The Bullhorn Acacia, Acacia cornigera, and Its Fearless Allies

Bullhorn acacia and its thorns.
Bullhorn acacia and its thorns.
A closer look on bullhorn acacia thorns.
A closer look on bullhorn acacia thorns.
Another look on thorns .
Another look on thorns .

The medium size tree, up to 15 m high, Bullhorn Acacia, Acacia cornigera, native of Central America, has long, strong and swollen thorns. The thorns are grouped in pairs at the base of the leaves and are connected at the base and pointed in opposite directions resembling the horns of a bull. Besides being extremely sharp which suggests one being careful when touching this plant these thorns are also feared for another reason. As it matures and becomes an adult, the mated acacia queen ant, Pseudomyrmex ferruginea, looks for these thorns and digs a hole at the base of the thorns. Its goal its to reach the hollow center of the thorns base where it will lay its eggs. After being born the young worker ants get out of their thorny nest and patrol the stem and young leaves nearby everyday. These stinging ants hunt down any insect that dares to eat the bullhorn acacia young fresh leaves. In fact, these ants are so aggressive that they attack any animal or even humans that touches or harms the tree. When bullhorn acacia is wounded it releases chemicals into the air as an alarm signal that the ants detect and respond to rapidly. The ants release an alarm pheromone whenever a threat is detected and rush out of their thorn "barracks" in great numbers. Scientists think that livestock and other foraging animals can apparently smell the pheromone and avoid these acacias. In this way the ants serve bullhorn acacia interests in keeping its leaves intact. As the queen ant continues to lay eggs at the base of each pair of thorns the colony grows and spreads to neighbouring branches. On doing this it is quite common that two different colonies of this ant species meet on the same tree. Then war begins. It is really a bloody war that can last for quite sometime and usually the biggest colony wins. The ant colonies are not just fighting for their thorny nests they also fight for the food that tree provides. The bullhorn acacia does not leave its valuable and tiny fearless guests with empty hands. The antes are in fact quite well rewarded by the plant. The bullhorn acacia provides two types of food. First, it has well located extraforal nectaries at the lower part of base of its leaves. These nectaries exude continuously sweet nectar, derived from phloem sap, all year round. Second, the bullhorn acacia supplies solid food in the form of protein-lipid nodules called Beltian bodies from its leaflet tips. These perfectly distinguishable orange nodules have no known function other than to provide food for the symbiotic ants and their larvae. In fact both species are so well adapted to each other that ant larvae have a special pouch, on the lower side of their bodies bellow the head, in which the worker ants put pieces of the Beltian bodies for the larvae to eat and store.

Beltian bodies on bullhorn acacia leaves.
Beltian bodies on bullhorn acacia leaves.

The protective nature of the ants towards the bullhorn acacia is not only against any harmful insect that threatens to eat the bullhorn acacia young and fresh leaves. The ants seem to do much more for the benefit of the host tree and not on their own interest. The ants also target any plant that stands in the bullhorn acacia way. The acacia ants travel along the tree base, trunk and main branches regularly, and they destroy any plant seedling within a radius of 30 cm from the acacia trunk keeping the acacia free or nearby competitors for water and nutrients from the soil. The ants are also aware of any plant that may come through air. Whenever a branch of a neighbouring plant touches the bullhorn acacia the ants pass to that neighbour plant and destroy the invading branch apex so it can no longer grows. It is hard to measure what benefits the most from this symbiotic and mutualistic relationship between the acacia ants and the bullhorn acacia. The main benefit that the tree gets is that it is free from insects that would damage its leaves and suck its sap. This enormous advantage was demonstrated by removing all the thorns and ant nests of the bullhorn acacia. Being thornless and without ants the bullhorn acacia showed itself vulnerable and life threatened when compared with a normally protected tree. One factor that may account for this disadvantage, when made thornless and without its protective ants, is that bullhorn acacias are deficient in the bitter alkaloids usually located in their leaves that defend them against ravaging insects and animals. Acacia ants, Pseudomyrmex ferruginea, also establish similar symbiotic relationships with tow other Central American acacia species, Acacia sphaerocephala and Vachellia collinsii, all collectively known as bullhorn acacias together with Acacia cornigera. It has also been observed that a single ant colony can spread over several bullhorn acacias. In Africa, a similar mutualism occurs between the Whistling Thorn acacia, Acacia drepanolobium, and ants from the species Crematogaster mimosae.

Bullhorn Acacia and Its Ants

Central America

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