Model parents: is full compensation for reduced partner nest attendance compatible with stable biparental care?

Previous models have suggested that biparental care will be evolutionarily stable when each parent only partially compensates for decreases in effort by their partner. We investigated a system where breeding success is an accelerating function of parental effort. This could occur in species with a high predation level—for example, in a dense sea-bird colony or in species where eggs or young are very prone to cooling. In these cases we found that parents will fully compensate for decreased partner effort, or else they will abandon the breeding attempt altogether. We use a second graphical model to show that biparental care can exist under a situation of full compensation for reduced partner effort if neither parent can do all the care alone. Each parent will abandon the breeding attempt if his or her condition falls below a certain threshold. If the participation of both parents is necessary for the breeding attempt to be successful, then neither parent will want to force their partner to abandon by making them work so hard that they fall below the condition threshold. Because abandonment by oen partner means the failure of the breeding attempt, each individual will do at least enough work so that the partner will not abandon, resulting in biparental care. There will be a region of conflict between the parents, within which the conflict can be resolved in various ways. Possible resolutions of this conflict, and the consequences and applications of the model, are discussed.