Effects of Male age and Size on Mating Success in the Bumblebee Bombus terrestris

Abstract

In social insects, the reproductive strategies adopted by colonies emerge as a complex property of individual behaviours, but as yet we are often unable to fully explain them in evolutionary terms. In bumblebees, colonies adopt either a short-lived strategy specializing in male production, or a longer-lived strategy in which mainly new queens are produced, but this results in males emerging long before mates are available; this strategy can only easily be explained if older males are at a significant reproductive advantage. Here we examine how age and morphological characters influence mating success of male bumblebees. In two separate experiments in which single virgin males and females were confined together, we found that young males and heavy males mated more swiftly and copulated for less time compared to old males or lighter males, respectively. However, in competitive situations age proved to be unimportant and the only factors to influence mating success were the lengths of the fore and hind tibiae, with strong directional selection for long leg length. Fore and hind legs are both used in courtship, so both traits are associated with plausible mechanisms under selection. It has previously been argued that, in times of food stress, bumblebee colonies should produce males as male size is less likely to be strongly correlated with fitness than female size. Our results suggest that this may not be so, since aspects of male size directly impact on their mating success. Our results leave unexplained the emergence of males many days before new queens.