LOVE, OP*; WYNNE-EDWARDS , KE; WILLIAMS, TD; Simon Fraser University; Queen\\\’s University; Simon Fraser University: Maternal condition, yolk corticosterone and sex-specific modulation of offspring phenotype: experimental studies in a wild bird

�Maternal effects� translate environmental conditions experienced by parents to offspring and can directly influence offspring phenotype. In particular, maternal condition at the time of laying can significantly influence the allocation of maternal hormones towards eggs influencing egg quality and hence can effect the fitness of the offspring. It has recently been hypothesized that corticosterone (CORT) may in fact be involved in phenotypic modulation of offspring via maternal transfer to eggs given its prominent role in homeostatic energy balance and since a decline in condition is strongly associated with increased plasma CORT levels in birds. Here we report that, in contrast to earlier work in domesticated and captive species, CORT is present in significant levels in the yolks of a wild bird, the European starling (Sturnus vulgaris). Moreover, experimental elevation of maternal plasma CORT via silastic implants in egg-laying female starlings resulted in a significant elevation of yolk CORT. Furthermore, this increase in yolk CORT resulted in a sex-specific bias in offspring phenotype favoring female offspring (the less costly sex for a female starling in poor condition to raise). Finally, we present recent experimental data on a direct manipulation of yolk CORT levels via egg injections and examine CORT’s role in the modulation of sex ratio. Overall, results indicate that maternal CORT is transferred to yolks potentially providing a hormonal link between maternal condition and offspring phenotype. The result may be a fine-tuning of the quantity and quality of offspring in relation to maternal condition and/or prevailing environments.