The social environment of reproducing females can induce changes in behaviour and physiology, with consequences for reproductive investment. Changes in reproductive investment, in turn, may modify the prenatal environment of the developing offspring and can thereby profoundly shape the offspring’s future phenotype. Such prenatal maternal effects may drive adaptive transgenerational plasticity, enabling mothers to prepare offspring for their future environmental conditions and thereby increasing their chances of survival. In the case of such anticipatory maternal effects, offspring that experience conditions that match the conditions predicted by the maternal phenotype are expected to perform better than offspring experiencing mismatching conditions. The maternal and offspring environments are thus expected to have interactive effects on offspring phenotypes. We tested for anticipatory maternal effects in a match/mismatch experiment by investigating the (interactive) effects of one important aspect of the social environment – group size – on maternal and offspring physiology, morphology reproduction and behaviour in a precocial avian species, the Japanese quail (_Coturnix japonica_).<br />

In the parental (P0) generation (chapter 2), the social environment of adult female Japanese quail was manipulated by housing the females in pairs (one female, one male) or groups (three females, one male). In previous studies, increased social density or social challenges have been linked to higher circulating androgen and glucocorticoid levels. Against our predictions, females housed in pairs had significantly higher concentrations of circulating androgens and tended to have higher concentrations of circulating corticosterone than females housed in groups. Although the female’s baseline hormone levels were affected by the social environment, we found no indication for effects on the response to endocrinological challenges of the main stress (hypothalamic-pituitary-adrenal) and reproductive (hypothalamic-pituitary-gonadal) axis. Furthermore, the social environment had no effects on female reproduction, suggesting that the effects on female endocrine physiology had little fitness consequences. Counter to our expectations, the social environment did not affect yolk testosterone levels, and we did not find a correlation between yolk testosterone levels and the females’ response to gonadotrophin releasing hormone (GnRH). We propose that our unexpected findings are due to differences in the exposure to males in our social treatments. In pairs, the male copulatory behaviour may have stimulated female circulating hormone levels more strongly than in groups where effects were diluted due to the presence of other females.<br />

Changes in social density have been shown to affect offspring sex ratio in previous studies, and variation in maternal hormone levels around conception have been suggested as a proximate mechanism underlying such effects. High maternal androgens have repeatedly been linked to increased investment in sons, whereas high glucocorticoid levels are usually related to increased investment in daughters. Even though maternal endocrine physiology was affected, we found no evidence for effects of the maternal social environment or maternal circulating androgen and corticosterone concentrations on offspring sex ratio or sex-specific juvenile survival (chapter 3). The maternal social environment did also not affect juvenile offspring growth and circulating androgen and corticosterone levels. Our negative results might be explained by the lack of effects on egg mass or yolk testosterone levels in the parental generation, since both are important mediators of maternal effects. Furthermore, differences between the type of social stimuli and the timing of changes in the social environment and hormones with respect to the reproductive cycle and meiosis might explain the contrasting results between studies.<br />

F1 adult females were housed under social conditions that either matched or mismatched their maternal social conditions with respect to group size (pairs of two females and groups of four females; chapter 4). This experimental setup allowed us to investigate the interactive effects of the maternal and adult F1 offspring social environments. We found an interaction effect between the maternal and own social environment on F1 female mass, in combination with a significant effect of the F1 social environment on growth. We initially predicted matched offspring to perform better, however, ‘mismatched’ group-housed daughters from pair-housed mothers turned out to be heavier overall than females from the other combinations of P0-F1 social environments. Our findings thus support the idea that maternal effects may emerge context-dependent, though the adaptive value of this match/mismatch effect remains speculative. Furthermore, in contrast to our findings in the P0 generation, the social environment of the F1 females did not affect their circulating hormone levels, but affected their growth and reproductive investment. F1 females housed in groups grew more than pair-housed females, which resulted in a maternal effect on egg mass, hatching success and F2 offspring mass at hatching (all increased compared to F1 pair-housed females; chapter 4). These effects on F2 hatch mass could have important consequences for their subsequent growth and survival, which should be further investigated in future studies.

The effects of social group size on female physiology, reproduction, and the next generation differed between the P0 and F1 generations. Differences in the sex ratios of the social environments between the P0 and F1 generation could partly explain these effects.<br />

Taken together, our results indicate that the social environment does affect female physiology and reproduction, and may induce maternal effects on the offspring’s phenotype in a context-dependent way. However, our results also indicate that different types of social stimuli induce different effects on females and their offspring. Furthermore, the timing of measurements and manipulations of the social environment or female and offspring physiology is likely an important factor explaining why results vary between studies. To gain a better understanding of the underlying mechanisms and the function of maternal effects of the social environment, it is important to establish which social stimuli are most important, and how effects of social stimuli may interact with each other. The studies described in this thesis point towards a number of factors that should be further investigated, in particular the effects of different adult sex ratios on females and their offspring. Moreover, it is important to further investigate what mediates maternal effects and at which time they manifest. This includes studying how resources accumulate in the yolk, and how environmental factors can influence these processes.