I was hoping to see and remember some of what I learned during my Tropical Biology course in college, and so I was extremely excited when I saw this plant today!! This is the Bullhorn Acacia... a plant that has a symbiotic relationship with the Annas ants (shown). The ants live in the large horns produced by the plant, and in return, the ants protect and defend the plant against predators that try to eat it. What an amazing thing!
A more complete explanation, borrowed from wikipedia:
Bullhorn Acacia is best known for its symbiotic relationship with a species of ant (Pseudomyrmex ferruginea) that lives in its hollowed out thorns. Unlike other acacias, Bullhorn acacias are deficient in the bitter alkaloids usually located in the leaves that defend against ravaging insects and animals. Bullhorn acacia ants fulfill that role.
The ants act as a defense mechanism for the tree, protecting it against harmful insects, animals or humans that may come into contact with it. The ants live in the hollowed-out thorns for which the tree is named. In return, the tree supplies the ants with protein-lipid nodules called Beltian bodies from its leaflet tips and carbohydrate-rich nectar from glands on its leaf stalk. These Beltain bodies have no known function other than to provide food for the symbiotic ants. The aggressive ants release an alarm pheromone and rush out of their thorn "barracks" in great numbers.