KeywordsTestosterone Level Male Offspring Maternal Stress Glucose Availability Chromosomal Composition
Chromosomal composition of an individual’s progeny
Offspring sex refers to the chromosomal composition of an individual’s progeny. In humans and most other mammals, this is determined by the joint contribution of either an X- or a Y chromosome-bearing sperm from a father and the X chromosome-bearing ovum from a mother. Males possess both an X and a Y chromosome, whereas females possess two X chromosomes. Offspring sex can be influenced both pre- and postconception, and this is affected by many biological and environmental factors. Evolutionary theory predicts that parents may be able to adjust the sexes of their offspring. Although the exact mechanism for this remains unknown, several physiological, psychological, and environmental factors have been implicated.
Facultative Adjustment of Offspring Sex
Sex ratio, or the ratio of the number of males to the number of females within a population, can be considered at both primary and secondary levels (tertiary sex ratio, or number of males to the number of females at sexual maturity in a population will not be discussed here). Primary sex ratio refers to the sex ratio at conception and secondary sex ratio refers to the sex ratio at birth. Although in humans the secondary sex ratio remains relatively stable at approximately 105–107 male births for every 100 female births, the primary sex ratio is more difficult to quantify.
Deviations from the typical male-biased secondary sex ratio have been observed in response to various large-scale events (e.g., wars) (reviewed in James 2009), but it was not until the Trivers-Willard hypothesis (Trivers and Willard 1973) was published that researchers began to untangle the potential mechanisms through which these changes might occur. Since then, parental manipulation of offspring sex has been studied from physiological, psychological, and environmental contexts, both pre- and postconception. However, in meta-analyses with ungulates and primates, respectively, both Sheldon and West (2004) and Cameron (2004) have demonstrated that facultative adjustment of offspring sex is most likely to occur close to conception rather than as a result of differential investment in offspring postconception. Thus, adjustment of sex ratios at conception may be the source of skewed secondary ratios in response to factors associated with the maternal environment.
The bulk of research involving facultative adjustment of offspring sex preconception involves maternal influences. Three physiological mechanisms have been proposed: stress, glucose availability, and testosterone levels (reviewed in Edwards et al. 2016). Maternal stress increases endogenous concentrations of glucocorticoids (e.g., cortisol), which may explain female-biased sex ratios in chronically stressed women. Maternal glucose availability, which is indicative of energy store, is positively associated with the frequency of male offspring. Similarly, maternal testosterone levels may also increase the probability of conceiving a male (Grant et al. 2011). Paternal influences on offspring sex, such as changes in the proportions of X- and Y chromosome-bearing sperm in seminal fluid, may also occur, but this has not consistently demonstrated.
Interestingly, maternal stress, glucose availability, and testosterone level may all be interrelated and under the influence of psychological mechanisms. Maternal dominance, or a combination of inherent female traits (e.g., influential, authoritative), that facilitate the changing of another’s view or actions, for example, has been associated with the production of male offspring in women (e.g., Grant 1990, 1994). It may also be associated with better access to resources (and thus, sources of glucose) and lowered feelings of stress, ultimately producing an intrauterine environment conducive to the conception of male offspring. Indeed, larger women as well as women with greater muscle mass in the upper arm – a possible proxy for nutritional status (i.e., resource acquisition ability) – also have more sons (Kanazawa 2005; Gibson and Mace 2003).
Offspring sex refers to whether an individual’s progeny is chromosomally male (i.e., XY) or female (i.e., XX). Although offspring sex can be determined at both primary (i.e., at conception) and secondary (i.e., at birth) levels, the majority of human research involves analysis of the secondary sex ratio. Facultative adjustment of offspring sex has been proposed and likely takes place at conception under the combined influence of both physiological and behavioral influences.