Growth velocity and weaning δ(15)N "Dips" during ontogeny in Macaca mulatta

A "dip" in the stable nitrogen isotope ratios (δ(15)N) of subadults in the late weaning/early post-weaning phase of growth and development has been observed. Speculatively, this is the mechanism of positive nitrogen balance operating among rapidly growing subadults. An alternate hypothesis for δ(15)N dips is that during weaning, subadults eat lower-(15)N foods than adults.

This study explores the role of positive nitrogen balance in affecting δ(15) N variation of growing subadults by comparing growth velocity with stable carbon isotope (δ(13)C) and δ(15)N ratios of blood serum from captive rhesus macaques (Macaca mulatta) (n = 14) with controlled diets during the first 10 months of life.

During the first six months, δ(15)N values are inversely correlated with growth in some of the anthropometrics (weight and sagittal circumference). Dips in some infants' δ(15)N values below their mothers' values are observed at the end of the weaning period. However, during this time frame, δ(15)N values of the infants are not correlated with anthropometric indices. Serum stable isotope ratios of lactating and non-lactating adult females differ significantly.

Growth in body mass and size explains some of the variation in infant δ(15)N values, but are not responsible for dips in the late weaning/early post-weaning phase. It is advised that future research evaluate the extent to which growth in other body systems affects nitrogen balance and δ(15)N dips during ontogeny, and expand on isotopic differences between lactating and non-lactating females.