Social spiders of the genus Anelosimus occur in wetter, more productive environments than non-social species
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Latitude, rainfall, and productivity have been shown to influence social organisation and level of sociality in arthropods on large geographic scales. Social spiders form permanent group-living societies where they cooperate in brood care, web maintenance, and foraging. Sociality has evolved independently in a number of unrelated spider genera and may reflect convergent evolutionary responses to common environmental drivers. The genus Anelosimus contains a third of approximately 25 described permanently social spider species, eight to nine species that all occur in the Americas. To test for environmental correlates of sociality in Anelosimus across the Americas, we used logistic regression to detect effects of annual rainfall, productivity, and precipitation seasonality on the relative likelihood of occurrence of social and non-social Anelosimus spiders. Our analyses show that social species tend to occur at higher annual rainfall and productivity than non-social species, supporting the hypothesised effects of these environmental variables on the geographical distribution of social species. We did not find support for the hypothesis that permanently social species occur in areas with low precipitation seasonality. High annual precipitation and, to less extent, high productivity favour the occurrence of permanently group-living Anelosimus spiders relative to subsocial and solitary species. These results are partially consistent with previous findings for the Old World spider genus Stegodyphus, where a link between high habitat productivity and sociality was also found. Unlike Anelosimus, however, Stegodyphus typically occur in dry habitats negating a general importance of high precipitation for sociality. Sociality in spiders thus seems to be strongly linked to productivity, probably reflecting the need for relatively high availability of large prey to sustain social colonies.
KeywordsSocial spiders Social environment Anelosimus Distribution range Precipitation Productivity
This study was supported by grants from The Danish Council for Independent Research to TB (grant number 09-065911) and a travel grant from the Oticon foundation to MM. We would like to thank Laura J. May-Collado for her help with entering the species occurrence data, and Brody Sandel and Michelle Greve for their help with the statistical computation. The authors would like to greatly thank the permanent and visiting members of Ingi Agnarsson's lab for their kind and friendly reception and academic discussion. Constructive comments from three anonymous reviewers helped improve the manuscript.
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