- by A Sunjian and Li H.
| Sunjian A and Li H. (2006) Do Soldier "Barracks" in Atta leafcutter ants exist and are they adaptive? Notes from Underground 11 (2) |
(revision date: 2006/10/3)
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| A soldier aggregation in an Atta cephalotes nest. Click the thumbnail above to see detailed image. |
The major workers of Atta leafcutter ants are mainly specialized for colony defense. At the first sign of nest disturbance, these large individuals boil out of the nest in a mass and attack any intruders.
During several trips to photograph the captive colony of Atta cephalotes in the Central Park Zoo in New York City, NY (USA) we noticed that many major workers inside the nest were not randomly distributed throughout the available space. Instead, they seemed to aggregate together in loose clusters, forming "barracks" of relatively immobile major workers surrounded by masses of smaller ants. We also noticed this behavior in another captive colony of Atta cephalotes in the National Zoo in Washington, DC.
In order to quantify this behavior, we marked off a two-dimensional 20 cm by 20 cm area in the A. cephalotes colony housed in the Invertebrate building of the Smithsonian National Zoo in Washington, DC on 2006/10/2. Within a two minute period, we noted down the locations of all major workers present in that area, as well as the presence of congregated groups or "barracks" of these major workers. In this case, "barracks" were defined as having at least 5 or more major workers grouped within 5 cm of one another.
We found 5 isolated major workers in the area, and groups consisting of approximately 8, 15, and 20 major workers that formed defined "barracks". Although the vast majority of major workers located in "barracks" remained unmoving during the short time period, there was some difficulty involved in precisely counting individuals because of the masses of smaller ants moving around them.
If such aggregations do exist in natural colonies, and are not simply an artifact of the man-made situation in captive colonies, then we propose that this possible emergent behavior may be adaptive for species with specialized major workers in their workforce.
The adaptive advantages of such soldier aggregations may be two-fold:
The first possible advantage is perhaps self-explanatory. It can be somewhat facetiously compared to the blitzkrieg tactics of WWII, where large numbers of tanks are brought to bear against an enemy in a ferocious and unstoppable mass, instead of hurling the armored behemoths individually at the enemy.
The second possible advantage deserves some explanation.
When disturbed or when in danger of attack, leaf-cutting ants exhibit an aggressive alarm reaction and release an alarm pheromone from their mandibular glands (Hughes and Goulson, 2001). Information about the possible danger is usually spread into the nest interior by workers who lay down pheromone trails in a process called alarm recruitment (Holldobler and Wilson, 1990). When they contact other members of the colony, they transmit the alarm information to these workers, who in turn may proceed to spread the alarm information themselves.
It is possible that the presence of clusters of major workers results in the faster spread of the alarm information to all these individuals compared to a situation where the major workers are randomly distributed among the general population.
In order to test this hypothesis, it will be necessary to create and run models of the competing processes. We propose to use NetLogo (Wilensky 1999) to create these models and test the hypothesis.
Literature Cited
Holldobler B, Wilson EO (1990). The Ants. Belknap Press, Cambridge, MA.
Hughes WOH, Goulson D. (2001). Polyethism and the importance of context in the alarm reaction of the grass-cutting ant, Atta capiguara. Behav Ecol Sociobiol (2001) 49:503–508.
Wilensky U (1999). NetLogo: http://ccl.northwestern.edu/netlogo/