Calcium Isotopic Composition in Reindeer (Rangifer tarandus) Bones and Tooth Enamel: A Scalable Tool to Quantify Nursing Frequency in Past and Modern Populations?

Abstract

Rationale

Populations of North American caribou and Eurasian reindeer (Rangifer tarandus sspp.), essential for Arctic ecosystems and human communities, are experiencing unprecedented declines due to climate change and anthropogenic pressures. While understanding their historical resilience and demographic dynamics is critical for conservation, long-term population trends and the mechanisms driving these declines remain partly unresolved.

Methods

This study explores if calcium (Ca) isotopic compositions (δ^44/42Ca) measured by MC-ICP-MS could help quantify the number of nursing periods experienced by female reindeer during their lifetimes, as proxy for monitoring fecundity throughout the Quaternary. This pilot study focuses on five reindeer specimens who lived in controlled environmental conditions with documented life histories (including periods spent nursing). Skeletal elements with specific mineralization timings (molars and jawbones) were targeted to help untangle the effect of nursing from confounding factors, such as specimen specific diet and physiological baselines.

Results

Despite this sampling approach, no clear effect of nursing was observable in jawbones δ^44/42Ca values. We hypothesize that this may be the consequence of differences in experienced nursing activity and isotopic fractionation compared to other cases from the literature or of a faster than expected turnover rate of Ca in Rangifer jawbones. In parallel, insights of age-related Ca imbalance from an undiagnosed pathology (e.g., osteoporosis) were found in one specimen, while generated data also provide complementary perspectives for investigating seasonal diet and weaning habits in Rangifer.

Conclusions

While exploring past fecundity might prove difficult using this approach, these results are promising regarding the study of paleo-diet and pathology. A path for future methodological improvements to address the shortcomings of our approach is outlined. This work notably underscores the urgent need for better quantifying elemental turnover rates in bones and for more controlled environment studies, two critical steps to develop robust geochemical proxies for wildlife conservation and paleoecology.