Games spiders play

Abstract

1. The predictions of game theory concerning the use of resource assessment strategies by animals in conflict situations were tested through study of the agonistic behavior of the funnel-web spider, Ageleropsis aperta, in disputes over web sites and associated territories. Specifically, contest cost was considered relative to resource value (i.e., the quality and availability of territories). 2. The contest methodology was similar to that described in Riechert (1978a) with weight controlled to eliminate size bias. In addition, relative contest costs were converted to joules using resting metabolism under field conditions as a reference. Estimates of territory quality and availability were made through the application of a model of spider reproductive success to the results of discriminant analyses of site characteristics. 3. The immediate and ultimate benefits of ownership of specific territories accounts for a significant amount of the within-population variability in dispute cost-the greater the benefit, the greater the total cost of the dispute. The territory owner is shown to determine the level of escalation reached in these contests, while the intruder merely adjusts its behavior to that of its opponent. 4. Contest behavior is also shown to vary according to the relative availability of territories: more energy is expended in maintaining and winning territories in a habitat near saturation (desert grassland) than where territories are in ready supply (desert riparian). 5. A greater range of benefits was available to spiders occupying specific sites in the desert grassland study area than to the riparian population. This was emphasized by the greater variability in contest cost, richness, and in the total number of acts exhibited in the sequences involving desert grassland spiders. Prey availability and ultimately reproductive success were found to be correlated with web-site quality in the desert grassland habitat, whereas differential utilization of specific habitat types by the riparian spiders corresponded not to reproductive potential but to the ease of web placement. 6. The correspondance existing between within-population contest cost and site quality reflects active resource assessment on the part of the territory owner, while population differences observed in the level of escalation achieved in disputes over territories is attributed to selection pressure for evolved differences in level of aggression. These findings are discussed in terms of optimization and relative to the predictions of game theory.