Wednesday 12 March 2014

The repeated evolution of sociality in a genus of Eresid Spiders.

Social Spiders are unusual in that the females form colonies of hundreds to thousands of individuals, which cooperate in the building of webs, capture of prey, defence of the colony and raising of young. Unlike in Insects, where social numerous highly successful species are known in several groups, most of which are exclusively social, of the approximately 41 000 known species of Spider only about 25 are social. However sociality appears to have arisen separately at least 18 times in tropical and subtropical Spiders, suggesting that under some circumstances there must be strong evolutionary pressure for Spiders to develop sociality.

In a paper published in the journal Frontiers in Zoology on 23 February 2013, Marija Majer, Jens-Christian Svenning and Trine Bilde of the Department of Bioscience at Aarhus University examined the distribution and occurrence of sociality in Eresid Spiders of the genus Stegodyphus, which is found across Africa, the Mediterranean Basin, The Middle East and the Indian Subcontinent, and which contains three social species (two in Africa and one in India) believed to have evolved the trait separately, as well as eighteen non-social species.

A colony of the Indian colonial Spider Stegodyphus sarasinorum tackling a large Moth. Spiderlab Aarhus University.

Majer et al. found that the social species were found in the areas with the greatest plant productivity and Insect biomass, suggesting that the abundance of prey is a driving factor in the evolution of sociality in these Spiders. High prey abundance is an extremely good thing for any Spider, and Majer et al. suggest the ability to secure large amounts of prey may be a determining factor in the adoption of sociality in Spiders, overcoming the disadvantages that would otherwise come from intra-specific competition, and enabling the Spiders to gain the advantages from communal breeding and defence.

Distribution map of Stegodyphus species on a continental gradient of vegetation productivity (unitless ratio). Three regions, defined to separate the distributions of the social species, are indicated by circles (region 1), triangles (region 2) and rectangles (region 3). Empty and filled symbols indicate the occurrences of social and solitary species, respectively (n = 366 occurrences in total). The darker the green the more productive the continental area is. Majer et al. (2013).

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