Bullhorn Acacias

Bullhorn Acacia (Acacia cornigera)

On my recent trip to Mexico, we saw lots of bullhorn acacias (Acacia cornigera), particularly in the sand dune areas at Cansaburro. They are attractive shrubs that are named for their prominent swollen thorns that resemble a bull's horns.

Yes, the thorns really do resemble a bull's horns.

Bullhorn acacias are noteworthy for their complex, mutualistic relationship with one particular ant species, Pseudomyrmex ferruginea. Most acacia species contain lots of bitter alkaloid compounds in their leaves and stems, which act to deter browsing by various herbivores. Bullhorn acacias lack these chemical defenses, but have lots of biting, stinging ants living on them. The ants hollow out the large thorns and nest in them.

Pseudomyrmex ferruginea

This hollowed-out thorn contained an active ant nest

The mutualistic relationship between the ants and the acacias was studied by biologist Thomas Belt, who described the interaction in his charming 1923 book The Naturalist in Nicaragua. The ants deter herbivores with their painful bites and stings. They also remove seedlings that sprout around the base of the tree, thus eliminating potential competitors for sunlight. In return the trees provide food for the ants. Protein-rich nodules called Beltian bodies form at the tips of newly-unfolding leaves. These are harvested and eaten by the ants.

Pseudomyrmex harvesting Beltian bodies

The acacia trees also provide carbohydrates from foliar nectaries that form at the base of some of the leaves. The ant/acacia relationship has a long evolutionary history, with both partners showing a considerable dependence on one another. Colonies of this species of ant are only found living on acacia bushes. Conversely, acacias that have been experimentally cleared of their ant colonies suffer considerably more herbivory than bushes that retain their ants.

Foliar Nectary

I was a bit surprised at the lackadaisical nature of the acacia ants that I encountered in Veracruz. Shaking the bushes, or even giving them a stiff whack with the handle of my butterfly net only produced a small amount of rather tepid ant activity. I contrast this with my encounter with another ant-protected tree some years ago in Costa Rica. In that case, the tree involved was a cecropia. When I struck the trunk just once with a stick, thousands of tiny ants boiled out of small holes in the bark.