Allomerus octoarticulatus – Puca curu

Allomerus ant

Another, less conspicuous ant-plant association occurs between Cordia nodosa (native name: huevo de gato) and Allomerus octoarticulatus. Actually, there are three known genera of ants that have a symbiotic relationship with this plant, Azteca, Allomerus and Myrmelachista, however the most abundant ant is the A. octoarticulatus.  This plant is the most abundant ant-plant, also known as a Myrmecophite plant, found in South-eastern Peru (Yu and Pierce 1997), growing less than 2 meters in size, it has specialized stem swellings in which the ants are housed, known as domatia.

The relationship between the ants and the plants begin when the plant is in its sapling stage. As it grows, the number of domatia increases, and so this can be used as a good indicator of plant and ant colony size (Yu and Pierce 1997). Each colony occupies all or most of the plants’ domatia (Wheeler 1929). Colonies begin occupying the plants when the first domatium is produced (Frederickson 2006). They then reach reproductive size when the plant has about 5 domatia and the plant is said to reach adult size when it has 11 domatia.  In exchange for the ants’ protection of the tree against herbivores and neighboring plant competitors, the plant provides the resident ant colony with food bodies and housing.

The queen initially penetrates the domatia by making a perforation in the thinner portion of the wall below the node, when the plant is about 3-5 ft tall, shortly after the walls begin to lignify. Once the initial hole is made by the queen, the first brood of workers often needs to reopen it to make their exit as it tends to close as the plant grows.

A strange behaviour is associated to the species of A. octoarticulatus. All is normal in the relationship between the ants and the plant, until it comes time for the plant to reproduce. When the flowers from the plant begin to appear, the ants attack the buds, before they can spread their seeds. The workers attack the flowers and fruiting bodies of their host plant, inhibiting its reproduction. This is a very good example of what is known as ‘cheating mutualism’ (Yu and Pierce 1997). This attacking behaviour seems illogical if you realize that this same ant is protecting the leaves of the plant, and especially so, when considering that the plant parts are not ingested by the insect. The buds of C. nodosa are attacked only once they reach full size and the ants can reduce the total fruit production of the plant by 80%. In an 11-month study by Yu and Pierce, where over 1000 C. nodosa plants were surveyed, about 70% of these had no fruit production.

The reason for this strange behaviour is that the castrated plants distribute the energy that they otherwise would have allocated to reproduction, to the plant’s growth. If you consider the relationship as one having some kind of conflict of interest, it might be simpler to understand. On one hand, the tree needs to provide the adequate conditions for the ant colony to survive and grow (i.e. food and shelter from domatium and fruiting bodies). On the other hand, the tree needs to ensure its own reproduction, by growing flowers and fruits. This means that the energy is split between the ants and the plant, which is not what the ants would prefer.

Therefore, the ants castrate the plant, causing all the energy to be allocated to the plants growth. With increased plant growth, there are more domatium and so the colony can grow larger and have greater overall fitness. So, with this behaviour, the ants gain more room to expand their colony, but unfortunately the plant suffers.

This behaviour might lead some to characterize the ants as parasites, however, since the plants are known to have a lifespan of 77 years, while the ant colonies only live about 7-14 years, this wouldn’t be considered as such. According to Frederickson, the ants might help the plant to survive and grow, until the next ant colony comes to inhabit the plant. This next colony would be one that would not castrate the plant, and thus allow it to reproduce.

References:

University of Chicago Press Journals. “When Industrious Ants Go Too Far.” ScienceDaily 1

May 2009. 13 July 2009  <http://www.sciencedaily.com/releases/2009/04/090428111535.htm&gt;.

Edwards, D.P. et al. (2009). Selection for Protection in an Ant-Plant Mutualism : Host

Sanctions, Host Modularity, and the Principle-Agent Game. Royal Society, 273: 595-602.

Edwards, D.P. et al. (2007). Protection in an Ant-Plant Mutualism : an adaptation or sensory trap?. Animal Behaviour, 74: 377-385.

Frederickson et al. (2009). Conflict over Reproduction in an Ant-Plant Symbiosis: Why

Allomerus octoarticulatus Ants Sterilize Cordia nodosa Trees. The American Naturalist, 173 (5): 675-681.

Frederickson, M.E. (2006). The Reproductive Phenology of an Amazonian Ant Species

Reflects the Seasonality Availability of its Nest Sites. Oecologia, 149:418–427.

Frederickson, M.E. (2006). Ant species confer different partner benefits on two neotropical myrmecophytes. Oecologia, 143: 387-395.

Solano, J-P. et al. (2005). Formation and structure of food bodies in Cordia nodosa. Comptes Rendus Biologies, 328: 642-627.

Wheeler, W.M. (1928). Two Interesting Neotropical Myrmecophytes (Cordia nodosa and C.

alliodora). International Congress of Entomology, 2: 341-353.

Yu, Douglas W., and N.E. Pierce. (1998). A Castration Parasite of an Ant-Plant Mutualism.

The Royal Society, 265: 375-382.