Thomas Belt was an engineer who spent five years supervising a gold mining operation in Nicaragua. While there, Belt kept a naturalist's journal. He was the first to describe the relationship between ants and bull's-horn acacia. Published in 1874, Belt's work was controversial.

Two years later, Francis Darwin (Charles Darwin's son) published his examination of the ant/acacia relationship. Theories that plants evolved to encourage specific insect inhabitants remained argued until the late 1960s. Daniel Janzen proved the mutualistic relationship between the ants and the Acacia trees.

Acacia sphaerocephala is one of the five species of "bull's-horn acacia". The ants, Pseudomyrmex ferruginea, are obligate on these species; they cannot exist without the trees. The trees barely exist without the ants; they do poorly and lose in competition to other plants.

A thorn is colonized by a single queen who chews a hole near the tip. Inside the thorn she will lay 15 to 20 eggs. As the colony grows, the ants inhabit other thorns. The colony becomes aggressive and protects the acacia when its population has reached about 400 members. The ants drive out other ant colonies and kill any insects browsing on the tree.

Pseudomyrmex act as caretakers. The ants keep the ground around the tree free of any other plants. They even prune branches of neighboring trees which threaten to shade the acacia. Janzen found that when ants were removed from Acacia, the trees became vulnerable to beetles, caterpillars, and other chewing insects. The acacias were overgrown by faster developing trees and vines.

Acacia sphaerocephala and other bull's-horns do more than provide homes for the ants. The compound leaves have a special gland near the base. These extrafloral (outside of a flower) nectaries exude sugar- and amino acid-rich nectar. The tips of the leaflets grow nutritive packets called Beltian bodies after Thomas Belt who first described these growths. The Beltian bodies are high in oils and proteins.

The larvae of the pseudomyrmecine ants have evolved a special pouch called a trophothylax. The trophothylax is located on the lower surface of the larva's thorax. Nurse ants place a Beltian body inside the trophothylax for the larva to feed itself.

The ants can recognize specific odors. They become 'scent-alert' when humans or cattle are in the vicinity of the tree. If the tree is touched, the ants swarm out of the thorns and sting the intruder.

To read more about these interesting insects, check out The Ants by Bert Holldobler and E.O. Wilson, (Cambridge: The Belknap Press of Harvard University Press, 1990)