ORIGINAL ARTICLE Leticia Avile s á Cora Varas á Eric Dyreson Does the African social spider Stegodyphus dumicola control the sex of individual offspring? Received: 21 October 1998 / Received in revised form: 23 March 1999 / Accepted: 26 April 1999 Abstract By scoring the chromosome number of de- veloping embryos, we show that the sex ratio bias of the African social spider Stegodyphus dumicola Pocock is the result of an overproduction of female embryos. Only 17% of 585 embryos sexed from 14 egg sacs were male, a signi®cant departure from a 1:1 sex ratio. We also explored the possibility of direct control of the sex of individual ospring in this species by examining the variance in the number of males per sac and the spatial distribution of male and female embryos within the sacs. We postulated that a variance in the number of males per sac lower than binomial (i.e., underdispersed or precise sex ratios) or a non-random distribution of male embryos within the sacs would suggest direct control of the sex of individual ospring. We found that the vari- ance in the number of males per sac was indistinguish- able from binomial and signi®cantly larger than expected under exact ratios. Likewise, the spatial distribution of male embryos within three sacs examined was no more clustered than expected by chance. The sex ratio biasing mechanism in this species, therefore, apparently only allows control of the mean sex ratio but not of its variance. We present randomization and Monte Carlo methods that can be applied to test for departures from a random spatial arrangement of male and female embryos in an egg mass and for departures from binomial or exact ratios when not all members of a clutch have been sexed. Key words Sex ratio á Primary sex ratio bias á Precise sex ratios á Social spiders á Randomization test á Monte Carlo test Introduction Among the most intriguing features in the biology of social spiders are the highly female-biased sex ratios that characterize, with a few exceptions, the non-territorial permanent-social species (reviewed in Avile s 1997). These biased sex ratios are thought to have arisen be- cause the strongly subdivided population structure of these spiders has allowed selection among the isolated colony lineages to override Fisherian selection within them (Avile s 1986, 1993, 1997; see also Elgar and Godfray 1987; Frank 1987). In two Neotropical and one Australian species (AvileÂs and Maddison 1991; Rowell and Main 1992), it has been con®rmed that biased sex ratios are the result of an overproduction of female embryos. Although a primary sex ratio bias is suspected, dierential mortality of the sexes still needs to be for- mally ruled out as the cause of the bias in the remaining species where only subadult or adult sex ratios have been reported (see references in AvileÂs 1997). In addition to the presence or absence of a bias, an additional question of interest concerns whether the sex ratios are binomially or more precisely produced (Green et al. 1982; Hardy 1992; Nagelkerke and Hardy 1994; Nagelkerke 1996; Hardy et al. 1998). Females that produce precise sex ratios are expected to have a selec- tive advantage over binomial females, especially when clutch size and foundress number are small and the av- erage sex ratio highly female biased (Green et al. 1982; Hardy 1992; Nagelkerke and Hardy 1994; Nagelkerke 1996). Under the latter conditions, binomial females have a non-negligible probability of producing maleless clutches, so that the daughters of solitary foundresses could remain unmated (e.g., Hardy and Cook 1995; Hardy et al. 1998). Several social spider species with Behav Ecol Sociobiol (1999) 46: 237±243 Ó Springer-Verlag 1999 L. AvileÂs (&) á C. Varas á E. Dyreson 1 Department of Ecology and Evolutionary Biology University of Arizona Tucson, AZ 85721, USA e-mail: laviles@u.arizona.edu Fax: +1-520-6219190 Present address: 1 Ecology and Evolutionary Biology Guyot Hall, Princeton University Princeton, NJ 08544, USA