Parental investment amplifies effects of genetic complementarity on growth rates in song sparrows, Melospiza melodia

Life history theory predicts that individuals paired with attractive mates may invest more in offspring. Such differential investment may amplify the effects of genetic quality on fitness. Attractiveness can include ‘good gene’ and ‘complementary gene’ components, but how the latter affects parental investment remains unknown. We found that nestling song sparrows with genetically dissimilar parents grew faster than did nestlings whose parents were genetically more similar to one another. A cross-fostering experiment revealed complementary gene effects on growth; nestlings produced by genetically dissimilar parents grew faster than their adoptive ‘siblings’ reared in the same nest but produced by parents that were more genetically similar. To explore whether parental investment exaggerates these complementary gene effects on growth, we monitored parental nest visits. Maternal visits were negatively related to genetic similarity between mates. The novel finding that females adjust levels of care according to the genetic diversity of their offspring suggests that parental investment can amplify complementary gene effects on fitness.