advantage is the standard explanation for ant-plant evolution.
Identifying ant-plants
If you ever visit a botanic garden with a tropical house, look for:
- Swollen, bladder-like structures where the leaf blade meets the stalk.
- Small entrance holes on or near these swellings.
- Ants moving in and out, especially if you disturb the plant gently.
Please do not poke or damage rare plants; observation is enough.
Related reading
For other special relationships between melastomes and animals, see buzz pollination and Axinaea's bird pollination. For the range of leaf forms and how to spot a melastome in general, see identifying a melastome by its veins.
Takeaways
- Ant-plants have built-in shelters called domatia.
- Several melastome genera, including Tococa and Maieta, have evolved them.
- Ants defend the plant, and the plant houses the ants.
- The details differ between species, and research is still refining the picture.