Explaining discrepancies in the study of maternal effects: the role of context and embryo

Prenatal maternal effects are increasingly recognized as important mediators in the development of individual differences during early sensitive or even critical periods. Hormone-mediated maternal effects in egg-laying species are a frequently used model to study such effects, mostly to test whether these increase maternal fitness. However, experimental evidence is inconsistent. This has led researchers to divert to other topics. In this review, we argue that from a Darwinian perspective one should however expect strong interactions between effects of maternal hormones with contextual cues, including environmental factors, embryonic modulation of maternal signals, offspring age and sex, and fathers’ influence. Taking these into account may explain the inconsistencies and new experiments should reveal how the benefits and costs of maternal hormones and prenatal maternal effects in general play out in different contexts.