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