Caste and division of labor in leaf-cutter ants (Hymenoptera: Formicidae: Atta)

Summary

More than 90% of media workers with head widths 1.8–2.2 mm, comprising the largest and energetically most efficient segment of the foraging force, were removed from four Atta cephalotes colonies containing approximately 8,000 workers. Contrary to expectation, the colonies did not respond by adding workers from adjacent size classes (head width ≦1.6 mm, ≧2.4 mm) to the foraging force (Table 1, Fig. 1). Yet the rate of leaf harvesting remained unaffected, due to the fact that excess workers in the adjacent size classes were already present on a stand-by basis in the foraging area (Table 2, Fig. 2). Moreover, the few survivors in the 1.8–2.2 mm group increased their individual activity by approximately 5x. In control and undisturbed colonies, the prime foragers in the 1.8–2.2 mm group tended to displace the others from the edges of the leaves, where most cutting takes place. When these individuals were removed, the auxiliaries participated more actively.

Brood development was followed through 2 full development cycles after the excision. No differential increase of the extirpated worker class (1.8–2.2 mm) could be detected in the experimental colonies when compared with sham-treated control colonies. As a consequence, this group remained underrepresented in the foraging arenas by about 50% at the end of the first cycle (8 weeks), but was fully replenished by the end of the second cycle (16 weeks).

The size-frequency distributions of the worker pupae belonged to one or the other of three patterns among both experimental and control colonies: unimodal at head width 0.8 mm, unimodal within 1.4–2.0 mm, or bimodal at these two respective positions (Fig. 3). Several colonies shifted from one pattern to another during a single brood cycle (Table 3). A hypothesis of the control of the size-frequency distribution has been developed consistent with these results.