Adaptation

The question of adaptation asks why a behavior is beneficial to an organism. It is necessary to study the animal's environment and the specific selective pressures present in order to determine a behavior's adaptive value. Understanding the adaptive value of sociality in spiders offers an explanation for why the behavior is observed.

Of the 39,000 know species of spiders only 20-25 have been found to participate in permanent sociality (10).  Due to the fact that such a small fraction of these species has evolved this social trait, the reasons why these select few have adapted this social structure gives insight to why other animals might participate in group living.  Two types of social groups have been distinguished in spiders; cooperative living is where the spiders share in capturing food, eating food and sometimes raising of the young, and colonial living is where a large group of spiders live in the same area but each stays to his or her respective web (13). 

Anelosimus may nest with incorporation of leaves. Image taken from Reference 3

Group Foraging in Colonial and Cooperative Spiders

Group living is hypothesized to have evolved via different pathways for each group.  The colonial or sub-social route is thought to have resulted from extended maternal care, and the cooperative or para-social path is thought to have evolved as a result of spiders aggregating around a resource like food or protection (13).  Colonial spiders usually hunt individually and have only been observed hunting together in situations with large prey. The ability to hunt larger prey confers an advantage by allowing access to a greater variety of prey.  In cooperative spiders, foraging as a group is usually done by young spiders and as the individuals learn to capture prey on their own, there is some variation observed in how much foraging is done by the community (13). Social foraging is especially advantageous to younger spiders who would normally be limited to smaller prey.

Benefits of Communal Webs

Colonial living offers its memebers reduced energy cost in web building. The close proximity allows the spiders attach individual webs to common places thus saving on silk production, lowering the cost of producing and maintaining the web (13). 

In addition to lowering web production cost, group living in spiders also offers protection and thus increases the survival of each member.  In some species, offspring survival may be enhanced by the physical structure of the web, which often incorporates leaves and sticks, protecting against environmental factors such as rain and predation (13).  Dispersion of social spiders usually results in a decrease in survival and it has been shown that large groups have higher success rates. 

Social Hierarchy

It has been demonstrated that some species may have a caste system within the group (11).  Rypstra (1993) et al. observed that spiders fed larger prey developed a more exaggerated caste system, with fewer, larger reproducing individuals, than those fed smaller prey. They hypothesized that the capture of larger prey, since it requires the cooperation of more individuals, leads to fiercer direct competition for food. This increased competition creates a situation where smaller spiders acquire less food, while larger spiders grow ever stronger and more dominant. Eventually this stratification leads to a few large spiders becoming the reproducers, leaving the smaller spiders only to help with hunting and maternal care (11).