A Time to Wean
Katherine A. Dettwyler, PhD
College Station, Texas, USA
From: NEW BEGINNINGS, Vol. 12 No. 3, May-June 1995, pp. 86-67
We provide articles
from our publications from previous years for reference for our Leaders and
members. Readers are cautioned to remember that research and medical information
change over time.
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 should 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 of 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 of 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 US 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 four to seven 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 non-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 seven 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
Charnov, E.L. and D. Berrigan.
Why do female primates have such long lifespans and so few babies? or
Life in the slow lane. Evol Anthropol 1993; 1:191-94
Fredrickson, D. University of Kansas, personal communication.
Harvey, P.H. and T.H. Clutton-Brock.
Life history variation in primates. Evolution 1985; 39:559-81.
Lawrence, R.A. Breastfeeding:
A Guide for the Medical Profession, 3rd edition. St. Louis: Mosby,
1989.
Lee, P.C., P. Majluf, and
I.J. Gordon. Growth, weaning and maternal investment from a comparative
perspective. J Zool Land 1991; 225:99-114.
Smith, B.H. Age of weaning
approximates age of emergence of the first permanent molar in nonhuman
primates, abstracted. Phys Anthropol Suppl 1991; 12:163-63.
This article is an excerpt from the August 1994 issue of BREASTFEEDING
ABSTRACTS, LLLI's quarterly publication for health professionals. The
BREASTFEEDING ABSTRACTS article is also available on our Web site.
Last updated 11/12/06 by jlm.
Page last edited Sun Oct 14 09:30:46 UTC 2007.
