Costs And Benefits Of Group Living In An Unusual Social Spider, Delena Cancerides

Abstract

Virtually all spiders are predators, and many are cannibalistic. Of the handful of species that tolerate conspecifics and sustain long-term associations, nearly all spin a web or silken retreat. Previous research on these social spiders showed that many of the benefits they derive from group living depend on these silken structures. The social huntsman spider of Australia, Delena cancerides, is the exception and only lives under the bark of trees. I studied the costs and benefits of group living in this species, given that many of the benefits ascribed to other species are impossible without a web, in three contexts: (1) predator defense, (2) foraging, and (3) dispersal. I examined predator defense by introducing potential predators into field colonies that had been manipulated to allow observations and into captive colonies in the laboratory. The single adult female of the colony was the primary defender of the colony, while younger spiders were ineffective at repelling predators. I examined foraging by observing natural prey capture in the field, introducing prey into field colonies, and recording the condition of field spiders at the time of collection. Spiders predominantly foraged individually; however, some prey captured inside the retreat was shared, and younger iii spiders benefited from sharing prey captured by their older siblings. I examined dispersal into neighboring colonies through a combination of allozyme analyses and direct observations in the field and laboratory. Young spiders can and do immigrate into foreign colonies; however, older spiders were usually attacked in foreign colonies, and thus their dispersal options are constrained. Through the use of nest boxes, I showed that competition for a new bark retreat is intense and that spiders should wait in their natal retreat until they are larger and better competitors for a new retreat. Together, these data show that the bark retreat is indeed critical to the social biology of D. cancerides. Without the web some forms of cooperation found in other social spiders never evolved, yet the reliance on a rare bark retreat also promotes group cohesion, as is found in many social vertebrates. iv