Commentary: Parental care and the proximate links between maternal effects and offspring fitness.

Maternal effects influence the phenotype of offspring through non-genetic mechanisms, and thus are important components of individual life-histories and act as drivers of and/or constraints on phenotypic evolution. A maternal effect common in egg-laying vertebrates is provisioning of the yolk with carotenoids, organic pigments that often color sexual ornaments and are hypothesized to play positive and substantial physiological roles. In a recent study, yolks of great tit (Parus major) eggs were directly supplemented with carotenoids, and the effects on offspring fitness proxies measured (Marri and Richner in Oecologia 176:371-377, 2014a). Nestlings from supplemented broods were heavier early in development and more likely to fledge, but otherwise equivalent to control nestlings. The authors consider in detail the potential physiological mechanisms that might underlie this result, and here I expand on their Discussion by considering a non-exclusive explanation: that parents provided higher quality care to broods that received supplemental carotenoids. I discuss the general non-independence of pre- and post-hatching/parturition maternal effects when parents care for offspring, and then briefly review evidence that carotenoids specifically are tied to the intensity of avian begging displays. Finally, I detail how inclusive fitness opportunities and constraints shape the adaptive landscape in which maternal effects operate, highlighting both theoretical and applied concerns surrounding questions about the adaptiveness of maternal effects.