- by A Sunjian and Li H.
| Sunjian A and Li H. (2005) Task partitioning during nest excavation in the leafcutter ant Atta mexicana. Notes from Underground 11 (1) |
(revision date: Nov 15, 2004)
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| Fig 1. Growing nest mound showing routes of primary (red) and secondary (green) soil transporters. |
This note details a possible instance of task partitioning during nest excavation in the leafcutter ant Atta mexicana in Nuevo Vallarta, Mexico, in November 2003.
Task partitioning refers to a phenomenon wherein tasks are subdivided into sequential subtasks linked by material transfer (Hart et al. 2002). Leafcutter ants have recently become a model organism for studying this component of work organization in social insects, with numerous papers detailing the use of task partitioning in the collection and transport of leaves (Roschard and Roces 2003; Hart and Ratnieks 2001a; Anderson and Jadin 2001); in leaf processing (Holldobler and Wilson 1990); in waste management (Hart and Ratnieks 2001b; Hart and Ratnieks 2002); and possibly in colony emigration (Hart et al. 2002). However, there have so far been no reports of task-partitioning in leafcutter ants during nest excavation, although such an activity has been discovered in a few other ants, including the army ant Neivamyrmex legionis (Anderson and Ratnieks 2000).
In November 2003, the authors spent two weeks filming the daily activities of numerous Atta mexicana colonies in Puerto Vallarta and Nuevo Vallarta using a Canon Ultra miniDV camcorder. Included in this comprehensive sampling was a segment showing a large colony in the preliminary stages of excavating a foraging or nest hole. The colony resided on the tree-filled border between the heavily-trafficked road of Paseo de las Palmas and a sidewalk. The new nest hole itself lay between two clusters of large nest mounds, and at the time of filming, the growing heap that half-surrounded it consisted of a crescent-shaped spray of piled soil with a radius of approximately 30 cm (see figure 1). Large numbers of ants carrying packed earth were exiting from the nest hole, climbing the growing half-mound, and then releasing their loads on the outward face of the structure.
The vast majority of soil transporters followed this set routine, with individual ants releasing their loads at the top or outer face of the heap, then going directly back into the nest hole. However, the authors also noted that there were several ants that deposited their burdens at the base of the inner face of the mound, before turning back and going into the nest (see figure 1, red arrow). More intriguingly, at least a couple of ants came down from the top of the mound, retrieved soil particles from this area, then returned to the top to deposit their new loads (see figure 1, green arrow). Unfortunately, due to the frenetic pace of the activity, the authors could not determine how many times these ants functioned as "secondary transporters" of the excavated material. Short clips in Quicktime format are available.
These observations suggest that the leafcutter ant A. mexicana utilizes task-partitioning during excavation of new nest or foraging mounds, although the seeming rarity of the process indicates this work organization is not obligatory for the activity. "Primary transporters" of excavated soil relieved themselves of their burdens near the nest hole, at the base of the inner surface of the growing mound, while "secondary transporters" collected packed material from this area and transported them to the top of the heap, where they were finally deposited or dropped.
The authors also noted some smaller minim ants on the outer surface of the growing mound who seemed to be wandering aimlessly and occasionally poking at the soil pellets dropped by their nestmates. It would be interesting to find out whether these workers were intent on reorganizing the excavated material, much like specialized workers in other leafcutters turn over internal and external waste heaps (Hart and Ratnieks 2002).
Literature Cited
Anderson C and JL Jadin (2001) The adaptive benefit of leaf transfer in Atta colombica. Insectes Soc 48:404-405
Anderson C and FLW Ratnieks (2000) Task partitioning in insect societies: novel situations. Insectes soc. 47 (2000) 198–199
Hart AG and FLW Ratnieks (2001a) Leaf caching in the leafcutting ant Atta colombica: organizational shift, task partitioning and making the nest of a bad job. Anim Behav 62:227-234
Hart AG and FLW Ratnieks (2001b) Task partitioning, division of labour and nest compartmentalisation collectively isolate hazardous waste in the leafcutting ant Atta cephalotes. Behav Ecol Sociobiol (2001) 49:387–392
Hart AG and FLW Ratnieks (2002) Waste management in the leafcuttng ant Atta colombica. Behav Ecol 13:224-231
Hart AG, Anderson C, and FLW Ratnieks (2002). Task partitioning in leafcutting ants. Acta Ethol 5:1-11
Holldobler B and EO Wilson (1990) The Ants. Harvard University Press/Belknap Press, Cambridge , Mass.
Roschard J and F Roces (2003) Cutters, carriers and transport chains: Distance-dependent foraging strategies in the grass-cutting ant Atta vollenweideri. Insect Soc 50:237-244