Congratulations to Joseph El Adli who successfully defended his dissertation on December 5, 2017
Advisor: Daniel C. Fisher
The end of the Pleistocene saw the extinction of many large vertebrate species across much of the Earth, including mammoths (genus Mammuthus). Despite many decades of work by various researchers, the cause(s) of the late Pleistocene extinction are still heavily debated, with climate change and human hunting being the two primary culprits. The problem of identifying the cause of this extinction is mostly due to the fact that changing climates and appearance of human hunters both coincide with the timing of mammoth extinction, making decoupling of one potential cause from the other difficult. This study bypasses the strictly chronological work of many previous studies and takes a different approach to addressing the cause of the end Pleistocene extinction by utilizing information about reproductive life history. Age of first conception and the average time between conceptions are expected to change both predictably and divergently under a climate- versus hunting-driven extinction, so assessment of changes in these aspects of life history towards 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. Using thin section and serial isotope analyses, this study documents the periodicity of X-ray density features observed in microCT sections of tusks. These attenuation features are found form annually in both Columbian and woolly mammoths (Mammuthus columbi and Mammuthus primigenius, respectively), but are semiannual in a gomphothere from South America (identified as Stegomastodon platensis). MicroCT scans of entire tusks are employed to provide a record of multiple decades of growth for several Siberian woolly mammoths. These records reveal a regular 3 to 6 year-long pattern of growth in all adult females that is absent in both males and juveniles. This pattern is interpreted as the signal for calving interval. The onset of this pattern is observed in several individuals occurring at an age approximating that of sexual maturity in extant elephants. Though limited in number of specimens, this dataset shows a minor decrease in age of maturation and calving interval duration towards the end of the Pleistocene, an expectation of the hunting-driven extinction hypothesis to the exclusion of climate change. This work contributes to our knowledge of mammoth life history and the patterns of tusk growth associated with aspects of reproduction, which may be key to our understanding of the cause of megafaunal extinction.