Mother–offspring conflicts, hormone signaling, and asymmetric ownership of information

Abstract

In sexually reproducing organisms, asymmetries in relatedness often result in deviating interests over the reproductive investment, that is, in parent–offspring conflicts. Because the costs and benefits of parental investment differ between parents and their offspring, selection will act antagonistically on them (e.g., Trivers 1974; Godfray 1995; Parker et al. 2002). Offspring are under selection to demand more resources and/or parental care than what is optimal for the parents, and the parents are selected to optimize their own fitness, which results in parental investment that is suboptimal for the offspring. To bring parental investment closer to their respective optima, both parents and offspring may use manipulative signals to influence offspring demand or parental investment. One particular type of signal that has received considerable attention within this framework is hormones (Haig 1993, 1996; Muller et al. 2007; Uller 2008). The function of yolk hormones as manipulative maternal signals in birds has been discussed comprehensively by Gil (2003, 2008) and Groothuis et al. (2005) but only recently has their role as mediators in parent–offspring conflicts been considered in detail (Muller et al. 2007). In particular, Muller et al. (2007) emphasize that because the evolutionary interests of the mother and offspring may differ, offspring may be selected to respond to maternal hormonal signals ‘‘in a way that is adaptive for themselves rather than for the mother.’’ Consequently, offspring development may not always be optimal from the mother’s point of view. Here, we further elaborate on the role of maternally derived hormones as mediators in parent–offspring conflicts in birds. We present a concept that may help to explain why offspring remain prone to be manipulated even when there is a potential for mother–offspring conflicts and why this maternal manipulation of offspring by hormone signaling may be evolutionary stable. Although we specifically discuss maternal yolk hormones, our arguments are general and relate to any potentially signaling substance that is transferred from the mother to the egg.