When I was in my last year of undergraduate education, my professor, Jack Longino, was a myrmecologist - an ant scientist. In Costa Rica, he showed us various "ant plants," that is, plants which have a symbiotic relationship with ants. One of the most common of these in Costa Rica was the bullhorn acacia.

This acacia produces thick thorns, resembling bulls' horns, in opposite pairs. Each pair of thorns has a hole at the junction. Look closely, and you can see little ants crawling in and out. This is the acacia's strategy for competing for space among all the vying plant species. The ants patrol up and down the stems and out along the ground in a circle around their home acacia, cutting off any competing plants. In this way, the acacia keeps a clear space around itself, where it can grow unhindered with no other plants crowding it. To keep the ants happy, the acacia not only provides hollow thorns for ant dwelling places, but also nutritious appendages on the tips of the leaf clusters.

Other plants in Costa Rica had similar habits, although none produced the large thorns. Instead, the usual ant plant has pithy stems which the ants hollow out. One of these ant species, Zacryptoceros, is known as the sombrero ant, because it has a white protuberance on its head in the shape of a sombrero, which it uses to plug the entrance hole when threatened. It inhabits only the hollowed twigs of the Cordia alliodora tree.

Other plant-dwelling ants protect their plants from more than just competing vegetation. In Venezuela, I first encountered the tree called palo-de-Maria. It is very distinctive among the forest trees, having very smooth bark. Look closely, and you will see a tiny hole in the trunk. Bang on the trunk with the flat side of your machete, and in a matter of seconds, the tree will be swarming with tiny black ants. These are the Pseudomyrmex, and despite their tiny size, they pack a bite that feels like the sting of a wasp! Thus, by providing a home for the Pseudomyrmex, the palo-de-Maria receives protection from any herbivorous animals, and even from tree-chopping humans!

Ant-plant symbioses are a fascinating aspect of life in the tropical rainforests. Professor Longino introduced me to them, but since then I have been on the lookout for then wherever I go. But plants are not the only organisms which make use of ant behavior. In the tropical parts of the Americas are a number of species of antbirds. These are rightly considered commensal rather than mutualistic, since they do not help the ants; they are merely hangers-on. Antbirds will accompany raiding columns of army ants. As the ants attack, their small insect prey attempts to flee - and runs right into the waiting birds.

As human beings reign supreme on our scale - the macro scale - on the meso scale the world may rightly be called the Empire of the Ants. Ants are the equivalent of civilization on their scale, building great cities and highways, and all other species of comparable size must adapt to ant activity in order to survive. Some plant and animal species have gone one better; rather than merely adapt, they have found ways to use the ants to their advantage.

Learn more about this author, Jason Hernandez.
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