Bad neighbors: hunger and dominance drive spacing and position in an orb-weaving spider colony

Abstract

The costs and benefits of group living can vary among group members depending on their physical location within the group. If individuals can anticipate poor positions and leave the group, the society may dissolve. Therefore, understanding the geometry of social groups is critical to understanding their stability. We examined social geometry in the colonial spider, Cyrtophora citricola (Araneidae), which builds long-lasting individual webs within a shared colony framework. Group foraging benefits are thought to be an ultimate cause for the evolution of colonial living in spiders, but conflict often arises over food and territory. To understand how foraging benefits of grouping interact with inter-individual conflict to shape group geometry, we examined the effects of feeding history and body size on inter-individual spacing in the laboratory. We also examined the effect of spider density and body size on individual position within colonies in a semi-natural setting. We found that spiders with prior food stress increased their inter-individual spacing, suggesting that competition for prey may override group foraging benefits. Larger spiders built their webs first, relegating smaller spiders to the margins of the space and sometimes preventing them from completing web construction. In a semi-natural setting, spiders did not maintain close spacing, but rather spread themselves out over the substrate, and larger spiders occupied the preferred side of the substrate to the exclusion of smaller spiders. Contrary to the hypothesis that foraging advantages to group living promote greater cohesion under food stress, food competition appeared to promote group instability, and exclusion of small spiders by larger and more dominant individuals seemed to determine position in the colony. In colonial spiders, foraging benefits are thought to be the primary driver of group living, yet there is often conflict over food and territory within the group. We tested the hypothesis that foraging benefits should promote group cohesion under food stress in the colonial spider Cyrtophora citricola. By manipulating prey abundance prior to trials, but minimizing prey cues during trials, we eliminated the influence of prey position on spiders, leaving interactions among individuals to determine spider location. Prior food deprivation caused spiders to space themselves farther apart, refuting the group cohesion hypothesis. Larger spiders prevented smaller spiders from constructing webs and relegated them to less preferred positions in the group, suggesting that aggressive interactions determined spacing and placement within the colony, but that these interactions are modulated by feeding history.