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

We present a simple, general model of how the optimal levelof intra-group aggression should vary in different social contexts.A key component of this model is the value of the recipientof aggression to a potential aggressor (i.e., the ratio of expectedlong-term group productivity with the recipient present to theexpected group productivity with the recipient absent). Therecipient's value measures its contribution to group reproductivesuccess. We demonstrate theoretically that if aggression increasesthe aggressor's share of the group's expected total reproductiveoutput, but at the same time decreases the magnitude of thisoverall reproductive output, then the optimal level of aggressiontoward a recipient will decrease with increasing recipient'svalue. This proof establishes a rigorous theoretical connectionbetween the level of aggression within a group and the benefitsof belonging to such a group and can be tested by experimentallymanipulating the values of group members to each other. We test,and thus illustrate the utility of, this model by examiningaggression within experimentally-manipulated foundress associationsof social wasps. We show that the value of co-foundresses toeach other in the social wasp Polistes fuscatus lies in theirability to provide insurance against colony failure caused bythe loss of all tending foundresses. Removals of worker-destinedeggs and pupae, which increase the value of co-foundresses,both lead to significant reductions in aggression by the dominantfoundress, despite the fact that the immediate, selfish benefitsof competitive aggression should increase when empty brood cellsare present Removal of reproductive-destined eggs, which doesnot affect co-foundress value, but increases the benefits ofselfish aggression, causes a significant increase in aggressionby beta foundresses. Finally, wing reduction of subordinateco-foundresses significantly increases aggression by dominantfoundresses, as expected since the subordinate's value is reduced.Our results indicate that foundress aggression is sensitiveto the value of future cooperation, as predicted by the optimalaggression model. The model may apply widely to both invertebrateand vertebrate societies