Quantitative reconstruction of weaning ages in archaeological human populations using bone collagen nitrogen isotope ratios and approximate Bayesian computation

Abstract

Background: Nitrogen isotope analysis of bone collagen has been used to reconstruct the breastfeeding practices of archaeological human populations. However, weaning ages have been estimated subjectively because of a lack of both information on subadult bone collagen turnover rates and appropriate analytical models.

Methodology: Temporal changes in human subadult bone collagen turnover rates were estimated from data on tissue-level bone metabolism reported in previous studies. A model for reconstructing precise weaning ages was then developed using a framework of approximate Bayesian computation and incorporating the estimated turnover rates. The model is presented as a new open source R package, WARN (Weaning Age Reconstruction with Nitrogen isotope analysis), which computes the age at the start and end of weaning, (15)N-enrichment through maternal to infant tissue, and [Formula: see text] value of collagen synthesized entirely from weaning foods with their posterior probabilities. The model was applied to 39 previously reported Holocene skeletal populations from around the world, and the results were compared with weaning ages observed in ethnographic studies.

Conclusions: There were no significant differences in the age at the end of weaning between the archaeological (2.80±1.32 years) and ethnographic populations. By comparing archaeological populations, it appears that weaning ages did not differ with the type of subsistence practiced (i.e., hunting-gathering or not). Most of [Formula: see text]-enrichment (2.44±0.90‰) was consistent with biologically valid values. The nitrogen isotope ratios of subadults after the weaning process were lower than those of adults in most of the archaeological populations (-0.48±0.61‰), and this depletion was greater in non-hunter-gatherer populations. Our results suggest that the breastfeeding period in humans had already been shortened by the early Holocene compared with those in extant great apes.

Figure 1. Schematic illustration of the bone turnover process.

The value for bone collagen at the unit time age of years is represented as .

Figure 2. Estimated temporal changes in bone mineral and collagen turnover rates.

Turnover rates of bone minerals and collagen are represented as discrete values, and that of collagen is fitted to QP plotted against age.

Figure 3. Relationships between MDEs and the logarithms of the probabilities of the four weaning parameters for the archaeological populations.

Dotted lines indicate the thresholds of probability. (A) and (B) The age at the start and end of weaning, respectively. (C) -enrichment from maternal to infant tissue. (D) The difference between the value of collagen synthesized entirely from weaning foods and the mean value for all adults. A regression line is also shown for (slope = −0.514, intercept = −3.403).

Figure 4. Histograms for the MDE distributions of the four weaning parameters for the archaeological populations.

See Figure 3 for the meanings of the labels.

Figure 5. Maximum density weaning ages plotted by the midpoint time period for the populations.

Figure 6. Boxplots for the weaning ages estimated in this study and observed in previous ethnographic studies.

Data were obtained from Barry and Paxson (1971): , Ford (1964): , and Sellen (2001): .

Figure 7. Schematic illustration of the terminology used for the subadult diet.

The ages at the start and end of weaning are represented as and , respectively.