Records of Growth and Weaning in Fossil Proboscidean Tusks as Tests of Pleistocene Extinction Mechanisms.

Abstract

Mammoths (Mammuthus primigenius) and mastodons (Mammut americanum) became extinct about 10,000 ago. Their disappearances coincide broadly with the timing of two possible causal factors: climate change leading into the current interglacial (Holocene) and overhunting by humans, who were spreading across the globe at the time. Mammoths and mastodons had lived through several previous interglacial intervals with climate similar to that of the Holocene. However, expanding human populations presented a new potential threat. Climate-related extinction hypotheses assert that conditions at the end of the most recent glacial period were somehow different enough from those during previous warming events that these animals could not cope with the changing environments. Hunting-based explanations suggest that increasing exploitation by humans caused mammoth and mastodon populations to fail. Fossil proboscidean tusks provide remarkable growth records that can be used to evaluate past events 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. Tusk analyses for Ziegler Reservoir mastodons (Snowmass Village, CO) show no evidence that populations were struggling during the previous interglacial (Sangamonian) when climate was similar to current conditions. Poor nutrition is likely to result in later weaning age in mammals. However, in the interval of warming leading up to their extinction, Siberian woolly mammoths were apparently weaning earlier than they had been during the last glacial maximum. The shift to earlier weaning at the end of the Pleistocene refutes climate-related nutritional stress as a mechanism for their extinction. Population pressure from human hunting, which is expected to result in earlier weaning, is a more likely explanation for mammoth population declines. Two reasons that the causes of the late Pleistocene extinctions are debated are (1) the poorly developed understanding of how populations fared during previous interglacials, and (2) the general lack of data that support one competing causal model to the exclusion of others. This study contributes to both of these aspects of the discussion and demonstrates methods that will help bring resolution to our understanding of the mechanisms behind these extinctions.