Building structures and aggregating can increase an animal's fitness, but the benefits come at a cost. Some orb web spiders build multiple structures or build in aggregations, which may have the same effect on prey capture success as the addition of a structure. As these structures often appear together, they may bestow interactive benefits not realized if the structures were added alone. We performed field experiments to investigate whether the multiple structures associated with the orb webs of two spider species provide interactive benefits. The orb web spider Nephila clavata adds barrier webs and prey carcass decorations to its webs. We manipulated their webs in the field by removing either, both or neither the barrier webs or the carcass decorations. We found that prey interception rate was greatest when neither barrier webs nor carcasses were present but, for the prey caught, the prey retention rate was greatest with both structures present. Another orb web spider, Cyclosa mulmeinensis, adds prey carcass decorations and forms aggregations. We manipulated the decorations and aggregations of C. mulmeinensis in the field to determine their interactive influences. In solitary webs and webs with decorations, prey capture rates were lower than those without structures. These negative foraging effects, however, did not exist in decorated webs that were in aggregations. Our results thus show that multiple structures, individually, are costly, but interactively they provide substantial benefits. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
