J. L. Reyes and J. Fernandez Haeger, in their recently published paper "Sequential co-operative load transport in the seed-harvesting ant Messor barbarus", Insectes Sociaux 46(1999):119-125, describe the ant as using bucket brigades to carry seeds back to the colony. The smallest, slowest ants forage out farthest. When carrying a seed back toward the nest, such an ant may be interrupted by a larger, faster ant, who wrests the seed from them and continues carrying it towards the nest. After the largest ant leaves the seed at the nest, she goes back out to get another.
Figure 1: In this figure from "Insectes Sociaux", the slower, smaller ants 1 and 2 are conveying seeds toward the nest, while ant 3, the fastest, has delivered a seed and is returning to take the seed from ant 2.
This raises an interesting question: How do bucket brigades arise? We offer an explanation in C. Anderson, J. J. Boomsma, and J. J. Bartholdi, III (2002); "Task partitioning in insect societies: bucket brigades", Insectes Sociaux 49(2). First we make the following assumptions, which are consistent with the observations of Reyes and Fernandez Haeger (and others).
Now under Assumptions 1 and 2, bucket brigades arise spontaneously if each forager follows this simple, myopic rule:
The Foraging Rule: If you are without a food item, run out along the foraging trail until you encounter one and then take it if you can, even if you must wrest it from another ant, and carry it back toward the nest.
Consider the experience of a large forager. As it leaves the nest, it is likely that the first returning forager it encounters is smaller, and so our ant will successfully wrest the food item away and return to the nest. It is unlikely to meet a still larger ant on the way back. Subsequent trips are likely to repeat this experience.
Similarly, consider the experience of a smaller ant. It is likely to have to travel for a long time before it encounters an even smaller forager that is returning with a food item; in fact, it may have to travel all the way out to the food source to get a food item. As our small ant returns with a food item, it is likely that any forager it encounters will be larger and will take the food item, after which our small ant will return to the food source. Again, subsequent trips are likely to repeat this experience.
The result is that the ants will sort themselves from slowest (smallest) to fastest (largest) along the direction of seed movement towards the nest.
In the following Java simulation the food source is at the left and the nest is at the right. Ants travel unladen to the source (right to left); and they return laden to the nest (left to right).
Larger ants are represented by larger marks. To be consistent with field observations:
Initially, all ants begin at random positions and directions (either toward or away from the nest). You can re-place the ants at random positions and directions by clicking anywhere on the simulation panel.