Reproductive Life Histories of Mammoths

Abstract

The end of the Pleistocene saw the extinction of many large vertebrate species, including mammoths (genus Mammuthus). Despite many decades of work by various researchers, the cause(s) of mammoth extinction are still heavily debated, with climate change and human hunting being the two primary hypothesized agents of extinction. One major problem with identifying the cause of this extinction is the fact that changing climates and movement of human hunters into ecosystems containing mammoths are both broadly associated with the time of extinction, making it difficult to decouple one potential cause from the other using only temporal data. This study bypasses the strictly chronological approach of many previous studies and instead investigates the cause of the end-Pleistocene extinction using information about reproductive life history. The age of first conception and the average time between conceptions are both expected to change predictably and divergently under the hypotheses of climate-driven extinction and hunting-driven extinction, so assessment of changes in these aspects of life history approaching the time of extinction could provide a test for cause of extinction. I use the record of growth within tusk dentin to identify patterns associated with reproductive life history in mammoths. Thin sections and serial isotope analyses document the periodicity of X-ray density features observed in microCT sections of tusks. These attenuation features form annually in both Columbian and woolly mammoths (Mammuthus columbi and Mammuthus primigenius, respectively), but form semiannually in a gomphothere from South America. MicroCT scans of entire tusks are employed to provide a record of multiple decades of growth for ten Siberian woolly mammoths. In eight of these specimens, all of them adult females, we observe a repeated 3- to 6-year-long cyclical pattern of regularly varying growth rate. This pattern was absent in both adult males and juveniles. We interpret this pattern as a record of calving in females, and its onset is observed in several individuals to occur at an age approximating that of sexual maturation in extant elephants. Our dataset shows a minor decrease in age of maturation and average calving interval near the end of the Pleistocene. This is predicted by a hunting-driven model of extinction but is not expected for extinction driven by climate change. This work contributes to our knowledge of the reproductive life history of mammoths, which we argue is key to understanding the cause of their extinction.