Male density affects large-male advantage in the golden silk spider, Nephila clavipes
Clare C. Rittschof
Across a variety of animal taxa, the outcome of male--male contests depends on male body size; winners are usually the larger males or the males with bigger weapons. However, high male density can either increase or reverse large-male advantage because density changes the frequency and intensity of male--male interactions. In the golden orb-web spider Nephila clavipes, large males have a competitive advantage in male--male contests. However, this species shows more than 2-fold variation in male body size and highly variable local male density (the number of males on a female's web). To test how male density affects large-male advantage, I manipulated male density on female webs. I measured how density affects the rate of male--male challenges and the body size of successful males. Large males have a reproductive advantage when male density is high and fighting among males is intense, but small and large males are equally likely to copulate successfully when male density is low, possibly because male--male interactions are infrequent at low densities. High variance in male density among female webs and the positive relationship between density and the strength of sexual selection for male body size create a heterogeneous mosaic of selection intensities. Spatial variation in selection intensity could act to maintain variation in male body size in this species. Copyright 2010, Oxford University Press.