Abstract
We present the isotopic discrimination between paired skin and bone collagen from animals of known life history, providing a modern baseline for the interpretation of archaeological isotopic data. At present, the interpretation of inter-tissue variation (Δ(skin–bone)) in mummified remains is based on comparisons with other archaeological material, which have attributed divergence to their contrasting turnover rates, with rapidly remodelling skin collagen incorporating alterations in environmental, cultural and physiological conditions in the months prior to death. While plausible, the lack of baseline data from individuals with known life histories has hindered evaluation of the explanations presented. Our analysis of a range of animals raised under a variety of management practices showed a population-wide trend for skin collagen to be depleted in 13C by –0.7‰ and enriched in 15N by +1.0‰ relative to bone collagen, even in stillborn animals. These results are intriguing and difficult to explain using current knowledge; however, on the basis of the findings reported here, we caution any results which interpret simply on differing turnover rates. We hypothesize that there may be a consistent difference in the routing of dietary protein and lipids between skin and bone, with potentially on-site synthesis of non-essential amino acids using carbon and nitrogen that have been sourced via different biochemical pathways.
Highlights
To test the null hypothesis that there is no significant difference between δ13C and δ15N values in skin and bone collagen within an individual, we report inter-tissue discrimination in 25 sheep (Ovis aries) and five pigs (Sus scrofa) with known life histories, examining the impact of sex, age and health status
The disparity between observed and theoretical C:N ratios in skin may indicate the presence of non-collagenous proteins such as elastin (C:N 5.8) and epithelial keratins (C:N 3.4) which account for 5% of the protein fraction [41]
Contrary to the assumptions made by previous studies, we reject the hypothesis that δ13C and δ15N variation between skin and bone collagen in an individual is the result of their contrasting turnover rates
Summary
With the δ13C and δ15N of proteinaceous tissue reflecting an individual’s dietary intake and physiological condition during development [1,2,3,4,5], the paired analysis of skin collagen—remodelling on the scale of weeks and months [6,7]—and bone collagen—on the order of years and decades [8,9]—has been used to examine short- and long-term trends in diet, migration, health and cultural practices [10,11,12,13,14,15,16,17,18,19]. Δ(skin–bone) discrimination has been attributed to alterations in cultural and environmental conditions in the months prior to death, including substantial dietary change and/or geographical movement, as well as physiological and nutritional stress [11,12,13,14,15,16,17,18,19]. While these interpretations are plausible, the lack of baseline data from individuals with known life histories confounds evaluation of the explanations presented. Δ(skin–bone) discrimination in archaeological populations may be influenced by differential decomposition patterns [26] and the presence of exogenous contaminants [27,28,29]
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