Congratulations to Mike Cherney who successfully defended his dissertation on December 3, 2015.

Advisor: Dan Fisher

Abstract:  American mastodons (Mammut americanum) and woolly mammoths (Mammuthus primigenius) went extinct approximately concurrently, as part of the late Pleistocene mass extinction event. Their disappearances overlap generally with two possible causal mechanisms, changing climate as glaciers receded and human hunting. Previous shifts to warm interglacial intervals had occurred without inducing mass extinctions of megafauna. In contrast, modern humans with advanced tool technologies had never before been so widely distributed across the globe. Climate-related extinction hypotheses assert that there was something different about the last glacial retreat that upset ecosystems more than previous warming events did. Hunting-based explanations suggest that exploitation pressure from increasing human presence caused populations to decline below sustainable levels. Fossil proboscidean tusks provide remarkable growth records that can be used to evaluate past conditions by revealing how individual life histories responded to environmental conditions. If climate was detrimental, we would expect to see evidence of slowed (and possibly more variable) growth, delayed maturation, and lower fecundity. Weaning age is a particularly informative life-history milestone that incorporates each of these parameters to some extent, and should increase when energy budgets are low or inconsistent. Tusk analyses for mastodons living in a montane setting during the previous interglacial (Sangamon) show no evidence that populations were struggling at a time when climate was similar to current conditions. In addition, weaning is interpreted as occurring later in Siberian woolly mammoths living during the last glacial maximum than during warmer periods, both in the prior interstadial and when glaciers retreated leading up to the time of extinction. This evidence suggests that terminal populations were not stressed by climate, and supports hunting by humans, which is expected to result in younger weaning, as a more likely explanation for population declines. Two reasons that the causes of the late Pleistocene extinctions are still hotly debated are the poorly developed understanding of how populations fared during previous interglacials and the general lack of data that support one competing model and exclude the others. This study contributes to both of these aspects of the discussion and demonstrates methods that may help resolve the mechanisms behind these extinctions.