The natural world is full of symbiotic relationships, where different lifeforms combine their powers. When this involves collaboration between animals and plants, we automatically give the credit to those with brains and the capacity to move. However, a new study has looked at the cooperation between certain plants and ants and found the plants may be driving the process.
"There are a number of different structures plants make that are specific for ant use," said Dr Matt Nelsen of the Field Museum in a statement. "Some plants have evolved features that persuade ants into defending them from attack from other insects and even mammals. These include hollow thorns that ants will live inside, or extra nectar on leaves or stems for the ants to eat. Some ants will just cheat and take the nectar and run, but some will stick around and attack anything that tries to hurt the plant.”
All this is in addition to getting ants to disperse seeds (instead of relying on the wind) by adding food packets tasty enough to justify transportation to the nest.
Nelsen was curious as to whether all this was initiated by the ants or the plants. It seems an impossible question to answer since the fossil records of both are so patchy from the Mesozoic when the relationship developed. However, Nelsen and colleagues compared the DNA of plant-dwelling ant species and the plants that host them, and created a family tree with different sorts of collaboration identified.
In Proceedings of the National Academy of Sciences, the team report that ants have needed plants much longer than plants have needed ants. The specialized structures some plants have developed to turn ants into their defensive army are more recent developments than the use of plants as an exclusive seed source for certain ants.
“Ants first began foraging arboreally, then incorporated plants into their diet, and then from there, they started nesting arboreally. While this stepwise shift towards an increased reliance on plants is intuitive, it still surprised us," Nelsen said.
The more beneficial an ecological niche is, the more diversification we usually see in the species that fill it. Ants have benefited well enough from the symbiosis to keep the relationship going over tens of millions of years, with 1,700 known species now participating. However, those taking part have not expanded any faster than others of this extraordinarily successful animal family.
In contrast, previous work has shown that plants that have evolved traits to make ants dependent on them have diversified more rapidly than their counterparts, and now number at least 10,000 species. In combination, these findings indicate the interaction is more beneficial to the hosts than the insects, indicating ants are the true servants and plants their masters.