Oecobius civitas

Oecobius civitas, the Mexican social spider, appears in a discussion of evolutionary stable strategies as a striking real-world case of a counterintuitive rule: a resident spider may retreat when an intruder enters its refuge. The resulting sequence of spiders displacing one another offers a possible natural example of the paradoxical strategy that the theoretical argument had nearly dismissed as unlikely.

Theoretical Context

The account follows an analysis of contests in which individuals differ in size and fighting ability. A seemingly sensible strategy is to attack smaller opponents and flee from larger ones. Yet when serious injury is costly, a paradoxical alternative can be stable: attack larger opponents and retreat from smaller ones. In a population dominated by this strategy, the larger contestant leaves, so nobody is injured; a mutant using the sensible approach risks serious fights against paradoxical opponents.

The book initially treats this outcome as mostly academic. For paradoxical strategists to have a higher average payoff, they would need to greatly outnumber sensible ones, a condition the author considers difficult to imagine arising in the first place. A slight shift toward sensible strategists could move the population into the other strategy’s zone of attraction, after which selection would draw it toward the sensible stable state. The spider example arrives as a pointed qualification to that expectation.

Observed Displacement Sequence

After writing that a natural example of a paradoxical evolutionary stable strategy seemed unlikely, the author notes that John Maynard Smith directed attention to a description by J. W. Burgess. When disturbed and driven from its retreat, an Oecobius civitas spider darts across a rock. If no empty crevice is available, it may enter the hiding place of another spider of the same species.

If that refuge is occupied, the resident does not attack. Instead, it darts out and looks for a new refuge of its own. The initial disturbance can therefore set off sequential displacement from web to web, continuing for several seconds and often shifting a majority of the aggregation from its home retreats into ones occupied by other spiders. Rather than a single contest ending in a straightforward victory, the observed pattern is a succession of retreats in which residency does not guarantee that the occupant will hold its place.

Significance and Limits

The example gives the paradoxical strategy an empirical foothold: an intruder can displace a resident, the reverse of the usual resident-wins pattern discussed in the book. The author had described the reverse strategy as self-destructive in a population, since individuals would have reason to avoid being residents and would have to keep moving. In the spider description, however, displacement is precisely what follows an initial disturbance, spreading through the aggregation.

The account is offered as a reported behavioral observation, not as a complete demonstration that Oecobius civitas constitutes a stable population-level strategy under all relevant conditions. Its importance is the contrast it creates between theoretical doubt and a documented sequence of behavior. More broadly, the book frames such animal examples as illustrations rather than sufficient evidence for general laws; the spider’s movements make the model vivid while leaving the question of evolutionary stability open to further analysis.