Towards a unified theory of the ecology of sociality

Background/Question/Methods

The geographical distribution of species with different social systems should provide clues as to the factors responsible for social evolution. Social spiders are notable for having a distinctly tropical distribution.  In the genus Anelosimus, which contains the largest number of social species of any spider genera, this latitudinal pattern is replicated altitudinally: social species are restricted to wet low to mid-elevation tropical areas, while subsocial species predominate at higher elevations and latitudes. We postulate that this pattern results from an interaction between the dense 3D webs characteristic of social spiders with two separate environmental gradients.

Results/Conclusions

That social species are restricted to the lower elevation wet tropics may reflect a gradient in insect size, which we show decreases with elevation and latitude. Large insects compensate for a decline in the number of prey caught per capita with increasing colony size that results from a declining surface area to volume ratio of the prey capture snares. Where large insects are abundant the spiders make up for this decline by cooperatively capturing larger insects in larger colonies.  The result is a biomass per capita that is maximal at intermediate colony sizes. Absence of subsocial species in the lowland rainforest, on the other hand, may reflect gradients on the intensity of precipitation and abundance of potential ant predators, which we show increase with proximity to the rainforest. Through transplant and rain exclusion experiments we show that dense 3D webs may be unsustainable for solitary living spiders in environments where intense rains cause their frequent destruction.  Large 3D webs, however, may provide better protection against predators, thus favouring large social groups in the lowland rainforest. We use a mechanistic model of prey capture by spider colonies in environments with different levels of disturbance and different ranges of insect sizes to show how broad scale patterns of sociality, and social evolution itself, can be explained as an interaction between intrinsic features of organisms and the environments in which they live.