Are there synergistic or antagonistic effects of multiple maternally derived egg components on offspring phenotype?

Eggs are 'multivariate' in that they contain multiple maternally derived egg components (e.g. hormones, antibodies, mRNA, antioxidants) which are thought to influence offspring phenotype. However, most studies have focused on single egg components and on short-term effects. Here, we simultaneously manipulated two egg components, maternally derived antibodies (MAb) and yolk testosterone, to assess potential synergistic or antagonistic effects on zebra finch offspring phenotype from hatching to sexual maturity. We found no evidence for short- or long-term effects of either MAb or yolk testosterone alone, or their interaction, on hatching mass, size at fledging (tarsus length), body mass at sexual maturity (day 82), chick survival, humoral immune function or any measured female reproductive trait at sexual maturity. There was a positive effect of yolk testosterone, but not MAb, on offspring phytohaemagglutinin (PHA) response at 26 days of age but at 82 days of age, MAb, but not yolk testosterone, had a positive effect on PHA response. There was also a MAb×sex interaction on 30 day chick mass, and a positive effect of yolk testosterone on male courtship behaviour at sexual maturity. However, we found no evidence for synergy, i.e. where offspring treated with both MAb and yolk testosterone had higher trait values than offspring treated with either MAb or yolk testosterone alone for any measured trait. Similarly, evidence for antagonistic (compensatory) effects, where offspring treated with both MAb and yolk testosterone had intermediate trait values compared with offspring treated with either MAb or yolk testosterone alone, was equivocal.