Acacia ants, in the genus Pseudomyrmex, and their acacia tree hosts, are a terrific example of coevolution between plants and insects. While yellow flowers and many species demonstrate a loose form of the coevolution between generalized pollinators and a large range of plants, the coevolution in this case is very specific, although there are other examples of Acacias with ants that have a looser association.
The species depicted here are likely Acacia cornigera and Pseudomyrmex ferruginea, native to Mexico and Central America. These photos were taken at Palo Verde National Park in Costa Rica.
While some instances of coevolution may be hard to demonstrate for certain, this case is definitive. First, a common name of this acacia species, bullhorn acacia, refers to the extremely large, swollen thorns shown here. The horns also happen to be hollow, and provide perfect chambers for nesting ants. (An ant on the left thorn can be seen entering a hole in it.)
Next, the tree provides food for the ants in two forms. The first is nectar, but this is not the generalized nectar production of flowers attracting pollinators. The acacia nectar is provided from extrafloral nectaries, found actually on the leaf petioles, as shown here (slightly out of focus). Because of the location and size of these nectaries, it is clear that the trees are obtaining a benefit apart from pollination.
Second, the tree also produces a unique protein source for the ants called Beltian bodies (after the naturalist Thomas Belt). These extraordinary structures are produced as part of new developing leaves. When the new leaves unfold and expand, there is a Beltian body on the tip of each leaflet. These are harvested by the acacia ants and provide most of their protein. Between the nectar and the Beltian bodies, and the housing provided by the thorns, the tree provides all an ant colony’s needs.
What does the tree get in return? It gets extremely aggressive defense from herbivores, both large and small. Pseudomyrmex have one of the nastier stings in the world of Hymenoptera (the order comprising ants, bees and wasps). Any insect that alights on the tree is instantly driven away or killed, and the ants are quite effective against potential vertebrate herbivores as well — to which I can attest first hand when I made the mistake of brushing against a branch while taking these pictures.
The ants are so aggressive that they also take care of potential plant competitors, by cutting down any seedlings that sprout in the vicinity of the tree. In this photo it should be clear that the acacia in the center is sitting in a circle of bare dirt, courtesy of its ant colony.
The relationship between the acacia tree and Pseudomyrmex ants is thus a true mutualism, in which both species have a large benefit from the association.