Parentage and the evolution of parental behavior

Parentage is the proportion of juveniles in a brood that are offspring of potential care givers. We analyzed how reductions in parentage affect the evolution of parental behavior using a static optimization model. The main benefit of parental effort was an increase in the survival of offspring, and the main costs were reduced opportunities to seek additional matings or to parasitize neighbors and or reduced survival. Both the costs and benefits included terms for relatedness to young. The effect of parentage depended on (1) whether parents responded in ecological time (facultative response) or in evolutionary time (nonfacultative response), (2) whether the cues enabling assessment of parentage permitted discrimination among offspring, and (3) whether parentage was the same among different groups of juveniles (unrestricted) or varied between them (restricted). When parents did not know their own parentage and mean parentage was the same for all matings, reduced parentage affected the costs and benefits equally, so, as in several previous models, there was no effect on the optimal level of parental effort. Parentage did affect optimal parental effort when mean parentage to the present brood differed from that to young from alternative or future matings. Lowered parentage reduced optimal parental effort when the cost of parenting was missed opportunities for extrapair copulations or brood parasitism or when parentage was consistently higher in alternative or future matings. Nonlinear changes in parentage with age gave complex trajectories of parental care, with individuals of different ages having similar parentage but exhibiting different levels of parental effort. Correlations between parentage and other variables in the model (such as opportunities for additional matings) sometimes masked, but never eliminated, the effects of parentage. When parents could discriminate their own young in a brood, overall parental effort was reduced, but nepotism was increased. When parents could not discriminate their own offspring but had general cues about average parentage to the brood, effects varied depending on the costs and benefits of parental behavior. When parental behavior was costly to care givers, parentage had more effect than when parenting was not costly. Likewise, parentage had less effect when care greatly increased offspring survival than when care was less necessary. Our analyses reconcile conflicting results from previous models and suggest a general framework for analyzing parental behavior within populations and among higher taxonomic groups.