Maternal effects and offspring performance: in search of the best method

Traditionally, maternal effects have been treated as a source of troublesome environmental variance that confounds our ability to accurately estimate the genetic basis of the traits of interest. However, the adaptive significance of maternal effects is currently at the centre of the attention of ecologists. Thus, in turn, the genetic basis of traits has become a troublesome source of the genetic resemblance that confounds our ability to accurately estimate the maternal effects of interest. This fact is, however, less widely realized among ecologists. We demonstrate this on the example of studies investigating egg-size effects on offspring performance in birds. Traditionally this relationship is being studied by cross-fostering of eggs or young and it is claimed that this design is able to separate the effects of egg size per se. However, a positive covariation between the direct effects of genes and the maternal effects exists for many studied traits, which may result in overestimation of the egg-size effects on offspring performance in cross-fostering studies. Within-clutch comparisons or direct experimental manipulation of the egg size are the approaches that do not suffer from such covariation and therefore give less biased estimates of the egg-size effects than cross-fostering studies.