Social spiders live in large, communal webs, in which they feed communally and cooperate in prey capture, web maintenance and brood care. Social spiders are highly inbred due to within-colony breeding and a lack of a pre-mating dispersal stage, they are therefore a perfect system to study inbreeding effects in wild animal populations.
Sociality in spiders and the subsocial route to permanent group-living
Sociality among spiders is a rare trait. Only about 60 out of 40,000 species are considered group-living (Lubin & Bilde 2007; Bilde & Lubin in press; Lubin 2010). In the category of group-living spiders different levels of sociality exists, and often the distinction is made between colonial or cooperative spiders – also known as permanently social. The colonial spiders share a common frame, but have individual webs where they catch their own prey, defend their own web, and breed independently aggregating around rich food resources (Lubin 2010, Whitehouse & Lubin 2005). Cooperative social spiders share a communal web and nest where the colonies can extend to a group size from a few to thousands of individuals and cooperate in prey capture and breeding (Whitehouse & Lubin 2005).
In the permanently social spiders there are approximately 25 species of cooperative spiders distributed across seven families (Avilés 1997, Lubin & Bilde 2007). The permanent social and cooperative spiders live their entire lives in a communal web and nest. As opposed to colonial spiders, the cooperative spiders do not defend specific territories within the web and they cooperate on prey capture, feeding on prey items, and brood care (Whitehouse & Lubin 2005). I will refer to the permanent cooperative spiders as social spiders.
The social behaviour is apparent in subsocial spiders that are non-territorial periodic-social (Avilés 1997). The social period of subsocial spiders is restricted to the juvenile stage (Whitehouse & Lubin 2005). The subsocial spiders are characterized by extensive maternal care. The social period consists of a prolonged period of maternal care where the nest contains the mother and her offspring. In some genera, the offspring will consume the mother (matriphagy) and stay within the nest (Lubin & Bilde 2007). They will cooperate in prey capture and foraging before premating dispersal to solitary living (Whitehouse & Lubin 2005). Juvenile dispersal is possibly triggered by resource competition (Lubin & Bilde 2007) or increasing body size. It was suggested in subsocial spiders from the genus Anelosimus that increasing body size of the spider results in a similar size between predator and prey and increased competition, hence the spider disperses and initiates individual foraging (Powers & Avilés 2007).
Social spiders are likely to have evolved from subsocial ancestors via the subsocial route (Agnarsson et al. 2006, Johannesen et al. 2007). It has been suggested that the transition from subsocial to social living requires a change in three behavioural traits: from premating dispersal to postmating dispersal, from outbreeding to inbreeding mating system, and from maternal care to cooperative breeding (Bilde & Lubin in press, Lubin 2010). The two genera, Anelosimus and Stegodyphus, contain both social and subsocial species with multiple independent origins of permanent sociality (Agnarsson 2006, Johannesen 2007). These genera are particularly well suited for comparative studies of causes and consequences of social evolution.