Nature 360, 156 - 159 (12 November 1992); doi:10.1038/360156a0

Sexual dimorphism and distorted sex ratios in spiders

Fritz Vollrath^* & Geoff A. Parker^†

^* Department of Zoology, University of Oxford, Oxford 0X1 3PS, UK; Zoologisches Institut, Rheinsprung 9, CH 4051 Basel, Switzerland; Smithsonian Tropical Research Institute, Box 20272, Balboa, Panama
^† Department of Environmental and Evolutionary Biology, University of Liverpool, Liverpool L69 3BX, UK

SEXUAL dimorphism in body size is widespread in the animal kingdom. Whereas male giantism has been studied and explained extensively^1,2, male dwarfism has not. Yet it is neither rare^3–7 nor without theoretical interest^8,9. 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 mechanisms^9,11 for the primary sex ratio.

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