Maternal corticosteroids influence primary offspring sex ratio in a free-ranging passerine bird

When fitness benefits of investment in sons and daughters differ, animals are predicted to manipulate the sex ratio of their offspring. Sex ratio manipulation occurs in many taxa, but the mechanisms underlying the phenomenon in vertebrates remain largely unknown. Factors favoring skewed sex ratios, such as reduced maternal condition or food availability, also induce elevated corticosteroids. Recent experimental studies support a causal relationship between corticosteroids and sex ratio. Evidence of a natural correlation between maternal corticosteroids and offspring sex ratio has been lacking, however. Without such evidence, the importance of corticosteroids in influencing sex ratios in natural populations was unknown. We measured baseline corticosteroids in 19 free-ranging female white-crowned sparrows (Zonotrichia leucophrys) and the sex ratios of their offspring. Females with high corticosteroids produced more daughters than females with low hormone levels. We then conducted a controlled, fieldbased experiment investigating the effects of moderately increased maternal corticosteroids on offspring sex ratios to determine if the observed correlation reflects a causal relationship between maternal corticosteroids and offspring sex ratio. Hormone-implanted females produced more female embryos than control females. These findings provide the first evidence of a natural correlation between maternal corticosteroids and offspring sex ratios in free-ranging birds, and the first experimental evidence of a causal link between moderate increases in corticosteroids and biased primary sex ratios.