Manipulative Signals in Family Conflict? On the Function of Maternal Yolk Hormones in Birds

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Abstract
The exciting discovery by Hubert Schwabl (Washington State University) in the early nineties that the yolk of bird eggs contains hormones originating from the mother opened up an extremely successful research area for endocrinologists and behavioral ecologists. Since then, knowledge of both the factors that influence the amount of hormones deposited in yolk by the female, and the consequences of exposure to these maternal hormones on offspring growth, physiology, and behaviour has accumulated rapidly. The field has been dominated by the idea that mothers use yolk hormones to adaptively adjust offspring development in their interest. However, this idea neglects that the (evolutionary) interests of mother and offspring may differ. Each offspring will try to maximize its share of parental investment while the mother wants to distribute her investment among all her offspring. When there is such parent-offspring conflict the offspring are selected to respond to the yolk hormones in a way that is b Parent-offspring conflict is not the only intra-familial strife that may shape the evolution of yolk hormone deposition. Parents disagree (evolutionarily) over the amount of care each of them is willing to provide to their young. Since yolk hormones affect offspring begging behavior, females may be able to use yolk hormone deposition to modify begging traits in a way that increases the amount of parental care provided by the male. The existence of sex differences in response to offspring begging The authors therefore suggest that for a full understanding of the evolution of hormone-mediated maternal effects it is essential to study both fitness consequences, and physiological mechanisms and constraints, from the perspective of all family members. Furthermore, the authors believe that the integration of evolutionary biology and endocrinology will lead to the most rapid advance in this field - from both an evolutionary and endocrinological point of view. It may therefore not be coinciden

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