The mating systems of social insects range from monandry, through low levels of polyandry to extremely high levels of polyandry. While the distribution of these mating systems across taxa is increasingly well-known, little is known about the proximate reasons behind the number of times social insect queens mate. Here we examined the role of mating duration and mating partner in predicting multiple mating in the naturally polyandrous bumble bee, Bombus hypnorum. We found that, as mating duration decreased, the likelihood of re-mating increased. Weak evidence suggested that male colony-of-origin may also influence re-mating. Dissections showed that males transferred a mating plug to queens. The plug does not last as long as in the congener B. terrestris, and males of different colonies varied in the allocation of resources to plug production. These results provide insight into the proximate factors behind multiple mating in bumble bees.