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Mothers know best? Fitness consequences and flexible adjustment of non-genetic inheritance in the zebra finch

Final Activity Report Summary - FANGI (Mothers know best? Fitness consequences and flexible adjustment of non-genetic inheritance in the zebra finch.)

The main aim of the project was to study the mechanism and function of maternal effects, both experimentally and theoretically. The focus was on the question regarding the extent in which different family members have control over and benefit from maternal effects and the way in which mechanisms of maternal effects influence their evolution.

The experimental work focussed on hormone-mediated maternal effects in birds, i.e. maternal effects because of the transfer of hormones to avian eggs. The first set of experiments studied the transfer and metabolism of two steroid hormones, testosterone and corticosterone, from the mother to the egg and from the egg to the embryo using radioactively labelled hormones. The experiments revealed that hormones were transferred from the maternal circulation to both albumen and egg white but over different time scales, suggesting that different information about the environment could be contained in the albumen and the yolk. It was also found that hormones were metabolised to different compounds during the transfer from the mother to the egg. This had both functional implications, since the metabolites could be physiologically active or inactive, and methodological implications, since measurements of hormones in the egg had to take into account the possibility of cross-reaction with metabolites.

The experiments further revealed that hormones were quickly taken up by the embryo in development and were metabolised into different compounds. This showed that the embryo had some potential control over the effects of maternal hormones by modifying the compounds to potentially inactive substances, a possibility that suggested some profitable avenues for future research. In conclusion, these experiments demonstrated that both mother and offspring had some control over the mechanism of hormone-mediated maternal effects, which in theory should allow both of them to achieve their optimal effects to some extent.

The second set of experiments focussed on the question regarding the extent to which mother and offspring benefited from variation in yolk hormone level, and involved experimentally manipulating yolk hormone levels and assessing the consequences for mother, offspring and siblings. Offspring exposed to the hormone and their siblings were negatively affected by the treatment while mothers did not seem to be affected, suggesting that natural variation in yolk did not benefit individual offspring at the cost of their siblings, but rather that maternal hormones resulted in maximisation of the quality of all offspring.

The third part of the project comprised theoretical work on the evolution of maternal effects in the context of conflict between family members. Conceptual work regarding hormone mediated maternal effects concluded that the actual mechanisms that allowed mother and offspring to regulate hormone-mediated maternal effects were crucial in determining whether mother or offspring benefited from these consequences. Furthermore, a novel game theoretical model was developed to investigate the adjustment of different components of female parental investment, prenatal versus postnatal, in relation to male parental investment. Previous models of biparental care typically predicted a reduction of female parental investment in response to an increase in male parental investment. The results of the new model, however, suggested that female postnatal investment could even increase in response to male assistance with care of young, because the presence of the male might select for reduced prenatal investment by the female.