There are all kinds of mutually beneficial relationships that exist in nature in which two seemingly unconnected species live in harmony and provide vital services for one another. This does not always result in both parties benefiting equally – but as long as it still works, it keeps on going. A recent study has shown that certain Central American ants live on and defend a particular tree not because they want to, but because enzymes in the tree’s nectar have made it impossible for them to get food anywhere else. The research team was led by Martin Heil of Cinvestav Unidad Irapuato in Mexico and was published in Ecology Letters.
The acacia tree is covered in pods that can be taken by insects. Colonies of Pseudomyrmex ferrugineus patrol the tree, protecting the pods. Some species of acacia tree even have thorns large enough for the ants to move into. In turn for their defense against predators like termites, the tree produces nectar for the ants. The nectar is rich in sucrose, a type of sugar. Using an enzyme called invertase, the ants break up the large sucrose molecules into smaller bits which can then be used to generate energy. About eight years ago, Heil’s previous research showed that the adult ants don’t even make invertase, but it is produced by the tree and can be found in the nectar. It is a classic textbook example of mutualism, or so we all thought. The truth, as it seems, is a bit more complicated.
Five years ago, Heil found out that young ants do produce invertase, but that ability is lost at some point during life. In recent years, he has searched for the answer which, he would find out, almost seems like one of the biggest betrayals in mutualism history: the tree stops the ants from producing their own digestive enzymes. Included in the sucrose and invertase is chitinase: an enzyme that blocks invertase production.
While it is true that the ants protect the tree in exchange for food, they do so because they have no other options. They are completely unable to eat from any other source, because they rely on the invertase from the acacia’s nectar. So, the tree gives them exactly what they need to live, but only because it made them invertase-deficient in the first place. The tree makes out like a bandit by having armies of ants to protect it, who will never be able to leave.
This methodology has vague (and highly anthropomorphized) connotations to Münchausen syndrome by proxy (MSbP) in which someone believes they are sick and have a dedicated caretaker, but it turns out that the caretaker was the one making them sick in the first place. In the case of the acacia tree, it is damning the ants to an eternity of servitude on top of blocking the invertase production to ensure it is the ants’ only food source.
On a very basic level, however, this is astoundingly impressive. Solely through genetic mutations over countless generations, the acacia tree has adapted a way to protect itself from predators and the ants won’t be able to leave, leaving the tree vulnerable. It just happened to evolve this way, which is absolutely amazing. You win this one, acacia tree.