Relationship of Acacia Tree & Ants
- The genus Acacia belongs to the larger plant family Fabaceae and has more than 1,300 species worldwide. Bull's horn acacia (Acacia cornigera) is native to Mexico and Central America, and is named for the pairs of thorns on the stem that closely resemble small bull horns. It is this species that is most often studied for its reciprocal relationship with ants. During the first couple of years of the tree's growth, there are rarely ants associated with the tree. At this time, it is most vulnerable to herbivores. Ants arrive at the plant after it has grown and developed the beneficial features that sustain them.
- Ants live inside plant structures called domatia.Photodisc/Photodisc/Getty Images
The thorns produced by the tree become homes, known as domatia or "little houses," for the ants. When a queen arrives at an unoccupied acacia, she bores a hole in the hollow thorns and lays her eggs inside. Over time, the colony grows to fill all of the thorns on the tree, which can house as many as 30,000 individual ants. - Not only does the tree provide a home for the ants, but it also makes food specifically for their use. Small glandular nodules called extrafloral nectaries are located on the stem near the domatia. They produce a sugary liquid on which the ants and their larvae feed. The plant also produces a solid food at the tip of the leaflets that contains the protein and fat necessary for larval development.
- The ant associated with this acacia species is known as Pseudomyrmex ferruginea. When the colony has grown large enough, worker ants will patrol outside the domatia in search of plant or insect invaders. If an unwelcome insect is found on the tree, the ants will viciously bite or sting to drive it away. The ants also protect the tree from encroaching plants. If a plant is touching the leaves or sprouting within a few feet of the base of the tree, the ants will use their sharp mandibles to cut it down. The tree and ant are interdependent, each relying on the other for survival.
