Tusk growth rate and season of death of Mammuthus columbi from Hot Springs, South Dakota, USA

Samples of dentin from the proximal end of mammoth tusks provide information on paleoclimate and tusk growth rate during the last years of life. We have used such data to determine season of death for multiple individuals and to characterize the cycle of seasonal variation in environmental conditions typical for the region of the Mammoth Site of Hot Springs, South Dakota, 26,000 yBP. We document patterns of change in oxygen isotope composition of dentinal hydroxyapatite and in thickness of circaseptan incremental features in tusk dentin. Oxygen isotope profiles show annually repeating patterns with a long autumn-winter phase of declining values and a shorter springsummer phase of rising values. Tusk growth rate profiles confirm the annual nature of these cycles and, in certain years, the isotope-based interpretation of season. Death seems to have had a bimodal distribution, occurring in a pattern implying elevated risk in autumn and spring. Moreover, death is usually preceded by an interval of reduced seasonal range in oxygen isotope values and of aseasonal (highly variable) patterns of tusk growth. We interpret this as reflecting a period of increased reliance on the isotopically stable water and food resources associated with one or more of the sinkhole-artesian spring systems that formed this site and others like it in the vicinity. Tusks thus record periods both before and after development of this site-fidelity.