Possible constraints on adaptive variation in sex ratio at birth in humans and other primates

Temperature-related birth sex ratio bias in historical Sami: warm years bring more sons

“…If mothers reproducing in warm years deliver particularly successful sons, this might provide evidence for the Trivers & Willard's (1973) hypothesis. However, a temperature-dependent physiological mechanism(s) may override maternal ability to facultatively adjust offspring sex, and thus produce neutral or even maladaptive sex ratio variation (James 2006).…”

Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception

“…However, though the TW paper has been cited more than 1000 times, tests of the hypothesis in some taxa (e.g. primates) have been successful in about only 50% of (roughly 100) attempts ( James 2006d). For this reason (among others), the author suggested, in that note, that parental hormones are not only responsible for adaptive variation of sex ratio, but also for constraints (processes which conflict with the adaptive processes) on that variation.…”

Decreased Levels of Plasma Testosterone/LH Ratio in Male Mice Exposed to Sodium Arsenite

“…Moreover, Egeland et al (1994) reported that men exposed to dioxin had low T and high gonadotropin levels (Egeland et al, 1994). There are a number of illnesses and occupations in which men have been reported to sire an excess of daughters and to display low T and/or high gonadotropin levels (James, 2006). Therefore, examination of T/LH ratio may provide more meaningful information than comparisons of mean LH and T levels examined separately (Foster et al, 1993).…”

“…More-fertile males sired a greater number of sons and less-fertile males sired more daughters (Gomendio et al, 2006). There is evidence that low parental T and/or high gonadotropin levels at conception are associated with the sex ratio (proportion of male at birth) of mammalian (including human) offspring (James, 2006). Testicular secretion of T is known to be impaired by many adverse chemical, occupational and medical exposures to men, and these are accompanied by low offspring sex ratios (proportion of male at birth) (James, 2006).…”

“…There is evidence that low parental T and/or high gonadotropin levels at conception are associated with the sex ratio (proportion of male at birth) of mammalian (including human) offspring (James, 2006). Testicular secretion of T is known to be impaired by many adverse chemical, occupational and medical exposures to men, and these are accompanied by low offspring sex ratios (proportion of male at birth) (James, 2006). It has been known that a substantially lower male/ female sex ratio at birth in the offspring of people exposed to high concentrations of dioxin (Mocarelli et al, 1996).…”