Fisher 9628063 Near the close of the Pleistocene epoch, 10,000 years ago, many of the largest and most impressive mammals inhabiting North America went extinct. Among the most conspicuous of these victims were mastodons and mammoths, both of which were relatives of elephants and members of the mammalian order Proboscidea. The cause of this "late Pleistocene megafaunal extinction" has been vigorously debated by paleontologist and archaeologists, with implications for evolutionary ecology, global climate change, and conservation of biodiversity. Opposing schools of thought attribute the extinction to climate change associated with the end of the last Ice Age, excessive hunting by humans , or possibly some combination of these factors. The project proposed here continues a relatively new approach to resolving this controversy, distinguished by its ability to associate clearly divergent predictions with the operation of contrasting extinction mechanisms. It focuses on the structure and composition of the tusks and molar teeth of mastodons and mammoths, revealing new information about these animal's growth rates throughout life, their individual reproductive histories, and the environments in which they lived. Mastodon and mammoth tusks, like those of elephants, began growing shortly after birth and continued throughout life, adding layer upon layer of new material. Microscopic analysis of tusks cross sections reveals structural and compositional laminae, somewhat like tree rings, representing regular intervals of time (daily, fortnightly, and annual). Records of growth in molar teeth are shorter but otherwise comparable. Variations in the thickness of these layers record changes in tusks growth rate and indirectly reflect the animal's nutritional and reproductive status. Variations in composition of layers, especially with respect to oxygen isotopes, reflect intra- and interannual climate changes and confirm annual features for use in age determination. Analyses of these patterns yield data o n seasonal mortality, age, gender, growth rates, age at sexual maturity, and calving. For example, sexual maturation in males is reflected as a pronounced drop in growth rate caused by nutritional stress following expulsion of maturing males from the matriarchal family units in which they were nurtured as calves. Calving histories of females appear as multiyear cycles of variations in tusk growth rate, reflecting a tradeoff between tusk growth on one hand and fetal bone development and milk production on the other . Life history traits such as age at maturity and calving interval are sensitive indicators of the nature and degree of ecological stress because they show predictable patterns of response to altered nutritional status and/or predation pressure. Deleterious climate and/or vegetational change tends to retard growth and reproductive schedules, while overhunting tends to accelerate them. The proposed research will use patterns of tusk/tooth lamination and composition to quantify changes in life history traits of North America proboscideans leading up to their time of extinction. Such trends, within species, represent a population's own response to changing conditions and thus comprise a sensitive record of the nature of local ecological stress. the goal of this work is to test the two major, competing causal hypotheses for the late Pleistocene extinction of mastodons and mammoths: overkill and climatic/vegetational change. In addition to contributing to the solution of a major problem in the history of the North American biota, the results of this study will clarify the nature and history of human interaction with proboscidean in North America and provide new information relevant to paleoclimate reconstruction.