Potential mechanisms for sex ratio adjustment in mammals and birds

Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.