In the Hobbit, young Bilbo Baggins fights to free his terrified companions from the webbed larder of a colony of giant spiders. But spiders that hunt cooperatively only exist in the realm of fantasy, right? Not so, according to renowned spider expert, Linda Rayor of Cornell University, New York. Speaking at Macquarie University today, Rayor explains that social spiders, though rare, are very real.
Only a tiny number of spiders (just 88 out of the 44,000+ known species) display what is termed ‘prolonged sub-social’ behaviour: A maternal-offspring relationship that extends beyond the customary maternal defence of the egg-sac and on to active food provision and protection of the young into adulthood . The relationship may be more tolerant than loving in nature, but in the world of spiders, anything less than cannibalism is remarkable.
So why have these spiders developed a penchant for group living rather than simply eating each other like their solitary counterparts?
The social theory around communal spiders is not well developed yet, says Rayor. But she speculates that unlike the empire building insects, bees and ants, which form societies around maximising reproduction, social spider communities are founded primarily upon communal foraging. The spiders benefit from both increased nutrition from co-operative hunting and prey-sharing and reduced individual energy costs by sharing silk web production. These benefits paradoxically outweigh the intensified competition for food that is intrinsic in sharing an abode with a hungry family.
Yet, interestingly, the species of social spider that Linda Rayor has come to Australia to study, Delena cancerides, lives communally without these benefits.
D.cancerides is a huntsman which clusters along the coastal regions of south-eastern Australia and Tasmania. It forms matriarchal colonies of a single breeding female and several overlapping clutches of her offspring. The spiders crowd into the slim spaces between loose bark and trunk on acacia, casuarina or gum trees . As Rayor shows in her slides, if you peel back the bark of a nest, you’ll find a thick mat of spiders, legs and bodies interlocked. An image likely to give any arachnophobe nightmares for weeks.
The offspring remain in the maternal nest until reaching sexual maturity, at which time, the eldest female siblings must decide whether to stand and fight to the death for control of the nest or take a punt on the perils of the outside world .
What makes D.cancerides unusual is that it is one of only a handful of social spiders that do not weave a web to capture prey . Rather, each individual spider forages for its own prey at night and is generally reluctant to share. Even the matriarch shares prey only with her very young progeny .
So, why do these spiders choose communal living if they don’t hunt or build webs together?
Well, firstly, it appears that despite the infrequency of prey sharing, young D.cancerides still gain fitness benefits from living with their older siblings. In a recent paper published in Animal Behaviour, Rayor and her colleague, Eric Yip, show that younger juveniles are significantly heavier if they live with an older sibling than if they don’t . These weight gains, Rayor claims, are due to rare prey-sharing events with older siblings, or scavenging from the older sibling’s off-casts. After all, tiny tidbits are all that is required for a small spider to grow. Big sister or brother presumably tolerate this occasional raiding because the costs are small compared to the energy required to chase the cheeky scabs away .
Another likely reason for continued communal living, according to Rayor, is the protection afforded by the nest itself . The D.cancerides nests are well concealed and easy to defend. The mother binds most of the openings of the bark nest closed with her silken thread, and sits astride the main entrance, a formidable bouncer, rebuffing predators and same species interlopers alike. Occasionally, this protection extends to impressive physical attacks on the predators while the offspring scurry away and hide .
However, these bark retreats are rare. In fact, Rayor and her colleagues have estimated that retreats are 100% saturated in many locations, meaning that D.cancerides really has no option but to share . Rayor says this makes it the first social spider exemplar of the ‘ecological constraints hypothesis’; a hypothesis which states that group living is promoted by habitat saturation and which has been demonstrated in many other species .
Recently, Raynor and her Australian colleagues have identified two more ‘prolonged, sub-social’ Delena species, the D.lapidicola and D.melanocheli . Although both species are a little less amiable than D.cancerides, the fact they are all so closely related has prompted Raynor to ask whether social behaviour evolved in each species individually, or whether it evolved once in an ancestral species and has been subsequently lost in their solitary descendants. We’ll eagerly be waiting for answers to these and many more questions, as Rayor continues her research into these fascinating spiders.
4. Agnarsson, I & Rayor, L.S. (2013) A molecular phylogeny of the Australian huntsman spiders (Sparassidae, Deleninae): Implications for taxonomy and social behaviour. Molecular Phylogenetics and Evolution. 69. 895-905