Ants and Acacias

The swollen thorn acacia (Acacia cornigera) is distributed throughout Central America entwined in an obligatory mutualistic relationship with a species of acacia ant (Pseudomyrmex ferruginea). Plant species that have colonies of ants living within the internal and external structures have been given the term myrmecophytes (Janzen, 1966). The acacia ant and plant provides essential benefits for each symbiont concerning protective and reproductive purposes.

The swollen thorn acacias acquire their name from key characteristic features including: enlarged stipular thorns, enlarged foliar nectaries, modified leaflet tips known as Beltian bodies which are consumed by the ants, and year round leaf production. It is to be noted that the obligate acacia ants are not species specific rather to the swollen acacia life form (Janzen, 1966). Initially a queen ant is attracted by a distinct odour and selects to nest inside the tree, specifically the hollow acacia horns. The queen then lays her eggs within the hollowed thorn whereby they hatch and become the first generation workers, the colony continues growing in size to adequately provide protection for the acacia from many insects and mammals that may wish to feed on the plant. The ants repel epiphytic vines by sectioning tendrils that come into contact with the acacia. A. cornigera provides sugar rich nectar from nectaries and beltian bodies which are collected and stored by the ants as a food resource for the ant larvae. The ants have also been known to remove small seedlings around the base of the acacia as potential competitors for sunlight and other essential resources (Risch et al., 1977).

The acacia ant colonies are continuously increasing in size until a population of over 4000 exists which takes a minimum of three years to establish. In recent experiments it has been shown that an acacia plant will become subject to large amounts of damage from herbivorous behaviour if the pair are separated (Hölldobler, 1990). The benefits each receive from the relationship are essential to each symbionts survival and success.

 Video with thanks to National Geographic

https://www.youtube.com/watch?v=Xm2qdxVVRm4

References:

HÖLLDOBLER, B. 1990. The ants, Harvard University Press.

JANZEN, D. H. 1966. Coevolution of mutualism between ants and acacias in Central America. Evolution, 249-275. 

RISCH, S., MCCLURE, M., VANDERMEER, J. & WALTZ, S. 1977. Mutualism between Three Species of Tropical Piper (Piperaceae) and Their Ant Inhabitants. American Midland Naturalist, 98, 433-444.