14 i Flora Unveiled
children can be breastfed until the age of three, and this in turn lowers the probability
of another pregnancy. An average of five children per woman keeps the population
substantially stable, because more than half will generally die at an early age.^6Breast- feeding can potentially inhibit pregnancy because stimulation of the nipple trig-
gers the secretion of prolactin, a hormone that inhibits ovulation. But don’t try this at
home! The effectiveness of this method of birth control depends heavily on the frequency
and duration of the nursing episodes. Hence, breast- feeding as a method of birth control
only works well in cultures in which infants are carried by their mothers more or less con-
tinuously and are nursed whenever they wish. For this reason, women in hunter- gatherer
or agricultural societies have had greater success using lactation as a method of birth con-
trol than have women in industrial societies. If Paleolithic mothers nursed their babies for
several years, as seems likely, the number of pregnancies resulting from sexual intercourse
would have been reduced.
There are, however, still gaps in our understanding of the contraceptive effect of lac-
tation, such as the relative importance of the intensity of breast- feeding versus maternal
nutrition. Valleggia and Ellison attempted to disentangle these two factors in studies of
well- nourished Toba women in Formosa, Argentina.^7 Here, they found that the correlation
between lactation- induced amenorrhea was not correlated with either nursing intensity or
maternal nutrition alone, but also could be at least partially explained by the ratio of food
energy intake to energy expenditure in individual women— which varied widely. Like the
variable time between conception and the awareness of quickening, this much wider varia-
tion could obscure the identification of cause and effect concerning pregnancy.
Changes in menstrual and hormonal function characteristic of nonlactationally induced
amenorrhea have been reported in very thin, dieting, and exercising Western women,
including athletes. Similar hormonal changes have been found in rural farming villages in
Nepal, where Catherine Panter- Brick and colleagues showed that fecundity was correlated
with the women’s energy balance— that is, the balance between caloric intake versus energy
expenditure as work.^8 They found that during the monsoon season when food was scarce,
the level of progesterone in the women’s bodies decreased, and so did their fertility. In win-
ter, energy expenditure decreased, progesterone levels increased, and the fecundity of the
women increased. Peter Ellison has argued that the hypothalamus of the brain tracks short-
term changes in energy balance, not just total energy reserves, tending to restrict conception
at times that are inappropriate to sustain the energy drain of pregnancy and lactation.^9
In a 2004 study in rural Poland, Jasienska and Ellison found that ovarian function was
suppressed when women engaged in hard physical labor, even when their caloric intake kept
up with the amount of energy expended. That is, ovarian function was suppressed by hard
work alone even when the total energy balance remained unchanged.^10
Jasienska and Ellison have attempted to apply their findings about lactational amenor-
rhoea in southern Poland to Paleolithic hunter- gatherers.^11 They speculated that because
the Paleolithic diet was relatively poor in calories, periods of high energy expenditure by our
human ancestors typically were not balanced by commensurate increases in caloric intake,
thus leading to a negative energy balance. Under such conditions, ovulation would be sup-
pressed and female fertility would decline. Alternatively, even when the energy expenditure