SEXUAL DIMORPHISM AND DISTORTED SEX-RATIOS IN SPIDERS

SEXUAL dimorphism in body size is widespread in the animal kingdom. Whereas male giantism has been studied and explained extensively1,2, male dwarfism has not. Yet it is neither rare3-7 nor without theoretical interest8,9. Here we provide experimental and comparative data on spiders to support the theory that dwarf males are associated with high differential adult mortality, with males at much greater risk. Species with sedentary (low-risk) females have dwarf, roving (high-risk) males. Life-history theory could readily explain dwarfing if juvenile, but not adult, male mortality were large. We present a new model in which high mortality of searching mature males reduces the adult sex ratio (males: females), relaxing male-male competition and reducing the importance of male body size to favour dwarfing by early maturation. Early maturity also reduces male juvenile mortality and thus opposes adult mortality. This provides a mechanism that buffers skews in adult sex ratio and which is quite distinct from Fisher's principle10 and allied mechanisms9,11 for the primary sex ratio.