Within-group aggression and the value of group members: theory and a field test with social wasps

Abstract

present a simple, general model of how the optimal level of intra-group aggression should vary in different social contexts. A key component of this model is the value of the recipient of aggression to a potential aggressor (i.e., the ratio of expected long-term group productivity with the recipient present to the expected group productivity with the recipient absent). The recipient's value measures its contribution to group reproductive success. We demonstrate theoretically that if aggression increases the aggressor's share of the group's expected total reproductive output, but at the same time decreases the magnitude of this overall reproductive output, then the optimal level of aggression toward a recipient will decrease with increasing recipient's value. This proof establishes a rigorous theoretical connection between the level of aggression within a group and the benefits of belonging to such a group and can be tested by experimentally manipulating the values of group members to each other. test, and thus illustrate the utility of, this model by examining aggression within experimentally-manipulated foundress associations of social wasps. show that the value of co-foundresse s to each other in the social wasp Poiistes fuscatus lies in their ability to provide insurance against colony failure caused by die loss of all tending foundresses. Removals of worker-destined eggs and pupae, which increase the value of co-foundresse s, both lead to significant reductions in aggression by the dominant foundress, despite die fact that the immediate, selfish benefits of competitive aggression should increase when empty brood cells are present Removal of reproductive-destined eggs, which does not affect co-foundress value, but increases the benefits of selfish aggression, causes a significant increase in aggression by beta foundresses. Finally, wing reduction of subordinate co-foundresses significantly increases aggression by dominant foundresses, as expected since the subordinate's value is reduced. Our results indicate that foundress aggression is sensitive to the value of future cooperation, as predicted by die optimal aggression model. The model may apply widely to both invertebrate and vertebrate societies. Kty words: aggression, animal societies, conflict, cooperation, eusodality, punishment, social contracts, social wasps, Potistts. [Bthav Ecol 8:75-82 (1997)]