Sexual cannibalism, competition, and size dimorphism in the orb-weaving spider Nephila plumipes Latreille (Araneae: Araneoidea)

The degree and direction of sexual dimorphism varies widely, but in several taxa of orb-weaving spiders, including Nephila, males may be less than one-tenth the size of females. This difference is commonly attributed to selection through precopulation sexual cannibalism: females may either fail to detect very small males, or ignore them as potential prey items. However, there is often the potential for male-male competition in these species because several males can be found on the web of a single female. We investigated experimentally the effects of sexual cannibalism and male-male competition on male body size and hence sexual dimorphism in the Australian golden orb-weaver (Nephila plumipes). Small males were less likely to be detected and cannibalized than larger males. However, larger males excluded small males from the central hub of the web, where mating takes place. The conflicting effects of sexual cannibalism and male-male competition may be responsible for the relatively large variation in male body size in this species.