The Ecology of Cooperation in Wasps: Causes and Consequences of Alternative Reproductive Decisions

Polistes wasps can initiate colonies by themselves, but spring foundresses often cooperate in constructing a shared nest and raising a common brood. The relative benefits for such cooperation vary across environmental and social contexts and thus foundresses should shift reproductive strategies when contexts change. An experimental study of 10 field populations of P. dominulus provides evidence for such strategy shifts. P. dominulus females displayed three basic alternative reproductive strategies: initiate a nest alone or as a dominant on a multifoundress nest, join as a subordinate, or refrain from nesting and instead "sit—and—wait" for reproductive opportunities to open up later in the season. In our populations, joining likely yields inclusive fitness benefits by increasing colony survival and resistance to usurpation. Nevertheless, joiners and other foundresses readily moved to improve their situations by (1) adopting orphaned nests, (2) switching to larger nests, and (3) usurping small nests, which may have had a weakened foundress or a foundress with relatively low motivation for nest defense. Joining occurring more often early in the season, and adopting and usurping occurring more often later were consistent with inclusive fitness maximization. Lost past investments in nests did not affect future decisions. Overall, residents of nests were larger than the wasps that joined them, but this difference decreased over the course of the season. Wasps that had cooperated in the past to form multifoundress associations were more likely to renest (usually together) after removal of their original nest than were single foundresses. After nest loss, single foundresses were more likely than multiple foundresses to take over another wasp's nest by adoption or usurpation. Cooperation within multifoundress associations is facultative and the balance between cooperation and conflict theoretically depends on the asymmetry in reproduction resulting from the asymmetry in dominance. P. dominulus cofoundresses closer in size (less asymmetrical in dominance) added cells to their nests more slowly and produced significantly smaller nests and fewer workers than associations with more size variation, as would be predicted by greater conflict between wasps that are closer in size. The decline in size differences between joining wasps and residents suggests that late—season joiners were relatively large wasps seeking to dominate residents. Conflict on the nest may also explain why multiple foundresses disappeared from nests at a higher rate than did single foundresses. In sum, the multiple reproductive options of P. dominulus lead to a dynamic and flexible balance between cooperation and conflict in their social interactions.