Some trees can't run, can't hide, and can't fight. So they hire an army.
These plants are called myrmecophytes, which means "ant plants." They grow hollow thorns, swollen stems or chambers called domatia for ants to live in, and many feed their ants with nectar or tiny food bodies. In return, the ants attack anything that tries to eat the plant. Hundreds of plant species shelter ants this way.
It sounds like a fair deal. But like any contract, it only works if both sides keep their end of it. And plants have come up with some surprising ways to make sure they do.
The company store: acacias that trap their ants
In Central America, swollen-thorn acacias house Pseudomyrmex ants in big hollow thorns and feed them nectar from glands on their leaves. The ants patrol day and night. They sting animals that try to browse, kill insects, and even chew down other plants that grow too close.
But the deal has a catch. Most nectar is full of sucrose, regular table sugar, which ants break down with an enzyme called invertase. Martin Heil and his colleagues found that the workers of the acacia ant Pseudomyrmex ferrugineus don't have working invertase, so they can't digest normal sucrose. The acacia makes its nectar pre-digested, with no sucrose, so its ants can drink it.
Here's the twist. The larvae can still digest sucrose. The workers lose that ability as they grow up, and Heil's team later found evidence that the tree's own nectar helps switch it off. In other words, the acacia feeds its ants a food that makes them unable to eat anything else. The more they drink, the more they depend on the tree.
The researchers called this "partner manipulation." It's a lot like a company town, where the only store that takes your wages is the one your boss owns.
Sanctions: punishing ants that slack off
Not every ant holds up its end of the deal. In the Amazon, the shrub Cordia nodosa houses the ant Allomerus octoarticulatus in swollen stems. The ants patrol the young leaves to keep herbivores away. But they also do something that hurts the plant: they destroy its flowers, which keeps the plant from reproducing and saves more of its energy for growing ant housing. Biologists call this "castration."
So how does the plant keep its ants working? David Edwards, Douglas Yu and their colleagues found that the plant ties the ants' housing to their job performance. When the researchers cut leaves off ant-patrolled shoots to imitate herbivores, the ant chambers on those shoots died more often and grew less. If the ants let the leaves get eaten, they lose their homes. It's a sanction, built right into how the plant grows.
The plant also fights back against castration. It makes about four times more flower buds on new shoots, where they escape the ants that destroy flowers. The plant can't fire its ants, so it hides its flowers from them.
Gardens the ants plant themselves
Sometimes the ants are the ones in charge. In Fiji, Philidris nagasau plants the seeds of its own host plants and fertilizes them. In Southeast Asia, the irritable carpenter ant builds treetop gardens and decides where the roots are allowed to grow. We cover both in Ants Invented Farming 66 Million Years Before Us.
When the guards disappear
The best way to see how much a plant's army matters is to watch what happens when it's gone. That's exactly what happened in Kenya.
On the plains of Laikipia, the whistling-thorn tree, Vachellia drepanolobium, covers huge stretches of savanna. It houses native acacia ants in swollen thorns, and those ants swarm and bite anything that tries to eat it, even elephants. The tiny ants are so good at this that they hold the whole landscape's tree cover in place.
Then, over the last couple of decades, a new ant arrived: the big-headed ant, Pheidole megacephala, an invasive species thought to come from an island in the Indian Ocean. Big-headed ants don't defend the trees. They kill the acacia ants.
A 2024 study in Science by Douglas Kamaru, Todd Palmer, Jacob Goheen and their colleagues traced what happened next:
- Without their bodyguards, the trees were browsed and broken by elephants five to seven times more often than in areas without big-headed ants.
- With fewer trees, the savanna opened up. Lions are ambush hunters, and they rely on tree cover to sneak up on zebras.
- In invaded areas, lions killed zebras nearly three times less often.
- The lions adapted by hunting more buffalo instead, which are slower but much more dangerous.
An ant a few millimetres long changed what lions eat. All it took was breaking one contract between a tree and its guards.
The lesson from ant plants
Cooperation between species isn't built on trust. It's built on contracts, and on ways to enforce them. Acacias make their ants dependent. Cordia punishes ants that don't do their job. Whistling thorns depend on their ants so heavily that losing them reshapes the whole savanna.
The deals that hold ecosystems together can look stable for millions of years. But they can fall apart fast when someone new shows up and doesn't play by the rules.
Further reading
- Kautz, S., Lumbsch, H. T., Ward, P. S. & Heil, M. (2009). How to prevent cheating: a digestive specialization ties mutualistic plant-ants to their ant-plant partners. Evolution.
- Heil, M. et al. (2014). Partner manipulation stabilises a horizontally transmitted mutualism. Ecology Letters.
- Edwards, D. P., Hassall, M., Sutherland, W. J. & Yu, D. W. (2006). Selection for protection in an ant-plant mutualism: host sanctions, host modularity, and the principal-agent game. Proceedings of the Royal Society B.
- Edwards, D. P. & Yu, D. W. (2008). Tolerating castration by hiding flowers in plain sight. Behavioral Ecology and Sociobiology.
- Kamaru, D. N. et al. (2024). Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey. Science.
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