The role of prey size and abundance in the geographical distribution of spider sociality.

1. Social species in the spider genus Anelosimus predominate in lowland tropical rainforests, while congeneric subsocial species occur at higher elevations or higher latitudes. 2. We conducted a comparative study to determine whether differences in total biomass, insect size or both have been responsible for this pattern. 3. We found that larger average insect size, rather than greater overall biomass per se, is a key characteristic of lowland tropical habitats correlating with greater sociality. 4. Social species occupied environments with insects several times larger than the spiders, while subsocial species nearing dispersal occupied environments with smaller insects in either high or low overall biomass. 5. Similarly, in subsocial spider colonies, individuals lived communally at a time when they were younger and therefore smaller than the average insect landing on their webs. 6. We thus suggest that the availability of large insects may be a critical factor restricting social species to their lowland tropical habitats.