Aggregations of male Bombus muscorum (Hymenoptera: Apidae) at mature nests. Incestuous brothers or amorous suitors?

Abstract

Aggregations of male bumblebees are occasionally observed at the nest entrances of conspecifics. It has always been assumed that they originate from nearby nests and are hoping to mate with emerging gynes. However, it is possible that they are males from within the nest that have not yet dispersed, or are hoping to mate with their sisters. Inbreeding in Hymenoptera with single locus complimentary sex determination (sl-CSD) is costly and should be avoided. Nevertheless, other taxa with sl-CSD have been found to inbreed in this way. We use microsatellites to compare aggregating males with workers from within a Bombus muscorum nest. All males have genotypes inconsistent with origination from within the nest. Estimates of F(is) are near zero, indicating low rates of non-random mating. Clearly male bumblebees can detect mature nests, and congregate in the hope of mating with virgin queens. These data suggest that mechanisms may exist to avoid brother-sister matings at the nest, and perhaps beyond.