Duration of Lactation
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The duration of lactation in most mammals lasts from birth to the eruption of the first permanent molar. The correlation coefficient between first molar eruption and weaning (cessation of lactation) is r = 0.9. This is also typical for most primates. However, both chimpanzees and humans deviate from this general mammalian pattern. Chimpanzees wean at a mean age of 4.8 years, which is about 1.7 years after first permanent molar eruption. Humans living in traditional foraging, horticultural, pastoral, and agricultural societies wean at a mean age of 2.8 years, which is about 3.4 years before first permanent molar eruption. There is at least one hypothesis to explain the chimpanzee-human difference, and the deviation from other mammalian species. The infant chimpanzee is totally dependant on its mother for feeding and the infant requires 4.5 years to learn how to find sufficient food to be independent of its mother. Chimpanzees must defer feeding independence until the juvenile stage of their life cycle. Humans infants, in contrast, are cared for by many members of the social group and by age 2.8 – 3.0 years enter the childhood stage of development when they are capable of being fed specially prepared complimentary foods by non-maternal caretakers.
Modern Human Life History: The Evolution of Human Childhood and Fertility, , The Evolution of Human Life History, Santa Fe, (2006)