Foraging specialization without relatedness or dominance among co-founding ant queens

Abstract

HYPOTHESES on the evolution of sociality in the Hymenoptera have focused on two non-exclusive selective processes. First, individuals may help relatives to enhance inclusive fitness (kin selection1,2). Second, group living may be so highly advantageous that competitively inferior individuals are forced into subordinate roles through social competition3–7; in this hypothesis, subordinates help dominants in the expectation that they may benefit from the group's resources if the dominants lose status or die. Many social Hymenoptera associate predominantly with close relatives8,9, which precludes an effective comparison of kin selection and social competition. Here we report on the existence of foraging specialists among unrelated co-foundresses of the leaf-cutter ant Acromyrmex versicolor; such task specialization leaves the forager at a relative fitness disadvantage within her foundress association. Contrary to the predictions of social competition theory, individuals specialize independently of competitive ability (as measured by relative body size) or reproductive status (as measured by ovarian condition) and without conflict. The selective basis of foraging specialization may lie in the intense competition that occurs among newly founded colonies engaged in reciprocal brood raiding.