Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers
 
 

 

 
 

The womb mate effect. For a detailed description, see Chapter 3 in Our Stolen Future, or examine the original scientific literature (references below)

If it's after breakfast, think about the uterus of a pregnant mouse (below).

 

 
 

The fetuses (up to 12) are arranged around the uterus one by one.

 

 
 

Each of them has either one or two neighbors, depending upon where they are in the womb. Those neighbors can be either male, or female.

Fred vom Saal discovered that each fetus creates a hormonal gradient around itself, with the nature of the gradient determined by the sex of the fetus. Those gradients affect their neighbors. Thus even within the same womb, maturing fetuses experience slight differences in their hormonal experience. For example, a female fetus with a female on each side (what vom Saal calls a Zero-Male female, or 0M, female; see diagram at right) experiences slightly more estrogen in the womb than a Two-Male (2M) female.

vom Saal and others have shown that these slight differences in fetal hormone experience have profound, life-long consequences for the mouse as it matures. 2M females, for example, once they reach adulthood are more aggressive than 0M females, and less attractive to males. 0M females mature faster than 2M females. 0M males have increased sexual activity compared to 2M males and are more likely to attack and kill baby mice when given the opportunity. In contrast, 2M males are more like females in parenting behavior than are 0M males.

What is astonishing about these results is how small the differences in fetal hormone experience actually are. 0M vs 2M females differ in experience by only 35 parts per trillion of estradiol (a form of estrogen) and 1 part per billion testosterone. vom Saal's evidence on the impact of these low level variations gave the first hints of the possibility that endocrine disrupting chemicals might also work at low exposure levels.

 

 
 

vom Saal, F, and F. Bronson. 1980. Sexual characteristics of adult female mice are correlated with their blood testosterone levels during prenatal development. Science 208:597-99.

vom Saal, F. 1984. The intrauterine position phenomenon: effects on physiology, aggressive behavior and population dynamics in house mice. in Biological Perspectives on Aggression, K. Flannelly, R. Blanchard and D. Blanchard (eds). Progress in Clinical Biology Research 169:135-179.

vom Saal, F., and J Vandenbergh. 1987. Regulation of puberty and its consequences on population dynamics of mice. American Zoologist 27:891-98.

vom Saal, F. 1989. Sexual differentiation in litter bearing mammals: influence of sex of adjacent fetuses in utero. Journal of Animal Science 67:1824-40.

 

 
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