Natal dispersal and demography of a subsocial Anelosimus species and its implications for the evolution of sociality in spiders

The transition to permanent-sociality in spiders is thought to have involved the suppression of the dispersal phase characteristic of hypothetical subsocial or periodic-social ancestral species. Extant periodic-social species may provide insights into this transition. The periodic-social Anelosimus jucundus in southern Arizona was found to form mother-offspring and sibling associations that disintegrate prior to the mating season. Following the breakdown of the social phase, more than twice as many females as males became established within a few metres of the natal nest. Given that the predispersal sex ratio was 1:1, a fraction of the males may have dispersed beyond the local area. The short dispersal distances of at least a fraction of individuals of both sexes, the clustering of nests in local areas, and at least two possible cases of sibling mating suggest, however, that dispersal may not eliminate the possibility of close inbreeding in this species. Estimated transition probabilities between life-history stages show that the heaviest loss of individuals occurs during dispersal. Once established, 41% of the females that reached maturity succeeded in producing grown progeny. We discuss the implications of these findings in terms of the transition from periodic to permanent sociality in spiders and of current models that consider the interplay between competition and inbreeding avoidance in the evolution of dispersal.