A conflicting-tendency model of spider agonistic behaviour: Hybrid-pure population line comparisons

Contests over web-sites in the spider, Agelenopsis aperta, have been simulated by a model in which the next act of a spider is determined by the value of two ‘drives’ or ‘tendencies’, to attack and to flee. Such a model can explain the fact that F₁ hybrids between two geographical races of A. aperta are not intermediate between their parents, but are more aggressive than either. The model also accounts for the following features of the observed contests: (1) contests consist of a series of bouts, during which the spiders perform increasingly aggressive acts; (2) contests are usually won by the larger spider; (3) if sizes are equal, contests are usually won by the owner; (4) contests over excellent-quality sites are more costly than over poor-quality sites; and (5) transition probabilities for de-escalation, matching and escalation with respect to prior acts show site quality and weight bias trends. It is argued that a two-tendency model is the simplest that would account for these observations. The model also accurately simulates spider behaviour in agonistic contexts not used in its development (i.e. round-robin contests involving opponents from different populations, and a group of contests involving unequally-matched hybrid opponents).The main discrepancy between the model and observed encounters is that the model predicts that, if the owner is larger, the cost of a contest will not vary with the value of the disputed site. This is because the smaller intruder, having no knowledge of site quality (Riechert 1984) is expected to withdraw early in the contest, regardless of site quality. In the actual contests over excellent sites, estimated costs are higher even when the owner is larger and the intruder loses. The change in intruder behaviour associated with site value occurs following the first series of actions, but within the first bout of the interactions: our analyses indicate that any information conveyed by the owner concerning site value is not through overt behaviour (i.e. through the frequencies with which specific acts are performed).