Evolution of polyandry in a communal breeding system

The evolution of polyandry requires an asymmetrical factor thatfavors more matings per breeding female than per breeding male,thus reversing Bateman's principle. Here a model is presentedfor the evolution of avian cooperative polyandry. The modelshows that polyandry can evolve if communal breeding is initiallyadvantageous and if increasing clutch size beyond an optimumis detrimental. The advantage of communal breeding favors theaddition of more breeders (either males or females) and thusselects against breeding as single pairs (monogamy). The optimaldutch size creates the asymmetry that favors adding male breeders(polyandry) over adding female breeders (polygyny). Adding femalesis detrimental because females must lay eggs to reproduce andcan therefore increase the clutch size of the group. On theother hand, males can reproduce by sharing paternity withoutincreasing dutch size. It is shown that cooperative polyandryevolves either because it maximizes both male and female fitnessor because polygyny and monogamy are behaviorally unstable.Data from acorn woodpeckers support the assumptions of the modeland suggest that cooperative polyandry evolved because it isbehaviorally more stable. The persistence of monogamy and polygynyin acorn woodpeckers (at a lower incidence than polyandry) isalso examined. Polygyny in these birds represents a case ofthe Prisoner's Dilemma