Mention social spiders and perhaps one of the first images that springs to mind is a family of spiders sitting around the dinner table sharing the day’s stories. Absurd as it sounds, it isn’t as far fetched as you might imagine. Dr Linda Rayor, from Cornell University has found that Delena cancerides (Australian Huntsman spider) engage in social interactions and communal living. These Huntsman spiders not only share their home with their mother and multiple offspring, but have also been found to share their prey with fellow siblings living in the same colony.
While cooperation and sharing of resources may not seem like such a big deal, social behaviour in spiders is very unusual and rarely seen. In a recent visit to Australia, Dr Rayor spoke in Sydney and explained that spiders are primarily solitary, cannibalistic and voracious predators. They undertake transient maternal care with very minimal, if any, attention provided for their young after eggs have been hatched. Of the 44,540 described species of spiders, only around 80 are social which gives us an indication of how rare this cooperative behaviour is. Of these 80 social species, almost all construct webs, a behaviour driven by the energy savings of shared silk production. In the case of D. cancerides they don’t produce webs which makes social living in this species even more intriguing.
Colonies of D. cancerides live under the bark of certain trees in what Rayor calls retreats. These retreats can be very constrained with numbers ranging from 2 to 113 in limited space. What is unique is that these colonies can consist of all age classes and sizes living together and Agnarsson and Rayor (2013) explain that the complexity and longer term of this behaviour is rarely seen. Suitable habitats are rare and very valuable and there can be enormous competition between spiders to obtain a fitting home. The rarity of habitats is driving young spiders to stay in the retreat getting stronger and fitter so that they can compete for an appropriate retreat when they leave the home. There is debate as to whether the social dynamics of huntsman spiders are a response to the habitat constraints and scarcity of retreats, while Rayor suggests that ecology may be important in shaping sociality, she does not believe that it is the driving factor for social behaviour in huntsman.
Given the rarity in social behaviour, there must be some benefits that are gained from overcoming the barriers to sociality. Whitehouse and Lubin (2005) suggest that living as a group must provide benefits that outweigh what would be obtained by living individually. The advantages that are gained depends on the primary function of the group whereby for reproduction, protection or foraging. Among insects sociality often brings reproductive advantages, however, Whitehouse and Lubin found that group characteristics of social spiders are driven by the foraging function providing benefits with prey and food.
Prey sharing is common amongst social spiders and provides benefits such as more efficient feeding and a means for the parents to care for offspring. Rayor explained that the lower metabolic rate of D cancerides means that less food is required to survive which is key to the success of sociality. In a study carried out by Yip and Rayor (2013a) there was evidence suggesting young spiders benefit from the presence of older siblings and prey sharing is the most likely method behind this benefit. While previous reports have demonstrated larger spiders sharing prey with smaller spiders, this study provides some of the first evidence that older siblings improve the fitness of younger siblings. Whether or not this is unique to D. cancerides is yet to be determined, however, it is certainly not commonplace amongst spiders.
Social behaviour in spiders provides an opportunity to look at a range of ecological concepts but also begs the question, when sociality is so successful, driving higher survival rates, increased growth rates and delayed mortality, why has it not evolved across more spiders? Is this behaviour a factor of physiology such as lower metabolic rate, or is it driven by community ecology and competition for suitable habitats? Furthermore, will anthropogenic disturbance by means of climate change and land clearing have an impact on these behaviours? All these questions are fascinating and require further investigation. Yip and Rayor (2013b) suggest that the reasons some organisms evolve socially and others do not is still not understood, however, the bond that is demonstrated between mother and offspring is a step towards more complex societies. These are interesting findings and are key in not only the behavioural ecology of spiders but other organisms as well. We look forward to further work which investigates the ecology and evolution of social behaviour.
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.
Whitehouse, M. E. & Lubin, Y. 2005. The functions of societies and the evolution of group living: spider societies as a test case. Biological Reviews, 80, 347-361.
Yip, E. C. & Rayor, L. S. 2013a. The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition? Animal Behaviour, 85, 1161-1168.
Yip, E. C. & Rayor, L. S. 2013b. Maternal care and subsocial behaviour in spiders. Biological Reviews.