Historically, many theorists have argued that the Mendelian nature of chromosomal sex determination limits opportunities for parental control of offspring sex ratio. However, adaptive adjustment of sex ratio has been found among many animals, including primates, red deer, and birds. The exact mechanism of such allocation is unknown, but several studies indicate that hormonal, pre-ovulatory control may be responsible. For example, higher levels of follicular testosterone in mothers, signifying maternal dominance, correlated with a higher chance of forming a male embryo in cows. Higher corticosterone levels in breeding female Japanese quails were associated with female-biased sex ratios at laying.