In the context of mammals, humans are part of a group known as primates, and we follow the fundamental pattern of this category in terms of breastfeeding and weaning. Weaning is the process through which any mother and infant will transition, and its physiology must also be known.

This pattern has developed over more than 65 million years of natural evolution to ensure an optimal survival rate for the primate offspring. It is presumed that this fundamental pattern has an essential genetic basis.

In addition, certain variables in the life cycle of non-human primates are linked to the age at which weaning occurs. What do these variables imply about the “natural” age of weaning in humans?

Weaning when the birth weight has multiplied three or four times

The notion that mammals wean their young when they reach three to four times their birth weight is widely discussed in lactation literature. Although this approach is valid for small mammals, it does not apply to larger mammals. Recent research has focused on the timing of weaning and growth in large mammals, including primates. The findings suggest that weaning occurs several months after birth weight has quadrupled rather than tripled. And when do infants in the United States quadruple their birth weight on average? Boys do so around 27 months, and girls at about 30 months.

Weaning at one-third of adult weight

Other research indicates that primates follow a similar pattern to other mammals, weaning their young when they reach about one-third the weight of an adult. Since humans have a wide range of body sizes, this approach would place weaning anywhere from 4 to 7 years of lactation. Generally, boys are breastfed for longer than girls. Where a population group is, on average, larger, they tend to prolong breastfeeding for longer than population groups with smaller average weight.

Weaning according to adult body size

A study was conducted about variables in the life cycle of primates, which included a formula to calculate the time of weaning based on the body weight of an adult female. According to this equation, weaning in humans would occur between 2.8 and 3.7 years, depending on the average weight of adult females. And population groups with larger individuals tend to prolong lactation.

Weaning according to pregnancy length

Literature often mentions that weaning in mammals tends to match approximately the length of the gestating period. Following this assumption, weaning in humans should occur after about nine months of lactation. However, this one-to-one correlation is strongly influenced by the size of the adult animal. In many small primates, the duration of lactation is shorter than the duration of gestation. In large primates, lactation may significantly exceed the average time of pregnancy. Compared with our close relatives, such as chimpanzees and gorillas, humans, who share more than 98% of their genetic material with them, would appear to be designed to be breastfed for at least six times the duration of pregnancy, which means about 4.5 years.

Weaning according to teeth development

Research indicates that many primates stop nursing their young when they develop their first permanent molar teeth. In modern humans, the first permanent molars usually appear around 5.5 to 6 years of age. Interestingly, humans also attain immunological autonomy when they are about six years old, suggesting that throughout our recent evolution, children would have had active immunity provided by breast milk until about that age.

For all these reasons, Dr. Dettwyler established the natural age for human weaning between two and a half and seven years. However, this type of weaning should not be confused with the so-called “respectful weaning,” which is a cultural construct that has been established subsequently and which advocates that the weaning process cannot be started until two and a half years of age.

References

Padró, A. (2021). Destete: final de una etapa. Grijalbo.