Biological Sciences,

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 off-spring, 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 trajectorie