Evolution of polyandry in a communal breeding system

The evolution of polyandry requires an asymmetrical factor that favors more matings per bleeding female than per breeding male, thus reversing Bateman's principle. Here a model is presented for the evolution of avian cooperative polyandry. The model shows that polyandry can evolve if communal breeding is initially advantageous and if increasing clutch size beyond an optimum is detrimental. The advantage of communal breeding favors the addition of more breeders (either males or females) and thus selects against breeding as single pairs (monogamy). The optimal clutch size creates the asymmetry that favors adding male breeders (polyandry) over adding female breeders (polygyny). Adding females is detrimental because females must lay eggs to reproduce and can therefore increase the clutch size of the group. On the other hand, males can reproduce by sharing paternity without increasing clutch size. It is shown that cooperative polyandry evolves either because it maximizes both male and female fitness or because polygyny and monogamy are behaviorally unstable. Data from acorn woodpeckers support the assumptions of the model and suggest that cooperative polyandry evolved because it is behaviorally more stable. The persistence of monogamy and polygyny in acorn woodpeckers (at a lower incidence than polyandry) is also examined. Polygyny in these birds represents a case of the Prisoner's Dilemma.