Dispersal behaviour and colony structure in a colonial spider

Group living in spiders is characterised by two principle modes, the cooperative social mode and the colonial non-cooperative mode. Kin-relationships due to reduced dispersal determine population genetic structure in social spiders, but the dispersal mechanisms underlying group structure remain poorly understood in colonial spiders. Assuming similar ecological benefits of group living, we address the question whether reduced dispersal shapes population structure in a colonial spider, Cyrtophora citricola (Araneidae). We analysed dispersal by studying settling decisions under semi-natural conditions in experimental trees with and without colonies, and in natural populations, we estimated dispersal and colony structure using population genetic analyses. The propensity to disperse decreased with increasing age in experimental colonies. Adult females did not disperse in the experiment. Sub-adult female spiders preferred trees with a colony to trees without a colony. Dispersal in third instar juveniles was influenced significantly by wind but not by the presence of a colony. Thus, we showed that being in a colony did not inhibit juvenile dispersal, but pre-mating females were philopatric. Genetic differentiation among colonies in natural populations was heterogeneous, colonies being either little or highly differentiated. The heterogeneous structure is likely caused by colony founding by one or a few females followed by dispersal among perennial colonies. Gene flow, however, was slightly male-biased. The experimental and indirect, genetic approaches combined showed that dispersal and the breeding system of C. citricola resemble that of solitary spiders, with juvenile dispersal occurring in both sexes, while the colonial distribution is maintained by female philopatry.