Conflict over male production in stingless bees
The social insects provide one of the best models for studying the evolution of cooperative behavior. Kin selection theory explains why a social insect might help relatives instead of trying to reproduce alone, but a more difficult problem remains unsolved. Individuals, once in groups, should often be selected to prefer being a reproductive rather than a helper. But if this is so, why is reproduction so often limited to one queen (or a few queens), particularly in large colonies where the queen cannot hope to physically dominate all her subordinates? A likely hypothesis is that average or collective worker interests determine who reproduces, including the possibility that workers often supress each other.
One test of this theory to date is in the honey bee, where extreme multiple mating by the queen causes workers to be more related to her sons than to the sons of other workers. The Strassmann/Queller group studied worker preferences with respect to male production in a related group, the stingless bees (Apidae: Meliponinae). They have previously found that stingless bee queens are generally singly mated, which causes workers to be less related to the queen’s sons than to each others¹ sons. The theory of collective worker control therefore predicts that stingless bees will differ from honey bees: workers should lay the male-destined eggs in stingless bees. We found that this was the case, with complications.
For each species, genotypes of highly variable microsatellite loci were used to confirm that the queen is singly mated, and to determine the fractions of males derived from the queen and from workers. These results were related to behavioral observations of conflict during the cell provisioning and oviposition process of each species, a period of highly ritualized behaviors that varies extensively among species. We confirmed the hypothesis that behavioral conflict is highest in species where both queens and workers produce some of the males, less high in species where both parties can produce males but only one does, and lowest in species where workers cannot produce males.
We discovered that:
- Predicted conflict between workers and the queen over male parentage because of relatedness asymmetries in stingless bees actually occurs and has different outcomes in different species, including workers opening closed cells to sneak in their own eggs (Tóth et al. 2002a, 2002b, 2003, 2004).
- Stingless bee queens mate with a single male, in contrast to honeybee queens, which mate with many males. In accord with theory, stingless bee workers tolerate virgin queens in the nest, attempt to produce the males, and a daughter queen leaves the nest during swarming (Peters et al. 1999).