Abstract
Some social insects produce workers of variable size and/or shape. One hypothesis to explain such variation is that it increases colony efficiency in performing diverse tasks, with greater efficiency leading to greater colony-level fitness. Using the granivorous ant Messor pergandei, which produces a unimodal distribution of worker sizes, I explored two corollaries of this “ergonomic” hypothesis: that there should be detectable task-matching and that the mean size should shift predictably with a change in task. In five experiments conducted in different years and sites in California deserts, colonies were baited with fragments of wheat or millet ground and sieved to different sizes. In six of eight colonies so baited the minimum linear dimension or mass of seeds carried by foragers was significantly related to forager size, with r2 values ranging from 3 to 30%. In four of the five experiments, colonies were subsequently baited with either large seed fragments alone or small fragments alone. In three of these four experiments I detected a rapid response, over a few hours or days, in which foragers deployed by colonies shifted to larger or smaller mean size in directions predicted by size of the bait. These results suggest that an ergonomic function is one contributor to worker size polymorphism in M. pergandei. Although task-matching did not explain a substantial amount of the variance in forager size in all experiments, the rapid size shifts provide behavioral evidence that task-matching is important to colonies.
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Waser, N.M. Task-Matching and Short-Term Size Shifts in Foragers of the Harvester Ant, Messor pergandei (Hymenoptera: Formicidae). Journal of Insect Behavior 11, 451–462 (1998). https://doi.org/10.1023/A:1020963000112
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DOI: https://doi.org/10.1023/A:1020963000112