Vachellia cornigera, commonly known as bullhorn acacia (family Fabaceae), is a swollen-thorn tree and myrmecophyte native to Mexico and Central America.

The common name of "bullhorn" refers to the enlarged, hollowed-out, swollen thorns (technically called stipular spines) that occur in pairs at the base of leaves, and resemble the horns of a steer.

Bullhorn acacia is best known for its symbiotic relationship with Pseudomyrmex ferruginea, an ant that lives in its hollowed-out thorns.

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 beltian bodies, small detachable tips on the pinnules of the bullhorn acacia, have evolved into multicellular structures to provide food for protective ant colonies.

The P. ferrugineus ants cut small holes in the thorns of the acacia where they lay eggs and care for larvae.

The secreted nectar plays an important role as plant indirect defense through the attraction of defending ants.

As long as the plants are inhabited by mutualistic ants, the extrafloral nectar will get secreted with a sharp diurnal peak (between 8-10am).

The nectary is the site of nectar synthesis, and the components that get synthesized include sugar, amino acids, and nectarines.

[7] The nectar secretion from nectaries and food bodies on leaves and shelter (hollow stipular spines at the base of a leaf) is known as swollen plant syndrome.

This syndrome is vital to the acacia plant's survival because it facilitates the animal-plant mutualism with the P. ferrugineus ants.

It has been reported that swollen thorn syndrome (production of specialized traits in the form of hollow stipular spines, beltian bodies, and extrafloral nectaries) was absent in the early development of the bullhorn acacia.

[8] In a study by Heil et al. in 2014,[9] the research team found that acacia hosts manipulate their ant inhabitants (pseudomyrmex) by inhibiting their sucrose invertase.

In El Salvador the horn-shaped thorns provide the legs for small ballerina seed dolls which are worn as decorative pins.