The bullís horn acacia is a tree that grows in South America. To deter grazing animals, the acacia has a pair of fearsome thorns, about 5 cm long and joined at the base. A queen ant bores a hole near the tip, just big enough to allow her to crawl into the hollow base and lays her eggs. When the worker ants are born, they come out to eat up any insects that may land on the acacia's leaves and stems.
The ants also depend on the nectar exuded from glands underneath the acacia's leaf stalks all year round. Small fat-rich orange beads on the tips of the leaflets also provide a nutritious meal for the ant larvae.
The acacia is rewarded for its hospitality in more ways than one.