Our 2017 study of calcium-normalized barium (Ba/Ca) patterns in four wild orangutan dentitions demonstrated a novel phenomenon of increased nursing on an approximately annual basis, which may relate to seasonal cycles of resource availability. Here we explore this further in their ܪrst molar tooth enamel by measuring oxygen isotope compositions (Ƚ18O), which vary with milk consumption, temperature, precipitation, and evaporation cycles during an animal’s development. We characterized molar development and developmental stress histologically, followed by trace element distributions and oxygen isotopes measured in situ on a weekly-basis over 3–5 years per tooth. Both Ba/Ca and Ƚ18O increased during the ܪrst year of life, consistent with ܪeld reports of exclusive nursing. Postnatal increases in Ƚ18O ranged from 3 to10 ‰, in excess of reported isotopic enrichment in human infants. Ƚ18O began to cycle before Ba/Ca, decreasing overall after the ܪrst year in three individuals. It is likely that these approximately annual Ƚ18O cycles reܫect meteoric water variation in plants consumed by infants, as Ba/Ca and Ƚ18O are rarely in phase after the ܪrst year of life, when orangutans typically begin to supplement their mother’s milk with solid foods. Subtle accentuated lines were observed on an approximately annual basis over 1–2 years, but there was no consistent relationship with trends in Ba/Ca or Ƚ18O, underscoring the multifactorial nature of developmental disruptions in teeth. Studies of juveniles with concurrent environmental and dietary records are needed to further clarify the enigmatic pattern of prolonged cyclical nursing in Bornean and Sumatran orangutans.