Permanent Signatures of Birth and Nursing Initiation are Chemically Recorded in Teeth

In 2013 we presented a model for identifying nursing behavior from primate teeth based on rapid postnatal concentration changes in the non-essential trace element barium. Here we leverage the permanent neonatal (birth) line in the enamel of several dozen primate M1 cusps to compare pre- and postnatal trends in barium, zinc, strontium, and oxygen, as each element is believed to evince developmental patterning. Barium and zinc are the most consistent biomarkers of nursing initiation; a majority of M1 cusps show concentration increases from prenatal to postnatal enamel, whereas strontium shows no change or decreases with equal frequency. Exceptions to the pattern of barium increase occurred in cusps that had been mineralizing for less than three weeks, suggesting that subsequent enamel maturation has only a minor impact on detecting real time events. Oxygen isotope compositions (δ18O) show marked fluctuations (~ 1–2 ‰) within a few weeks of birth in 92% of M1 cusps (n = 22/24). This is likely due to measurements of hypomineralized perinatal enamel, as well as physiological changes in the body water of newborn infants. Ongoing work integrating elemental concentration gradients with isotopic variation will help establish the degree to which milk intake may cause elevated δ18O in teeth. We show that chemical identification of pre- to postnatal transitions may be robust to slight planar deviations that often obscure growth increments under light microscopy, and could help validate the identification of potential neonatal lines, making this approach a useful complement to bioarchaeological studies and public health investigations.