Sperm competition and small size advantage for males of the golden orb-web spider Nephila edulis

Patterns of sexual dimorphism in dioecious organisms depend on the relative strengths of a variety of selection pressures (Andersson, 1994; Mùller, 1994). For example, selection may favour large female size if this ensures greater fecundity, parental care or dominance in contests over resources. Typically, sexual selection is thought to in ̄uence sexual size dimorphism in the context of female choice and/or male±male competition, and most studies have focussed on the bene®ts of large body size (Blankenhorn, 2000). Thus, large male body size may be favoured if females prefer larger males, or larger males dominate in physical contests over females (Andersson, 1994). Under conditions of scramble competition, sexual selection may favour small body size if smaller, early emerging males are more likely to ®nd females than later males (Andersson, 1994). However, multiple mating by females allows sexual selection to persist after mating has taken place (Mùller, 1998). If females store sperm from several mating partners, then these sperm may directly compete for fertilization inside the female. Numerous studies have identi®ed an extraordinary diversity of mechanisms by which males attempt to ensure that the eggs of their mating partners are not fertilized by rival males (see Birkhead & Mùller, 1998). However, the relationship between sperm competition and male body size is still poorly understood (Simmons & Siva-Jothy, 1998).