Sex-Ratio Bias and Possible Group Selection in the Social Spider Anelosimus eximius

Abstract

A demographic study on the social spider Anelosimus eximius (Araneae: Theridiidae) demonstrates no differential mortality of the sexes during the age of reproduction and no large difference in their maturation times to explain the highly female-biased sex ratios in adults. Moreover, sex ratios within the range of 0.04 to 0.40 males per female are already present at the earliest stage at which sexes can be distinguished in the field. Fisher's theory predicts a 1:1 sex ratio as evolutionarily stable. How, then, are the observed ratios attained and maintained? It is suggested that the unique population structure and dynamics of this social spider resulted in a change of balance between the opposing forces of group and individual selection, making evolutionarily stable a sex ratio that increases colony survival and proliferation.