Maternal body condition and plasma hormones affect offspring sex ratio in peafowl

In theory, females that can afford to do so may increase their fitness by investing in offspring of the sex with the greater probability of attaining high reproductive success. This has been observed in a wide variety of mammalian, reptilian and avian species although the proximate mechanism remains a mystery. Using a captive population of peafowl, Pavo cristatus, we investigated the relation between maternal quality, offspring sex ratio and plasma concentrations of the reproductive hormones testosterone and 17β-oestradiol and the principal avian stress hormone corticosterone. Each peacock was paired with three peahens that differed with respect to their relative body condition, creating a condition and dominance hierarchy within each pen and thus a situation in which we would predict investment in offspring to vary between hens. We found significant intercorrelations between maternal body condition (but not dominance rank or clutch size), maternal plasma levels of corticosterone and testosterone (but not 17β-oestradiol) and clutch sex ratio, such that good maternal condition, low plasma corticosterone and high levels of testosterone were associated with male biases in the sex ratio and increased investment in male eggs. The observed biases were probably present at laying, and thus add to the growing number of studies showing primary sex ratio adjustment in response to maternal body condition, and furthermore may indicate a role for corticosterone and testosterone in the avian sex manipulation process.