Mother's prior intrauterine position affects the sex ratio of her offspring in house mice.

Abstract

Sex ratio alterations related to environmental factors occur in several mammals, but no mechanism has been identified to explain the adjustment. Intrauterine position (IUP) may provide the context in which such alterations occur. Previous studies on house mice and gerbils reveal that the position of a fetus in the uterus in relation to the sex of its neighbors influences its later anatomy, physiology, and behavior. The anogenital distance (AGD) of females located between two males (2M) is longer than that of females not between two males (OM). We have found that the IUP, as determined by cesarean section and by an index of the AGD, correlates with the sex ratio of the litters produced by female mice. The sex ratio of the first Litter born to 2M females was 58% males, for iM females was 51% males and for OM females was 42% males. The effect on sex ratio continues into the second litter. The number of pups produced by mothers of different HUPs in her first two litters did not differ, suggesting that the sex ratio adjustment occurs prior to parturition. These results provide a basis for the natural variability observed in sex ratios of litter-bearing ammals, and suggest that one or more intrauterine mechanisms may be responsible for environmentally related sex ratio alterations.Parental reproductive fitness would increase if the sex ratio of their offspring could be adjusted to relate to environmental conditions. Trivers and Willard (1) hypothesized that under favorable environmental conditions males would be favored and that under adverse conditions females would be favored in polygynous species because a few strong males could sire many grandchildren, whereas females have a more sure but limited reproductive potential.A local resource competition model has been proposed (2, 3) to explain sex ratio alterations specifically for those species in which females show fidelity to their natal area and males leave the natal area to breed elsewhere. In this model, daughters of high-ranking mothers may achieve high reproductive success through inheriting the mother's high rank. However, since males breed in different groups, their reproductive success is presumably not influenced by the mother's rank. Thus, the local resource competition model predicts that, for certain species, high-ranking females would produce a female-biased sex ratio (3).Among invertebrates, breeding adults can adjust the sex ratio oftheir offspring to maximize their fitness (4). Data from mammals have been more controversial. Clutton-Brock and Iason (5) review a number of studies conforming to a high standard of evidence and conclude that significant variation in sex ratios at birth in nonhuman mammals exists and that sex ratio varies with the costs or benefits of producing male or female offspring. Investigators have also shown that manipulating food can influence sex ratios at birth. Experimentally supplementing the food supply of wild opossums (Didelphis marsupialis) results in male-biased sex ratios (6), and restricting the food available ...

Maternal diet composition alters serum steroid and free fatty acid concentrations and vaginal pH in mice

“…For example, female mice born between two males (2M) tend to be masculinized and have more male offspring (Vandenbergh & Huggett 1995, Vom Saal et al 1999. Dominant women with elevated serum testosterone concentrations are reported to have significantly more sons than submissive females with lower testosterone concentrations (Grant 1994, Singh & Zambarano 1997, Grant & France 2001, and James (1990) has implicated high testosterone concentrations in the mother around the time of conception as a factor likely to favor sons.…”

Androgens and environmental antiandrogens affect reproductive development and play behavior in the Sprague-Dawley rat.

“…Interestingly, in both gerbils and mice, the intrauterine position of females has significant influence on their subsequent litter sex ratios. Females that develop in utero between two males produce more males in their litters than do females gestated between two females (Clark and Galef 1995;Vandenbergh and Huggett 1994). The mechanism for this alteration is uncertain but is suspected to occur either at the level of fertilization or implantation, as there is no evidence of selective postimplantation or postnatal loss (Clark and Galef 1995;Vandenbergh and Huggett 1994).…”

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

“…Lastly Vandenbergh & Huggett (1994) reported highly significant evidence for such a phenomenon in the mouse. Taken together, these data provide some of the most powerful support for the author's hypothesis.…”