A Time to
Wean
Katherine A. Dettwyler,
PhD Department of Anthropology Texas A & M University
College Station Texas
from Breastfeeding Abstracts,
August 1994, Volume 14, Number 1, pp. 3-4.
In the United States, women
receive conflicting advice about when to wean their children completely
from breastfeeding. The American Academy of Pediatrics recommends one
year, while WHO and UNICEF recommend at least two years. Many physicians
consider six months to be "extended" breastfeeding, and some
health professionals question the motives of women who nurse for more
than a year. In turn, women may hide the fact that they are still nursing
an older child from disapproving health care professionals or family
members. From anthropological research, we know that in many non-Western
cultures children are routinely nursed for three to four years. Are
they eccentric, or are we? Can we look to other animals to determine
what the natural age at weaning would be in modern humans if it was
not modified by cultural beliefs?
Like all mammals, humans
have mammary glands that function to nurture their offspring. Within
the class Mammalia, humans are members of the order Primates, and have
the basic primate pattern of breastfeeding and weaning activity that
has been molded by more than 65 million years of natural selection to
ensure the best possible survival rate of primate offspring. This basic
pattern is assumed to be primarily genetically based. In addition, a
number of life-history variables are also associated with age at weaning
in the non-human primates. What do these variables suggest about the
"natural" age of weaning in humans?
Weaning according to tripling
or quadrupling of birth weight.
The idea that mammals wean
their offspring when they have tripled their birth weight is widely
reported in the breastfeeding literature (Lawrence 1989). This rule
of thumb holds true for small-bodied mammals, but not for larger ones.
Recent research has looked at age at weaning and at growth among large
mammals, including primates. The research shows that weaning occurs
some months after quadrupling of the birth weight, rather than tripling
(Lee, Majluf and Gordon 1991). When do U.S. infants typically quadruple
their birth weight? For males, the average age is around 27 months,
and for females, around 30 months.
Weaning according to attainment
of one-third adult weight. Other studies suggest that
primates are like other mammals in weaning each offspring when they
reach about one-third their adult weight (Charnov and Berrigan 1993).
Humans come in different sizes, but 4 to 7 years of nursing would be
the weaning age for humans using this method of comparison, with boys
generally being nursed longer than girls, and large-bodied populations
nursing longer than small-bodied groups.
Weaning according to adult
body size. Harvey and Clutton-Brock
(1985) published a study of life-history variables in primates, including
a formula for calculating age at weaning based on adult female body
weight. The equation predicts an age at weaning for humans at between
2.8 and 3.7 years, depending on average adult female body weight, with
larger-bodied populations nursing the longest.
Weaning according to gestation
length. It is often reported in the
literature that, among mammals in general, weaning age is approximately
the same as the length of gestation (Lawrence 1989). By this criterion,
weaning in humans might be expected to take place after only nine months
of breastfeeding. However, this one-to-one relationship is greatly affected
by the adult size of the animal. For many small-bodied primates, the
duration of breastfeeding is shorter than the length of gestation. Among
large-bodied primate species, the duration of breastfeeding far exceeds
the average length of gestation. For humankind's closest relatives,
the chimpanzee and the gorilla, the duration of breastfeeding is more
than six times the length of gestation. Humans are among the largest
of the primates, and share more than 98 percent of their genetic material
with chimpanzees and gorillas. Based on these comparisons, an estimated
natural age at weaning for humans would be a minimum of six times gestational
length, or 4.5 years.
Weaning according to dental
eruption. According to the research
of Smith (1991), many primates wean their offspring when they are erupting
their first permanent molars. First permanent molar eruption occurs
around 5.5 to 6.0 years in modern humans. It is interesting to note
that achievement of adult immune competence in humans also occurs at
approximately six years of age, suggesting that throughout our recent
evolutionary past, the active immunities provided by breast milk were
normally available to the child until about this age (Fredrickson).
Our evolutionary past has
produced an organism that relies on breastfeeding to provide the context
for physical, cognitive, and emotional development. The human primate
data suggest that human children are designed to receive all of the
benefits of breast milk and breastfeeding for an absolute minimum of
two and a half years, and an apparent upper limit of around 7 years.
Natural selection has favored those infants with a strong, genetically
coded blueprint that programs them to expect nursing to continue for
a number of years after birth and results in the urge to suckle remaining
strong for this entire period. Many societies today are able to meet
a child's nutritional needs with modified adult foods after the age
of three or four years. Western, industrialized societies can compensate
for some (but not all) of the immunological benefits of breastfeeding
with antibiotics, vaccines and improved sanitation. But the physical,
cognitive, and emotional needs of the young child persist. Health care
professionals, parents, and the general public should be made aware
that somewhere between three and seven years may be a reasonable and
appropriate age of weaning for humans, however uncommon it may be in
the United States to nurse an infant through toddlerhood and beyond.
References
[Note: the version of
this article that appeared in the publication NEW BEGINNINGS (May-June
1995, pp. 86-87) is also available on our Web site.]
Page last edited Sun Oct 14 09:32:41 UTC 2007.
