A new independently derived social spider with explosive colony proliferation and a female size dimorphism

Permanent cooperative sociality is rare in spiders. Here we describe sociality in a cobweb (Theridiidae) genus with no other known social species. In five areas of eastern Ecuador, we found nests of Theridion nigroannulatum containing from a single to several thousand adults living together in a communal web. Spiders cooperated in prey capture and shared their food. Subadult to young adult sex ratios were highly female biased, suggesting a strongly subdivided population structure as in other permanent-social spiders. Unusual aspects of T. nigroannulatum's biology include the existence of adult females of two discrete sizes, an extreme boom and bust pattern of colony growth, and the presence, within larger colonies, of populations of a predatory spider (Faiditus spp., Argyrodinae) that may play the role of a colony-level parasite. The existence of females of two sizes may be the result of alternative female reproductive strategies, intracolony competition for resources, or an incipient caste system in this species. A redescription and phylogenetic analysis corroborate the placement of this species in Theridion, indicating that sociality has evolved independently in at least three theridiid genera.