Ritualised fighting in Iridomyrmex purpureus

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The meat ant species Iridomyrmex purpureus is one the most abundant and obvious ants across much of southern Australia. They form large pebble-covered nests, with colonies consisting of numerous individual nests, these nests often connected by well-worn "highways". Colonies normally consist of about 7 individual nests but this can vary from only a single nest up to over a dozen nests, with a single queen per colony[1]. Nests are generally placed near food sources, minimising foraging times and making the colony more efficient, and colonies will abandon nests as food sources shift location[2][3].

It is well known that these ants are highly territorial with well defined boundaries being established between colonies[4]. These boundaries are not physical but are maintained by workers of neighboring colonies through highly stereotyped ritualised fighting, these confrontations sometimes lasting months and repeated over a number of years. These interactions are rarely fatal with the occasional minor injury being the most serious consequence for individual ants. While the vast majority of interactions between meat ant colonies are ritualised (some 95%), a few involve lethal fighting[5]. This lethal fighting seems to occur in situations where a foreign ant has invaded the territory of another colony and the fighting is initiated by the resident ant defending its colony. Thus these ants normally undertake ritualised fighting to establish and maintain colony boundaries with minimal injury to the colonies, but they will escalate to lethal fighting if their colony is under direct threat or attack.

Ritualised Fighting

Below is a flowchart of the interactions seen between workers involved in ritualised fighting. It should be noted that this is a rough guide and is not always followed. For example, appeasement can occur at most stages with the conflict ending, or stages can be undertaken for extended periods. In some cases stages may be skipped. Therefore, the following should be taken as a guide only and treated as typical but not the only pattern seen.

Iridomyrmex-purpureus-territorial-behaviour.jpg Shattuck 14452-web, Iridomyrmex purpureus territory defense, Canberra.jpg Antennation: Workers confront each other to determine if they are nestmates or non-nestmates. If non-nestmates are detected then ritualised displaying begins. Workers also stretch upwards to appear as large or tall as possible (although there is no evidence that size determines the outcome of the interaction, with small workers winning as often as large workers).
Shattuck 14433-web, Iridomyrmex purpureus territory defense, Canberra.jpg Mandibles gaped, antennation intensifed: Initial interactions include intense antennal contact and threatening mandiblular movements, but minimal direct contact. This is the first test of the strength of each worker.
Shattuck C27752, Iridomyrmex purpureus territorial display, near Bungendore.jpg Grab with mandibles: The workers then escalate the interactions with mandibular contact. There is little movement during this phase with workers standing relatively still while nipping at each other.
Shattuck C27725, Iridomyrmex purpureus territorial display, near Bungendore.jpg Side to side posture: If neither worker backs down, they move to a side-to-side position and "box" each other with their front legs. Again, there is minimal movement with each workers standing her ground.
Shattuck C27748, Iridomyrmex purpureus territorial display, near Bungendore.jpg Present tip of gaster: The side to side position increases with the involvement of the gasters. The gaster contains the defensive glands, a series of foul-smelling compounds. During this phase the workers slowly circle each other.
Shattuck C27788, Iridomyrmex purpureus territorial display, near Bungendore.jpg Appeasement: To acknowledge defeat, a worker will lower its position while the dominant worker remains stretched upwards. The interaction ends with both workers moving apart.
Shattuck 14443, Iridomyrmex purpureus territory defense, Canberra-web.jpg Break contact, groom: After the interaction ends both workers take a few minutes to groom before searching for another ant and beginning the cycle again.

References

  1. van Wilgenburg, E., Mulder, R.A., Elgar, M.A. 2006. Intracolony relatedness and polydomy in the Australian meat ant, Iridomyrmex purpureus. Australian Journal of Zoology, 54, 117–122.
  2. Greaves, T., Hughes, R.D. 1974. The population biology of the meat ant. Journal of the Australian Entomological Society, 13, 329-351
  3. van Wilgenburg, E., Elgar, M.A. 2007. Colony structure and spatial distribution of food resources in the polydomous meat ant Iridomyrmex purpureus. Insectes Sociaux, 54, 5–10.
  4. Ettershank, G., Ettershank, J.A. 1982. Ritualised fighting in the meat ant Iridomyrmex purpureus (Smith) (Hymenoptera: Formicidae). Journal of the Australian Entomological Society, 21, 97-102.
  5. van Wilgenburg, E., van Lieshout, E., Elgar, M.A. 2005. Conflict resolution strategies in meat ants (Iridomyrmex purpureus): ritualised displays versus lethal fighting. Behaviour, 142, 701-716.