Reproductive competition in the context of a superorganism

Abstract

Population density is a fundamental ecological factor that shapes the intensity of competition within a species. Increased density often intensifies competition for resources and mates, driving the evolution of diverse adaptive responses in behaviour, morphology and physiology. A key area where density-mediated competition manifests is in sperm competition. This occurs when sperm from multiple males compete to fertilise the ova of a female. Species facing intense competition exhibit a range of adaptations, including adjustments in sperm production, ejaculate characteristics, and mating strategies. Elevated population density is a major factor that can exacerbate sperm competition, further amplifying these evolutionary pressures. Within this context, this thesis investigates the effects of apiary density on reproductive strategies and drone physiology in the honey bee (Apis mellifera), a highly social insect that exhibits a complex interplay between individual and colony- level responses. Apiary density, characterised by the number of colonies within a defined area, was manipulated across three treatments: low (8 colonies), medium (60-68 colonies), and high (120 colonies). Drone brood production and comb allocation (both drone and worker) were measured monthly to assess colony-level investment in reproduction versus worker production. Total sperm count and the proportion of viable sperm were assessed in drones collected from each density treatment to evaluate sperm competition dynamics. Heat Shock Protein 70 (HSP70) levels, a biomarker of cellular stress, were quantified in drones from each treatment using an enzyme-linked immunosorbent assay. Colonies in high-density apiaries exhibited significantly increased drone brood production and higher total sperm counts compared to those in low-density apiaries. However, worker comb production was significantly reduced in high-density apiaries, suggesting a trade-off between resource allocation towards reproduction and colony maintenance. Sperm viability varied across densities and seasons, with low-density treatments having a higher proportion of viable sperm in early summer and medium-density treatments showing higher viability in late summer. This suggests that while high-density may favour increased sperm quantity, lower densities might facilitate greater investment in sperm quality. Apiary density significantly predicted HSP70 concentration in drones, with drones from high-density apiaries displaying significantly elevated levels, indicating increased physiological stress. These findings suggest that colonies in high-density environments experience intensified sperm competition, responding by increasing drone and sperm production. However, the observed variation in sperm viability and reduced worker comb production implies that this augmented reproductive investment may compromise overall colony function. The elevated HSP70 levels further corroborate that drones in high-density apiaries are subjected to greater physiological stress. Crucially, this study provides a unique perspective by demonstrating that apiary density, a factor often controlled in beekeeping, acts as a significant driver of reproductive strategies and physiological stress at both the colony and individual drone level within a superorganism. This highlights the profound impact of a managed environmental parameter on the evolutionary dynamics of a highly social species, offering critical insights for sustainable apicultural practices.